<code>Monad</code> Tutorial

Monad Tutorial

References: <1745863.rgDDbheD5J@localhost> Message-ID: > By the way, not all databases supported by Persistent have the ability to > represent NUMERIC with perfect precision. I'm fairly certain the SQLite > will just cast to 8-byte reals, though it's possible that it will keep the > data as strings in some circumstances. According to the documentation, SQLite stores whatever you give it, paying very little heed to the declared type. If you get SQLite to *compare* two numbers, it will at that point *convert* them to doubles in order to carry out the comparison. This is quite separate from the question of what it can store. From radical at google.com Sat Jan 26 07:00:29 2013 From: radical at google.com (Alvaro Gutierrez) Date: Sat, 26 Jan 2013 01:00:29 -0500 Subject: [Haskell-cafe] ANN: monad-bool 0.1 In-Reply-To: References: <20130124074045.63cf39dd@tritium.streitmacht.eu> <510313D9.9000902@freegeek.org> Message-ID: A brief stylistic note: to me, defunct has a connotation similar to that of deprecated, just stronger; meaning, it implies something closer to "NoLongerOnHackage" rather than wren's more general "NotOnHackage." In this case, the distinction is moot, because the code did happen to exist on Hackage, but I imagine there are cases one might want to warn against without this condition being true. On Fri, Jan 25, 2013 at 6:32 PM, David Thomas wrote: > I think it needs to be both places. I know when I'm searching, I often > just go to google with "site:hackage.haskell.org inurl:latest" I would > be likely to miss it if it were just in the cabal file (although in the > modules it could be as simple as a note saying "this is defunct - view the > main page for an explanation of why). Alternatively, if there is much > reasoning specific to a particular module it could certainly go there, > while the cabal gets the overview... > > > On Fri, Jan 25, 2013 at 3:23 PM, wren ng thornton wrote: > >> On 1/24/13 1:40 AM, Ertugrul S?ylemez wrote: >> >>> David Thomas wrote: >>> >>> We could even set up NotOnHackage: a "package" repository just like >>>>> Hackage, except the packages are just documentation on why there is >>>>> no such package. Implementation-wise it's just a wiki; but the idea >>>>> is about how to organize the wiki. NotOnHackage should be organized >>>>> and searchable similar to Hackage itself, so that people can look >>>>> there when nothing looks promising on Hackage. >>>>> >>>> >>>> Couldn't this be actually on hackage, so one search turns up what you >>>> want? >>>> >>> >>> Yes. There is no reason to put up a second Hackage for that one. >>> Without changing anything in the current system, packages can just >>> update their categories, so that they will be displayed below "Defunct" >>> or something like that. This is fine, as only the categories of the >>> latest version are significant. >>> >>> If you think this is a good idea, I will start with some of my >>> packages. =) >>> >> >> We've had package deprecation for a while, so the big trick IMO is the >> documentation. Good descriptions of why the package is defunct and >> suggestions on how people can do things better. >> >> If we're going to do it on Hackage itself, I think the big question is >> one of style: should the documentation be all in the cabal file (i.e., on >> the package description page, with no modules in the package); or should we >> put the documentation into modules? >> >> >> -- >> Live well, >> ~wren >> >> ______________________________**_________________ >> Haskell-Cafe mailing list >> Haskell-Cafe at haskell.org >> http://www.haskell.org/**mailman/listinfo/haskell-cafe >> > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From s9gf4ult at gmail.com Sat Jan 26 07:03:42 2013 From: s9gf4ult at gmail.com (s9gf4ult at gmail.com) Date: Sat, 26 Jan 2013 12:03:42 +0600 Subject: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ? In-Reply-To: <1359174588.hlpHosRzO2@localhost> References: <1359174588.hlpHosRzO2@localhost> Message-ID: <1925154.SGYH3JLzHb@localhost> http://ideone.com/v2CrAm I has posted to ideone to show what is wrong. -------------- next part -------------- An HTML attachment was scrubbed... URL: From alex.solla at gmail.com Sat Jan 26 07:17:57 2013 From: alex.solla at gmail.com (Alexander Solla) Date: Fri, 25 Jan 2013 22:17:57 -0800 Subject: [Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ? In-Reply-To: <3063314.mL3bUc6uXO@localhost> References: <1359174588.hlpHosRzO2@localhost> <3063314.mL3bUc6uXO@localhost> Message-ID: On Fri, Jan 25, 2013 at 12:39 PM, wrote: > ** > > > This has the code smell of trying to use typeclasses for OOP. That > won't work. (Yes, really.) > > > > I am not trying to use OOP, I am just writing some typecasting at all. > > > > > This would be correct. Constraints on an instance are applied *after* > the instance is selected, so when Haskell is looking for an instance, these > two are identical. > > > > I didn't understand why these two instances are identical ? The > constraints are different and OverlappingInstances should permit > overlapping typeclasses in constraints and select more specific instance > clause. > They are identical because constraints don't "count" for deciding that a type is in a class. For the purposes of deciding if a type is in a class, instance Foo (Bar a) instance Fizz a => Foo (Bar a) instance Fuzz a => Foo (Bar a) are exactly the same, and all three are therefore overlapping instances. None is more specific, because they all refer to the same type -- (Bar a). Also, you can just use Typeable instead of that downcasting stuff. -------------- next part -------------- An HTML attachment was scrubbed... URL: From s9gf4ult at gmail.com Sat Jan 26 07:21:02 2013 From: s9gf4ult at gmail.com (s9gf4ult at gmail.com) Date: Sat, 26 Jan 2013 12:21:02 +0600 Subject: [Haskell-cafe] How to store Fixed data type in the database with persistent ? In-Reply-To: References: <1745863.rgDDbheD5J@localhost> Message-ID: <6151944.YT1pq8IXuI@localhost> > According to the documentation, SQLite stores whatever you give it, > paying very little heed to the declared type. If you get SQLite to > *compare* two numbers, it will at that point *convert* them to doubles > in order to carry out the comparison. This is quite separate from the > question of what it can store. CREATE TABLE t1(val); sqlite> insert into t1 values ('24.24242424') ...> ; sqlite> insert into t1 values ('24.24242423') ...> ; sqlite> select * from t1 order by val; 24.24242423 24.24242424 sqlite> select * from t1 order by val desc; 24.24242424 24.24242423 sqlite> select sum(val) from t1; 48.48484847 it seems Sqlite can work with arbitrary percission data, very good ! Persistent must have ability to store Fixed. From mle+hs at mega-nerd.com Sat Jan 26 09:24:33 2013 From: mle+hs at mega-nerd.com (Erik de Castro Lopo) Date: Sat, 26 Jan 2013 19:24:33 +1100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication Message-ID: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Hi all, I am in the process of writing a Haskell program consisting of two threads, one for performing a calculation and one for an Ncurses UI (haskell-ncurses). The calculation thread needs to feed a stream of numbers back to the UI thread (about 1 value per second) and the UI needs to take input from the user and will pass parameter changes to the calculation thread using an IORef and atomicModifyIORef. However, I'm not sure how to hande the UI thread. The UI thread would normally wait for Ncurses input using getEvent, but I need advice on how to the the data from the calculation thread. Any advice or things to try? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/ From evohunz at gmail.com Sat Jan 26 10:46:04 2013 From: evohunz at gmail.com (Thiago Negri) Date: Sat, 26 Jan 2013 07:46:04 -0200 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: Do you need advice on what? I didn't understand your last phrase. Em 26/01/2013 06:25, "Erik de Castro Lopo" escreveu: > Hi all, > > I am in the process of writing a Haskell program consisting of two > threads, one for performing a calculation and one for an Ncurses UI > (haskell-ncurses). > > The calculation thread needs to feed a stream of numbers back to the > UI thread (about 1 value per second) and the UI needs to take input > from the user and will pass parameter changes to the calculation > thread using an IORef and atomicModifyIORef. > > However, I'm not sure how to hande the UI thread. The UI thread would > normally wait for Ncurses input using getEvent, but I need advice on > how to the the data from the calculation thread. > > Any advice or things to try? > > Cheers, > Erik > -- > ---------------------------------------------------------------------- > Erik de Castro Lopo > http://www.mega-nerd.com/ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From es at ertes.de Sat Jan 26 10:56:26 2013 From: es at ertes.de (Ertugrul =?UTF-8?B?U8O2eWxlbWV6?=) Date: Sat, 26 Jan 2013 10:56:26 +0100 Subject: [Haskell-cafe] ANN: monad-bool 0.1 References: <20130124074045.63cf39dd@tritium.streitmacht.eu> <510313D9.9000902@freegeek.org> Message-ID: <20130126105626.3a78f9bb@tritium.streitmacht.eu> wren ng thornton wrote: > > Yes. There is no reason to put up a second Hackage for that one. > > Without changing anything in the current system, packages can just > > update their categories, so that they will be displayed below > > "Defunct" or something like that. This is fine, as only the > > categories of the latest version are significant. > > > > If you think this is a good idea, I will start with some of my > > packages. =) > > We've had package deprecation for a while, so the big trick IMO is the > documentation. Good descriptions of why the package is defunct and > suggestions on how people can do things better. > > If we're going to do it on Hackage itself, I think the big question is > one of style: should the documentation be all in the cabal file (i.e., > on the package description page, with no modules in the package); or > should we put the documentation into modules? I think the package should be included in full, and the package documentation should clarify why the package shouldn't be used. The idea is that people can still download the code and see how not to do it. It also helps to keep legacy code working, because "bad idea" doesn't necessarily mean, "you could die if you use this". You might go as far as implementing special support for this in the cabal-install tool in the form of a flag like --allow-defunct. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 836 bytes Desc: not available URL: From tab at snarc.org Sat Jan 26 11:08:56 2013 From: tab at snarc.org (Vincent Hanquez) Date: Sat, 26 Jan 2013 11:08:56 +0100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: <20130126100855.GA24156@snarc.org> On Sat, Jan 26, 2013 at 07:46:04AM -0200, Thiago Negri wrote: > Do you need advice on what? I didn't understand your last phrase. IIUC, it has to do which how to interrupt a blocking call to ncurses event handling, from another thread (the calculation thread) to let the UI thread refresh the UI with something to display (the calculation). I've been confronted with the same problem in the past, and i haven't found a satisfactory solution with ncurses. Erik, I think the only way to do that with ncurses, is to wrap the ncurses getevent with a select loop on the terminal fd and an arbitrary fd to be able to "ping" the select loop for arbitrary processing. You can also replace the arbitrary fd with a SIGALARM to yourself which i believe would interrupt select too. I don't think there's any solution that integrate nicely with the haskell way, but i'ld be happy to be wrong on this. In my case, i switched to Vty which solved this problem, by allowing to send arbitrary strongly-typed event to widgets. -- Vincent From mle+hs at mega-nerd.com Sat Jan 26 11:28:34 2013 From: mle+hs at mega-nerd.com (Erik de Castro Lopo) Date: Sat, 26 Jan 2013 21:28:34 +1100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: <20130126212834.e12246f7e15a965567b1f9c8@mega-nerd.com> Thiago Negri wrote: > Do you need advice on what? I didn't understand your last phrase. Well I have data from two sources, stdin and the calculation thread. If I was doing this in C, I'd probably use a pipe for the calculation data and then do select on the two file descriptors. There is a select package: http://hackage.haskell.org/package/select but I was wondering if there was a more idiomatic Haskell way of dealing with inputs from more than one source. Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/ From es at ertes.de Sat Jan 26 11:46:21 2013 From: es at ertes.de (Ertugrul =?UTF-8?B?U8O2eWxlbWV6?=) Date: Sat, 26 Jan 2013 11:46:21 +0100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> <20130126212834.e12246f7e15a965567b1f9c8@mega-nerd.com> Message-ID: <20130126114621.058a01f9@tritium.streitmacht.eu> Erik de Castro Lopo wrote: > > Do you need advice on what? I didn't understand your last phrase. > > Well I have data from two sources, stdin and the calculation > thread. If I was doing this in C, I'd probably use a pipe for the > calculation data and then do select on the two file descriptors. > > There is a select package: > > http://hackage.haskell.org/package/select > > but I was wondering if there was a more idiomatic Haskell way of > dealing with inputs from more than one source. Of course. Threads communicate through MVars, Chans and other abstractions. If you have more complicated scenarios like one thread communicating with multiple other threads in a selective fashion you may also be interested in software transactional memory (STM). If you are using separate threads to perform calculations, you should make sure that you only pass fully evaluated values. In many cases it suffices to evaluate to WHNF, in which case you can simply write: putMVar var $! x This writes the WHNF of 'x' to the MVar 'var'. If the values you pass around are of more complicated data types with non-strict parts (like (Integer, Integer) instead of just Integer), you can also evaluate to NF: import Control.DeepSeq putMVar var $!! x Another option is go without concurrency entirely. Since this is about parallel calculations, I suppose that your computations are actually pure. Let me give you an example: let xs :: [Integer] xs = map (^1000000) [2..1000] mapM_ print xs To parallelize this you can simply use parallel strategies: import Control.Parallel.Strategies let xs :: [Integer] xs = parMap rseq (^1000000) [2..1000] mapM_ print xs This calculates the list values in parallel and is as simple as replacing "map" by "parMap rseq" or "parMap rdeepseq" (if the elements have non-strict parts like (Integer, Integer) instead of just Integer). Remember that for pure values you can always just say, "please evaluate this in parallel". No need at all to mess around with threads. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 836 bytes Desc: not available URL: From petr.mvd at gmail.com Sat Jan 26 11:51:56 2013 From: petr.mvd at gmail.com (Petr P) Date: Sat, 26 Jan 2013 11:51:56 +0100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: Hi Erik, there seem to be several options. Tie simplest one is that you'll call getEvent with some reasonable timeout in a loop and check some communication channel an update from the calculation thread. Probably the correct way how to handle it involves three threads: (1) The computation thread writes its updates to a channel of type Either Event ComputationUpdate using Right. (2) Another thread calls getEvent repeatedly and writes received events to the channel using Left. (3) A consuming thread (the one for Ncurses UI) consumes the channel and reacts both to computation updates and Ncurses events. However, I'm not sure if Ncurses doesn't mind calling getEvent from a different thread than the one that updates UI. For the communication channel, I'd probably use TQueue (or TBQueue, if there is a risk of writing values too fast). Best regards, Petr Pudlak 2013/1/26 Erik de Castro Lopo > Hi all, > > I am in the process of writing a Haskell program consisting of two > threads, one for performing a calculation and one for an Ncurses UI > (haskell-ncurses). > > The calculation thread needs to feed a stream of numbers back to the > UI thread (about 1 value per second) and the UI needs to take input > from the user and will pass parameter changes to the calculation > thread using an IORef and atomicModifyIORef. > > However, I'm not sure how to hande the UI thread. The UI thread would > normally wait for Ncurses input using getEvent, but I need advice on > how to the the data from the calculation thread. > > Any advice or things to try? > > Cheers, > Erik > -- > ---------------------------------------------------------------------- > Erik de Castro Lopo > http://www.mega-nerd.com/ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From jims at stuttard.org Sat Jan 26 14:47:25 2013 From: jims at stuttard.org (jims at stuttard.org) Date: Sat, 26 Jan 2013 06:47:25 -0700 Subject: [Haskell-cafe] aeson-0.6.1.0 deriveJSON error Message-ID: <20130126064725.15843zl9s0j3avsd@stuttard.org> ghc doesn't seem to be unifying deriveJSON (String->String) parameter with id :: a -> a. examples/TemplateHaskell.hs:22:14: Couldn't match expected type `Data.Aeson.TH.Options' with actual type `a0 -> a0' In the first argument of `deriveJSON', namely `id' In the expression: deriveJSON id ''Coord ghc --make: /usr/hs/ghc/7.4.1/bin/ghc failure (return code=1) ghc-7.4.1 x86_64 Centos-6 [jim at localhost aeson]$ ghc-pkg list --package-conf=../cabal-dev/packages-7.4.1.conf ../cabal-dev/packages-7.4.1.conf HTTP-4000.2.6 aeson-0.6.1.0 attoparsec-0.10.3.0 attoparsec-conduit-0.5.0.3 base-unicode-symbols-0.2.2.4 blaze-builder-0.3.1.0 blaze-builder-conduit-0.5.0.3 case-insensitive-0.4.0.4 conduit-0.5.6 dlist-0.5 hashable-1.2.0.5 http-types-0.7.3.0.1 lifted-base-0.2.0.2 monad-control-0.3.1.4 nats-0.1 network-2.4.1.0 network-conduit-0.6.2.2 parsec-3.1.3 primitive-0.5.0.1 resourcet-0.4.4 semigroups-0.9 simple-sendfile-0.2.10 transformers-base-0.4.1 unix-compat-0.4.1.0 unordered-containers-0.2.3.0 vault-0.2.0.4 vector-0.10.0.1 void-0.5.11 wai-1.3.0.1 warp-1.3.7.1 From v.dijk.bas at gmail.com Sat Jan 26 15:20:16 2013 From: v.dijk.bas at gmail.com (Bas van Dijk) Date: Sat, 26 Jan 2013 15:20:16 +0100 Subject: [Haskell-cafe] aeson-0.6.1.0 deriveJSON error In-Reply-To: <20130126064725.15843zl9s0j3avsd@stuttard.org> References: <20130126064725.15843zl9s0j3avsd@stuttard.org> Message-ID: On 26 January 2013 14:47, wrote: > ghc doesn't seem to be unifying deriveJSON (String->String) > parameter with id :: a -> a. It seems you're using aeson HEAD. Note that the deriveJSON from the released aeson-0.6.1.0 as the type: deriveJSON :: (String -> String) -> Name -> Q [Dec] But in aeson-HEAD it has the following type: deriveJSON :: Options -> Name -> Q [Dec] Cheers, Bas From v.dijk.bas at gmail.com Sat Jan 26 15:34:19 2013 From: v.dijk.bas at gmail.com (Bas van Dijk) Date: Sat, 26 Jan 2013 15:34:19 +0100 Subject: [Haskell-cafe] aeson-0.6.1.0 deriveJSON error In-Reply-To: References: <20130126064725.15843zl9s0j3avsd@stuttard.org> Message-ID: On 26 January 2013 15:20, Bas van Dijk wrote: > But in aeson-HEAD it has the following type: > > deriveJSON :: Options -> Name -> Q [Dec] Note that I'm currently working on extending the encoding Options record: * I added a constructorNameModifier :: String -> String which is applied to constructor names and is handy for lower-casing them for example. * I extended the sumEncoding with a ObjectWithSingleField constructor which causes a constructor to be encoded to an object with a single field named after the constructor (modified by the constructorNameModifier) and the value will be the contents of the constructor. * I'm also modifying the GHC Generics code to take the encoding Options into account. This work is happening in my parameterize-generic-encoding-with-options branch: https://github.com/basvandijk/aeson/commits/parameterize-generic-encoding-with-options Bas From jims at stuttard.org Sat Jan 26 16:23:48 2013 From: jims at stuttard.org (jims at stuttard.org) Date: Sat, 26 Jan 2013 08:23:48 -0700 Subject: [Haskell-cafe] aeson-0.6.1.0 deriveJSON error In-Reply-To: References: <20130126064725.15843zl9s0j3avsd@stuttard.org> Message-ID: <20130126082348.16856flqgicdj9qc@stuttard.org> Quoting Bas van Dijk : > On 26 January 2013 14:47, wrote: >> ghc doesn't seem to be unifying deriveJSON (String->String) >> parameter with id :: a -> a. > > It seems you're using aeson HEAD. Note that the deriveJSON from the > released aeson-0.6.1.0 as the type: Thanks for the instant help :) Now I know that versions numbers on github don't necessarily get bumped if there's a breaking change. I got confused when I found the hackage documentation was different . Cheers > deriveJSON :: (String -> String) -> Name -> Q [Dec] > > But in aeson-HEAD it has the following type: > > deriveJSON :: Options -> Name -> Q [Dec] > > Cheers, > > Bas > From jims at stuttard.org Sat Jan 26 16:40:54 2013 From: jims at stuttard.org (jims at stuttard.org) Date: Sat, 26 Jan 2013 08:40:54 -0700 Subject: [Haskell-cafe] aeson-0.6.1.0 deriveJSON error In-Reply-To: References: <20130126064725.15843zl9s0j3avsd@stuttard.org>

Message-ID: <20130126084054.12295bcrauf4i1om@stuttard.org> Quoting Bas van Dijk : > On 26 January 2013 15:20, Bas van Dijk wrote: >> But in aeson-HEAD it has the following type: >> >> deriveJSON :: Options -> Name -> Q [Dec] > Note that I'm currently working on extending the encoding Options record: > > * I added a constructorNameModifier :: String -> String which is > applied to constructor names and is handy for lower-casing them for > example. > > * I extended the sumEncoding with a ObjectWithSingleField constructor > which causes a constructor to be encoded to an object with a single > field named after the constructor (modified by the > constructorNameModifier) and the value will be the contents of the > constructor. > > * I'm also modifying the GHC Generics code to take the encoding > Options into account. > > This work is happening in my > parameterize-generic-encoding-with-options branch: Both hackage and your branch build TemplateHaskell.hs ok. I'll try and understand what you've done :) Cheers > https://github.com/basvandijk/aeson/commits/parameterize-generic-encoding-with-options > > Bas > From allbery.b at gmail.com Sat Jan 26 17:10:03 2013 From: allbery.b at gmail.com (Brandon Allbery) Date: Sat, 26 Jan 2013 11:10:03 -0500 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: On Sat, Jan 26, 2013 at 5:51 AM, Petr P wrote: > However, I'm not sure if Ncurses doesn't mind calling getEvent from a > different thread than the one that updates UI. > I don't know if the Haskell bindings change this, but at the C level as long as automatic echo to a window is turned off, it should be safe to do reads from one thread and writes from another. -- brandon s allbery kf8nh sine nomine associates allbery.b at gmail.com ballbery at sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net -------------- next part -------------- An HTML attachment was scrubbed... URL: From dan at fluffynukeit.com Sat Jan 26 17:33:53 2013 From: dan at fluffynukeit.com (Daniel Austin) Date: Sat, 26 Jan 2013 11:33:53 -0500 Subject: [Haskell-cafe] Writing config files for configurator library Message-ID: Hello Haskellers, What is the typical way to write out configuration files for use by the configurator library? From asking around, seems like the library does not (obviously) support writing out config files to disc from scratch. What I want to do is write out a default config file to disc, or write out the configuration settings that a user selects. I want to avoid having the user edit the config files by hand in a text editor, as well as avoid rolling my own function to write it out if possible. Both of those solutions leave a sour taste in my mouth. Has there been any work toward this end? Thanks much, Dan -------------- next part -------------- An HTML attachment was scrubbed... URL: From michael at snoyman.com Sat Jan 26 18:16:19 2013 From: michael at snoyman.com (Michael Snoyman) Date: Sat, 26 Jan 2013 19:16:19 +0200 Subject: [Haskell-cafe] How to store Fixed data type in the database with persistent ? In-Reply-To: <6151944.YT1pq8IXuI@localhost> References: <1745863.rgDDbheD5J@localhost> <6151944.YT1pq8IXuI@localhost> Message-ID: Very nice to see, I'm happy to stand corrected here. We'll definitely get some support for fixed into the next major release. On Saturday, January 26, 2013, wrote: > > According to the documentation, SQLite stores whatever you give it, > > paying very little heed to the declared type. If you get SQLite to > > *compare* two numbers, it will at that point *convert* them to doubles > > in order to carry out the comparison. This is quite separate from the > > question of what it can store. > > CREATE TABLE t1(val); > sqlite> insert into t1 values ('24.24242424') > ...> ; > sqlite> insert into t1 values ('24.24242423') > ...> ; > sqlite> select * from t1 order by val; > 24.24242423 > 24.24242424 > sqlite> select * from t1 order by val desc; > 24.24242424 > 24.24242423 > sqlite> select sum(val) from t1; > 48.48484847 > > it seems Sqlite can work with arbitrary percission data, very good ! > Persistent must have ability to store Fixed. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From felipe.lessa at gmail.com Sat Jan 26 20:43:24 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Sat, 26 Jan 2013 17:43:24 -0200 Subject: [Haskell-cafe] Use forM_ with Maybe, it's Foldable! Message-ID: Hey, Just wanted to share this little insight I had that I don't think is a common idiom. Often times I need to define: whenJust :: Monad m => Maybe a -> (a -> m b) -> m () whenJust (Just x) f = f x whenJust Nothing _ = return () It always bugged me that you can't find this function anywhere. Actually, you can find it on at least a couple of packages [1], but you can't find it on a standard place. Or can you? A few days ago I decided to hoogle the type of whenJust [2] and what I discovered is that import Data.Foldable (forM_) whenJust = forM_ For example, instead of: mfoo <- doSomething case mfoo of Just foo -> ... Nothing -> return () You may just write: forM_ mfoo $ \foo -> ... There you go. I hope this is useful to someone =). Cheers, [1] http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenJust [2] http://www.haskell.org/hoogle/?hoogle=Maybe%20a%20-%3E%20(a%20-%3E%20m%20b)%20-%3E%20m%20() -- Felipe. From evohunz at gmail.com Sat Jan 26 21:34:16 2013 From: evohunz at gmail.com (Thiago Negri) Date: Sat, 26 Jan 2013 18:34:16 -0200 Subject: [Haskell-cafe] SDL and ALUT on Windows: "stdin Bad file descriptor" Message-ID: I'm trying SDL on Windows, and things are getting really weird. I can compile the code (links on the end). When I run it, if I try using `stdin` the program crashes with this message: : hGetLine: invalid argument (Bad file descriptor) Is it something to do with SDL itself? What am I doing wrong? :( Code: http://hpaste.org/81321 Cabal file: http://hpaste.org/81322 Thanks. -------------- next part -------------- An HTML attachment was scrubbed... URL: From allbery.b at gmail.com Sat Jan 26 21:52:03 2013 From: allbery.b at gmail.com (Brandon Allbery) Date: Sat, 26 Jan 2013 15:52:03 -0500 Subject: [Haskell-cafe] SDL and ALUT on Windows: "stdin Bad file descriptor" In-Reply-To: References: Message-ID: On Sat, Jan 26, 2013 at 3:34 PM, Thiago Negri wrote: > I'm trying SDL on Windows, and things are getting really weird. > I can compile the code (links on the end). > When I run it, if I try using `stdin` the program crashes with this > message: > > : hGetLine: invalid argument (Bad file descriptor) > You haven't even touched SDL at that point in that source code. If I had to guess, something (possibly a linker option embedded in the SDL binding) is causing your program to be built as a Windows GUI (as opposed to Windows Console) so it has no stdin. I couldn't tell you much more than that, though; the ways of Windows development are foreign to me. -- brandon s allbery kf8nh sine nomine associates allbery.b at gmail.com ballbery at sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net -------------- next part -------------- An HTML attachment was scrubbed... URL: From sol at typeful.net Sat Jan 26 22:39:06 2013 From: sol at typeful.net (Simon Hengel) Date: Sat, 26 Jan 2013 22:39:06 +0100 Subject: [Haskell-cafe] cabal install choosing an older version In-Reply-To: References:

Message-ID: <20130126213906.GA3618@x200> Hi Ozgur, I'm missing some context here, but I'll release an updated version of hspec ASAP ;) Cheers, Simon From petr.mvd at gmail.com Sat Jan 26 23:03:51 2013 From: petr.mvd at gmail.com (Petr P) Date: Sat, 26 Jan 2013 23:03:51 +0100 Subject: [Haskell-cafe] catamorphisms and attribute grammars Message-ID: Dear Haskellers, I read some stuff about attribute grammars recently [1] and how UUAGC [2] can be used for code generation. I felt like this should be possible inside Haskell too so I did some experiments and I realized that indeed catamorphisms can be represented in such a way that they can be combined together and all run in a single pass over a data structure. In fact, they form an applicative functor. [1] http://www.haskell.org/haskellwiki/Attribute_grammar [2] Utrecht University Attribute Grammar Compiler To give an example, let's say we want to compute the average value of a binary tree. If we compute a sum first and then count the elements, the whole tree is retained in memory (and moreover, deforestation won't happen). So it's desirable to compute both values at once during a single pass: -- Count nodes in a tree. count' :: (Num i) => CataBase (BinTree a) i count' = ... -- Sums all nodes in a tree. sum' :: (Num n) => CataBase (BinTree n) n sum' = ... -- Computes the average value of a tree. avg' :: (Fractional b) => CataBase (BinTree b) b avg' = (/) <$> sum' <*> count' Then we can compute the average in a single pass like runHylo avg' treeAnamorphism seed My experiments together with the example are available at https://github .com/ppetr/recursion-attributes I wonder, is there an existing library that expresses this idea? Best regards, Petr Pudlak -------------- next part -------------- An HTML attachment was scrubbed... URL: From felipe.lessa at gmail.com Sun Jan 27 00:09:22 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Sat, 26 Jan 2013 21:09:22 -0200 Subject: [Haskell-cafe] Use forM_ with Maybe, it's Foldable! In-Reply-To: References: Message-ID: Herbert Valerio actually pointed out to me in PVT that funnily enough I've actually *forgotten* and *rediscovered* this idiom! =) http://www.haskell.org/pipermail/libraries/2011-November/017257.html Cheers! On Sat, Jan 26, 2013 at 5:43 PM, Felipe Almeida Lessa wrote: > Hey, > > Just wanted to share this little insight I had that I don't think is a > common idiom. Often times I need to define: > > whenJust :: Monad m => Maybe a -> (a -> m b) -> m () > whenJust (Just x) f = f x > whenJust Nothing _ = return () > > It always bugged me that you can't find this function anywhere. > Actually, you can find it on at least a couple of packages [1], but > you can't find it on a standard place. Or can you? > > A few days ago I decided to hoogle the type of whenJust [2] and what I > discovered is that > > import Data.Foldable (forM_) > whenJust = forM_ > > For example, instead of: > > mfoo <- doSomething > case mfoo of > Just foo -> ... > Nothing -> return () > > You may just write: > > forM_ mfoo $ \foo -> > ... > > There you go. I hope this is useful to someone =). > > Cheers, > > [1] http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenJust > [2] http://www.haskell.org/hoogle/?hoogle=Maybe%20a%20-%3E%20(a%20-%3E%20m%20b)%20-%3E%20m%20() > > -- > Felipe. -- Felipe. From hjgtuyl at chello.nl Sun Jan 27 00:17:45 2013 From: hjgtuyl at chello.nl (Henk-Jan van Tuyl) Date: Sun, 27 Jan 2013 00:17:45 +0100 Subject: [Haskell-cafe] SDL and ALUT on Windows: "stdin Bad file descriptor" In-Reply-To: References:

Message-ID: On Sat, 26 Jan 2013 21:52:03 +0100, Brandon Allbery wrote: > On Sat, Jan 26, 2013 at 3:34 PM, Thiago Negri wrote: > >> I'm trying SDL on Windows, and things are getting really weird. >> I can compile the code (links on the end). >> When I run it, if I try using `stdin` the program crashes with this >> message: >> >> : hGetLine: invalid argument (Bad file descriptor) >> > > You haven't even touched SDL at that point in that source code. > > If I had to guess, something (possibly a linker option embedded in the > SDL > binding) is causing your program to be built as a Windows GUI (as opposed > to Windows Console) so it has no stdin. I couldn't tell you much more > than > that, though; the ways of Windows development are foreign to me. > You probably used -optl-mwindows in the compile command, to prevent the DOS-shell appearing behind your window. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming -- From roma at ro-che.info Sun Jan 27 01:20:25 2013 From: roma at ro-che.info (Roman Cheplyaka) Date: Sun, 27 Jan 2013 02:20:25 +0200 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: <20130127002025.GA4119@sniper> * Petr P [2013-01-26 23:03:51+0100] > Dear Haskellers, > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > can be used for code generation. I felt like this should be possible inside > Haskell too so I did some experiments and I realized that indeed > catamorphisms can be represented in such a way that they can be combined > together and all run in a single pass over a data structure. In fact, they > form an applicative functor. > > ... > > My experiments together with the example are available at https://github > .com/ppetr/recursion-attributes Very nice! This can be generalized to arbitrary arrows: {-# LANGUAGE ExistentialQuantification #-} import Prelude hiding (id) import Control.Arrow import Control.Applicative import Control.Category data F from to b c = forall d . F (from b d) (to d c) instance (Arrow from, Arrow to) => Functor (F from to b) where fmap f x = pure f <*> x instance (Arrow from, Arrow to) => Applicative (F from to b) where pure x = F (arr $ const x) id F from1 to1 <*> F from2 to2 = F (from1 &&& from2) (to1 *** to2 >>> arr (uncurry id)) Now your construction is a special case where 'from' is the category of f-algebras and 'to' is the usual (->) category. I wonder what's a categorical interpretation of F itself. Roman From evohunz at gmail.com Sun Jan 27 02:30:31 2013 From: evohunz at gmail.com (Thiago Negri) Date: Sat, 26 Jan 2013 23:30:31 -0200 Subject: [Haskell-cafe] SDL and ALUT on Windows: "stdin Bad file descriptor" In-Reply-To: References:

Message-ID: I didn't use `-optl-mwindows`. I guess the dependency on SDL is doing this. Anyway, I was trying to print some debug messages to see what was going wrong. Replaces the debugs message with on-screen messages using `SDL-ttf` package. Thanks, Thiago. 2013/1/26 Henk-Jan van Tuyl > On Sat, 26 Jan 2013 21:52:03 +0100, Brandon Allbery > wrote: > > On Sat, Jan 26, 2013 at 3:34 PM, Thiago Negri wrote: >> >> I'm trying SDL on Windows, and things are getting really weird. >>> I can compile the code (links on the end). >>> When I run it, if I try using `stdin` the program crashes with this >>> message: >>> >>> : hGetLine: invalid argument (Bad file descriptor) >>> >>> >> You haven't even touched SDL at that point in that source code. >> >> If I had to guess, something (possibly a linker option embedded in the SDL >> binding) is causing your program to be built as a Windows GUI (as opposed >> to Windows Console) so it has no stdin. I couldn't tell you much more >> than >> that, though; the ways of Windows development are foreign to me. >> >> > You probably used -optl-mwindows in the compile command, to prevent the > DOS-shell appearing behind your window. > > Regards, > Henk-Jan van Tuyl > > > -- > http://Van.Tuyl.eu/ > http://members.chello.nl/**hjgtuyl/tourdemonad.html > Haskell programming > -- > -------------- next part -------------- An HTML attachment was scrubbed... URL: From johnw at fpcomplete.com Sun Jan 27 06:55:53 2013 From: johnw at fpcomplete.com (John Wiegley) Date: Sat, 26 Jan 2013 23:55:53 -0600 Subject: [Haskell-cafe] Use forM_ with Maybe, it's Foldable! In-Reply-To: (Felipe Almeida Lessa's message of "Sat, 26 Jan 2013 17:43:24 -0200") References: Message-ID: >>>>> Felipe Almeida Lessa writes: > A few days ago I decided to hoogle the type of whenJust [2] and what I > discovered is that > import Data.Foldable (forM_) > whenJust = forM_ You can also use "for_", if you want to use Applicative instead of Monad. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net From alexander.vershilov at gmail.com Sun Jan 27 07:55:17 2013 From: alexander.vershilov at gmail.com (alexander.vershilov at gmail.com) Date: Sun, 27 Jan 2013 10:55:17 +0400 Subject: [Haskell-cafe] How to store Fixed data type in the database with persistent ? In-Reply-To: <6151944.YT1pq8IXuI@localhost> References: <1745863.rgDDbheD5J@localhost> <6151944.YT1pq8IXuI@localhost> Message-ID: <20130127065517.GA2412@localhost.gtn.ru> Sat, Jan 26, 2013 at 12:21:02PM +0600, s9gf4ult at gmail.com wrote > > According to the documentation, SQLite stores whatever you give it, > > paying very little heed to the declared type. If you get SQLite to > > *compare* two numbers, it will at that point *convert* them to doubles > > in order to carry out the comparison. This is quite separate from the > > question of what it can store. > > CREATE TABLE t1(val); > sqlite> insert into t1 values ('24.24242424') > ...> ; > sqlite> insert into t1 values ('24.24242423') > ...> ; > sqlite> select * from t1 order by val; > 24.24242423 > 24.24242424 > sqlite> select * from t1 order by val desc; > 24.24242424 > 24.24242423 > sqlite> select sum(val) from t1; > 48.48484847 > > it seems Sqlite can work with arbitrary percission data, very good ! > Persistent must have ability to store Fixed. > It's not correct. SQLlite stores any value, but it will use arithmetic operations only with double presicion: sqlite> select val from t1; 1 0.000001 0.00000001 0.0000000001 0.000000000001 0.00000000000001 0.0000000000000001 0.000000000000000001 0.00000000000000000001 0.0000000000000000000001 sqlite> select sum(val) from t1; 1.00000101010101 as you see it has 14 degree. Let's check another well known floating point problem: sqlilte> create table t2 ('val') sqlite> insert into t2 values ('0.7'); sqlite> update t2 set val = 11*val-7; t2 should remain a const sqlite> update t2 set val = 11*val-7; -- 4 times sqlite> select val from t2; 0.699999999989597 sqlite> update t2 set val = 11*val-7; -- 10 times mote sqlite> select val from t2; 0.430171514341321 As you see you have errors. So SQLlite doesn't support arbitrary presision values. As for me Persistent should at least support a Money type and use correct backend-specific type for them, either a native for big integer. -- Best regards Alexander Vershilov -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 490 bytes Desc: Digital signature URL: From chrisyco+haskell-cafe at gmail.com Sun Jan 27 08:20:07 2013 From: chrisyco+haskell-cafe at gmail.com (Chris Wong) Date: Sun, 27 Jan 2013 20:20:07 +1300 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: Hi Petr, Congratulations -- you've just implemented a Moore machine! [1] I posted something very much like this just last year [2]. It's a very common pattern in Haskell, forming the basis of coroutines and iteratees and many other things. Edward Kmett includes it in his machines package [3]. His variation, like mine, hides the state inside a closure, removing the need for existentials. pipes 2.0 contains one implemented as a free monad [4]. [1] http://en.wikipedia.org/wiki/Moore_machine [2] http://hackage.haskell.org/packages/archive/machines/0.2.3/doc/html/Data-Machine-Moore.html [3] http://www.haskell.org/pipermail/haskell-cafe/2012-May/101460.html [4] http://hackage.haskell.org/packages/archive/pipes/2.0.0/doc/html/Control-Pipe-Common.html Chris On Sun, Jan 27, 2013 at 11:03 AM, Petr P wrote: > Dear Haskellers, > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > can be used for code generation. I felt like this should be possible inside > Haskell too so I did some experiments and I realized that indeed > catamorphisms can be represented in such a way that they can be combined > together and all run in a single pass over a data structure. In fact, they > form an applicative functor. > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > [2] Utrecht University Attribute Grammar Compiler > > To give an example, let's say we want to compute the average value of a > binary tree. If we compute a sum first and then count the elements, the > whole tree is retained in memory (and moreover, deforestation won't happen). > So it's desirable to compute both values at once during a single pass: > > -- Count nodes in a tree. > count' :: (Num i) => CataBase (BinTree a) i > count' = ... > > -- Sums all nodes in a tree. > sum' :: (Num n) => CataBase (BinTree n) n > sum' = ... > > -- Computes the average value of a tree. > avg' :: (Fractional b) => CataBase (BinTree b) b > avg' = (/) <$> sum' <*> count' > > Then we can compute the average in a single pass like > > runHylo avg' treeAnamorphism seed > > My experiments together with the example are available at > https://github.com/ppetr/recursion-attributes > > I wonder, is there an existing library that expresses this idea? > > Best regards, > Petr Pudlak > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > From roma at ro-che.info Sun Jan 27 08:49:01 2013 From: roma at ro-che.info (Roman Cheplyaka) Date: Sun, 27 Jan 2013 09:49:01 +0200 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: <20130127074901.GA6290@sniper> Hi Chris, While the two things solve similar problems and have other similarities, they are still quite different. It's hard to call Petr's type a machine ? there's no dynamics in it. It's just a pair of an f-algebra and finalizer. Values of your type return themselves ? it is exactly this recursion that makes them "machines". And the problem solved are different. Catamorphisms are not the same as folds in the sense of Data.Foldable. Folds get elements one at a time, while a catamorphism sees the structure of the tree. For instance, it is possible to write an evaluator for an expression AST using catamorphisms, but not using Moore machines[*], because you won't know what the operations are. [*] without linearizing the tree beforehand Roman * Chris Wong [2013-01-27 20:20:07+1300] > Hi Petr, > > Congratulations -- you've just implemented a Moore machine! [1] > > I posted something very much like this just last year [2]. It's a very > common pattern in Haskell, forming the basis of coroutines and > iteratees and many other things. > > Edward Kmett includes it in his machines package [3]. His variation, > like mine, hides the state inside a closure, removing the need for > existentials. pipes 2.0 contains one implemented as a free monad [4]. > > [1] http://en.wikipedia.org/wiki/Moore_machine > [2] http://hackage.haskell.org/packages/archive/machines/0.2.3/doc/html/Data-Machine-Moore.html > [3] http://www.haskell.org/pipermail/haskell-cafe/2012-May/101460.html > [4] http://hackage.haskell.org/packages/archive/pipes/2.0.0/doc/html/Control-Pipe-Common.html > > Chris > > On Sun, Jan 27, 2013 at 11:03 AM, Petr P wrote: > > Dear Haskellers, > > > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > > can be used for code generation. I felt like this should be possible inside > > Haskell too so I did some experiments and I realized that indeed > > catamorphisms can be represented in such a way that they can be combined > > together and all run in a single pass over a data structure. In fact, they > > form an applicative functor. > > > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > > [2] Utrecht University Attribute Grammar Compiler > > > > To give an example, let's say we want to compute the average value of a > > binary tree. If we compute a sum first and then count the elements, the > > whole tree is retained in memory (and moreover, deforestation won't happen). > > So it's desirable to compute both values at once during a single pass: > > > > -- Count nodes in a tree. > > count' :: (Num i) => CataBase (BinTree a) i > > count' = ... > > > > -- Sums all nodes in a tree. > > sum' :: (Num n) => CataBase (BinTree n) n > > sum' = ... > > > > -- Computes the average value of a tree. > > avg' :: (Fractional b) => CataBase (BinTree b) b > > avg' = (/) <$> sum' <*> count' > > > > Then we can compute the average in a single pass like > > > > runHylo avg' treeAnamorphism seed > > > > My experiments together with the example are available at > > https://github.com/ppetr/recursion-attributes > > > > I wonder, is there an existing library that expresses this idea? > > > > Best regards, > > Petr Pudlak > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe at haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe From petr.mvd at gmail.com Sun Jan 27 09:40:31 2013 From: petr.mvd at gmail.com (Petr P) Date: Sun, 27 Jan 2013 09:40:31 +0100 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: Hi Chris, thanks for insightful links. At the first glance, I think the main difference is that machines and iteratees process streams of data, while catamorphisms work on general recursive data structures. (I used "count" + "sum" in the example, which could lead to the impression that it's list oriented.) However, it seems to me that there is some connection between cata/anamorphisms and free (co)monads generated by a functor. I'm just guessing - perhaps using such a monad in a monadic pipe would lead to a similar result? BTW, while it seems that using existentials in by Cata data type is natural, I'd like to know if I could do it without them. If you have any ideas, please let me know. Best regards, Petr PS: Is there actually anything left that ekmett hasn't implemented? 2013/1/27 Chris Wong > Hi Petr, > > Congratulations -- you've just implemented a Moore machine! [1] > > I posted something very much like this just last year [2]. It's a very > common pattern in Haskell, forming the basis of coroutines and > iteratees and many other things. > > Edward Kmett includes it in his machines package [3]. His variation, > like mine, hides the state inside a closure, removing the need for > existentials. pipes 2.0 contains one implemented as a free monad [4]. > > [1] http://en.wikipedia.org/wiki/Moore_machine > [2] > http://hackage.haskell.org/packages/archive/machines/0.2.3/doc/html/Data-Machine-Moore.html > [3] http://www.haskell.org/pipermail/haskell-cafe/2012-May/101460.html > [4] > http://hackage.haskell.org/packages/archive/pipes/2.0.0/doc/html/Control-Pipe-Common.html > > Chris > > On Sun, Jan 27, 2013 at 11:03 AM, Petr P wrote: > > Dear Haskellers, > > > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > > can be used for code generation. I felt like this should be possible > inside > > Haskell too so I did some experiments and I realized that indeed > > catamorphisms can be represented in such a way that they can be combined > > together and all run in a single pass over a data structure. In fact, > they > > form an applicative functor. > > > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > > [2] Utrecht University Attribute Grammar Compiler > > > > To give an example, let's say we want to compute the average value of a > > binary tree. If we compute a sum first and then count the elements, the > > whole tree is retained in memory (and moreover, deforestation won't > happen). > > So it's desirable to compute both values at once during a single pass: > > > > -- Count nodes in a tree. > > count' :: (Num i) => CataBase (BinTree a) i > > count' = ... > > > > -- Sums all nodes in a tree. > > sum' :: (Num n) => CataBase (BinTree n) n > > sum' = ... > > > > -- Computes the average value of a tree. > > avg' :: (Fractional b) => CataBase (BinTree b) b > > avg' = (/) <$> sum' <*> count' > > > > Then we can compute the average in a single pass like > > > > runHylo avg' treeAnamorphism seed > > > > My experiments together with the example are available at > > https://github.com/ppetr/recursion-attributes > > > > I wonder, is there an existing library that expresses this idea? > > > > Best regards, > > Petr Pudlak > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe at haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From petr.mvd at gmail.com Sun Jan 27 09:58:32 2013 From: petr.mvd at gmail.com (Petr P) Date: Sun, 27 Jan 2013 09:58:32 +0100 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: <20130127002025.GA4119@sniper> References: <20130127002025.GA4119@sniper> Message-ID: Roman, this is interesting. Is this arrow generalization in some library already? And does it have a name? Best regards, Petr Pudlak 2013/1/27 Roman Cheplyaka > * Petr P [2013-01-26 23:03:51+0100] > > Dear Haskellers, > > > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] > > can be used for code generation. I felt like this should be possible > inside > > Haskell too so I did some experiments and I realized that indeed > > catamorphisms can be represented in such a way that they can be combined > > together and all run in a single pass over a data structure. In fact, > they > > form an applicative functor. > > > > ... > > > > My experiments together with the example are available at https://github > > .com/ppetr/recursion-attributes > > Very nice! This can be generalized to arbitrary arrows: > > {-# LANGUAGE ExistentialQuantification #-} > > import Prelude hiding (id) > import Control.Arrow > import Control.Applicative > import Control.Category > > data F from to b c = forall d . F (from b d) (to d c) > > instance (Arrow from, Arrow to) => Functor (F from to b) where > fmap f x = pure f <*> x > > instance (Arrow from, Arrow to) => Applicative (F from to b) where > pure x = F (arr $ const x) id > F from1 to1 <*> F from2 to2 = > F (from1 &&& from2) (to1 *** to2 >>> arr (uncurry id)) > > Now your construction is a special case where 'from' is the category of > f-algebras and 'to' is the usual (->) category. > > I wonder what's a categorical interpretation of F itself. > > Roman > -------------- next part -------------- An HTML attachment was scrubbed... URL: From cobbe at ccs.neu.edu Sun Jan 27 16:07:57 2013 From: cobbe at ccs.neu.edu (Richard Cobbe) Date: Sun, 27 Jan 2013 10:07:57 -0500 Subject: [Haskell-cafe] Where did the Cabal manual go? Message-ID: <20130127150757.GB389@weatherwax.local> I'm not able to access the cabal manual today: links to my local copy and links to the copy at haskell.org result in a 404. I'm running the Haskell Platform, 2012.4.0.0, 64-bit, OS X 10.8.2. On my first attempt this morning, I loaded file:///Library/Haskell/doc/start.html in my browser and clicked on the "Cabal" link. The browser can't find file:///Library/Haskell/doc/ghc-doc/Cabal/index.html, and indeed the directory /Library/Haskell/doc/ghc-doc/Cabal doesn't exist. Ok, I think, so my local copy's gotten corrupted. One "sudo /Library/Haskell/bin/uninstall-hs --remove thru 7.4.2" and reinstall later, I try again; no change. I also can't find any docs by doing find /Library/Haskell -iname \*cabal\* find /Library/Frameworks/GHC.framework -iname \*cabal\* either. I also get a 404 when I click on the "Cabal" link from . I can probably recover a copy from system backups, but what's going on? Richard From michael at snoyman.com Sun Jan 27 16:20:10 2013 From: michael at snoyman.com (Michael Snoyman) Date: Sun, 27 Jan 2013 17:20:10 +0200 Subject: [Haskell-cafe] How to store Fixed data type in the database with persistent ? In-Reply-To: <20130127065517.GA2412@localhost.gtn.ru> References: <1745863.rgDDbheD5J@localhost> <6151944.YT1pq8IXuI@localhost> <20130127065517.GA2412@localhost.gtn.ru> Message-ID: On Jan 27, 2013 8:46 AM, wrote: > > Sat, Jan 26, 2013 at 12:21:02PM +0600, s9gf4ult at gmail.com wrote > > > According to the documentation, SQLite stores whatever you give it, > > > paying very little heed to the declared type. If you get SQLite to > > > *compare* two numbers, it will at that point *convert* them to doubles > > > in order to carry out the comparison. This is quite separate from the > > > question of what it can store. > > > > CREATE TABLE t1(val); > > sqlite> insert into t1 values ('24.24242424') > > ...> ; > > sqlite> insert into t1 values ('24.24242423') > > ...> ; > > sqlite> select * from t1 order by val; > > 24.24242423 > > 24.24242424 > > sqlite> select * from t1 order by val desc; > > 24.24242424 > > 24.24242423 > > sqlite> select sum(val) from t1; > > 48.48484847 > > > > it seems Sqlite can work with arbitrary percission data, very good ! > > Persistent must have ability to store Fixed. > > > > It's not correct. SQLlite stores any value, but it will use arithmetic > operations only with double presicion: > > sqlite> select val from t1; > 1 > 0.000001 > 0.00000001 > 0.0000000001 > 0.000000000001 > 0.00000000000001 > 0.0000000000000001 > 0.000000000000000001 > 0.00000000000000000001 > 0.0000000000000000000001 > > sqlite> select sum(val) from t1; > 1.00000101010101 > > as you see it has 14 degree. > > Let's check another well known floating point problem: > > sqlilte> create table t2 ('val') > sqlite> insert into t2 values ('0.7'); > sqlite> update t2 set val = 11*val-7; > > t2 should remain a const > sqlite> update t2 set val = 11*val-7; -- 4 times > sqlite> select val from t2; > 0.699999999989597 > sqlite> update t2 set val = 11*val-7; -- 10 times mote > sqlite> select val from t2; > 0.430171514341321 > > As you see you have errors. So SQLlite doesn't support arbitrary > presision values. > > As for me Persistent should at least support a Money type and use > correct backend-specific type for them, either a native for big integer. Let me clarify a bit: 1. Persistent will currently allow you to create a `Money` datatype which internally stores as an integer. 2. What Persistent currently lacks is a PersistValue constructor for arbitrary-precision values. As a result, during marshaling, some data will be lost when converting from NUMERIC to Double. 3. The upcoming change we're discussing for Persistent would just be to add such a constructor. We could theoretically provide some extra PersistField instances as well, but that's not really what's being discussed. HTH, Michael -------------- next part -------------- An HTML attachment was scrubbed... URL: From felipe.lessa at gmail.com Sun Jan 27 18:47:38 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Sun, 27 Jan 2013 15:47:38 -0200 Subject: [Haskell-cafe] Where did the Cabal manual go? In-Reply-To: <20130127150757.GB389@weatherwax.local> References: <20130127150757.GB389@weatherwax.local> Message-ID: Do you mean the Cabal User Guide? http://www.haskell.org/cabal/users-guide/ On Sun, Jan 27, 2013 at 1:07 PM, Richard Cobbe wrote: > I'm not able to access the cabal manual today: links to my local copy and > links to the copy at haskell.org result in a 404. > > I'm running the Haskell Platform, 2012.4.0.0, 64-bit, OS X 10.8.2. On my > first attempt this morning, I loaded file:///Library/Haskell/doc/start.html > in my browser and clicked on the "Cabal" link. The browser can't find > file:///Library/Haskell/doc/ghc-doc/Cabal/index.html, and indeed the > directory /Library/Haskell/doc/ghc-doc/Cabal doesn't exist. > > Ok, I think, so my local copy's gotten corrupted. One "sudo > /Library/Haskell/bin/uninstall-hs --remove thru 7.4.2" and reinstall later, > I try again; no change. I also can't find any docs by doing > find /Library/Haskell -iname \*cabal\* > find /Library/Frameworks/GHC.framework -iname \*cabal\* > either. > > I also get a 404 when I click on the "Cabal" link from > . > > I can probably recover a copy from system backups, but what's going on? > > Richard > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -- Felipe. From cobbe at ccs.neu.edu Sun Jan 27 19:48:06 2013 From: cobbe at ccs.neu.edu (Richard Cobbe) Date: Sun, 27 Jan 2013 13:48:06 -0500 Subject: [Haskell-cafe] Where did the Cabal manual go? In-Reply-To: References: <20130127150757.GB389@weatherwax.local> Message-ID: <20130127184806.GC389@weatherwax.local> On Sun, Jan 27, 2013 at 03:47:38PM -0200, Felipe Almeida Lessa wrote: > Do you mean the Cabal User Guide? > http://www.haskell.org/cabal/users-guide/ Yes, that's it, and I'm in the process of downloading a copy now (so I can work without a WiFi connection). I'm still curious about why there's a broken link on , and what on earth I did to delete the copy of the user's guide that used to be on my hard drive as part of the Haskell Platform installation (or, worse, what was silently done on my behalf). It's possible I missed a notification that this was happening, but I certainly don't remember seeing anything to that effect. Richard From cobbe at ccs.neu.edu Sun Jan 27 20:09:06 2013 From: cobbe at ccs.neu.edu (Richard Cobbe) Date: Sun, 27 Jan 2013 14:09:06 -0500 Subject: [Haskell-cafe] Where did the Cabal manual go? In-Reply-To: <20130127184806.GC389@weatherwax.local> References: <20130127150757.GB389@weatherwax.local> <20130127184806.GC389@weatherwax.local> Message-ID: <20130127190906.GB325@vimes.home> On Sun, Jan 27, 2013 at 01:48:06PM -0500, Richard Cobbe wrote: > On Sun, Jan 27, 2013 at 03:47:38PM -0200, Felipe Almeida Lessa wrote: > > Do you mean the Cabal User Guide? > > http://www.haskell.org/cabal/users-guide/ > > Yes, that's it, and I'm in the process of downloading a copy now (so I can > work without a WiFi connection). By the way, if the Cabal users guide is no longer to be packaged with the Haskell Platform, can I suggest adding a link to http://www.haskell.org/cabal that would allow folks to download a tgz with the HTML docs, much as is provided for Happy and Alex? As it is, one has to download 4 html files and a css file and hope that's everything. Richard From doaitse at swierstra.net Sun Jan 27 21:12:56 2013 From: doaitse at swierstra.net (Doaitse Swierstra) Date: Sun, 27 Jan 2013 21:12:56 +0100 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: <4C1179B8-3267-4C79-ACBF-215E522A35A3@swierstra.net> You have applied the so-called banana-split theorem, as described e.g. in http://www.cs.ox.ac.uk/jeremy.gibbons/publications/acmmpc-calcfp.pdf Indeed you are right in noticing the correspondence between AG's and catamorphims; actually I see AG's as a domain specific language for constructing algebras. Since we believe in embedded domain specific languages we have developed a library for constructing attribute grammars in Haskell, which is described in our ICFP paper: @inproceedings{1596586, Address = {New York, NY, USA}, Author = {Viera, Marcos and Swierstra, S. Doaitse and Swierstra, Wouter}, Booktitle = {ICFP '09: Proceedings of the 14th ACM SIGPLAN international conference on Functional programming}, Date-Added = {2009-10-05 22:06:26 +0200}, Date-Modified = {2009-10-05 22:06:26 +0200}, Doi = {http://doi.acm.org/10.1145/1596550.1596586}, Isbn = {978-1-60558-332-7}, Location = {Edinburgh, Scotland}, Pages = {245--256}, Publisher = {ACM}, Title = {Attribute grammars fly first-class: how to do aspect oriented programming in Haskell}, Year = {2009}} where you will find the problem you are solving done using the library. On March 8 2013 Marcos Viera hopes to defend his Ph.D. thesis at Utrecht University. His thesis contains the progress we have made in this area in recent years. You can find it at the bottom op the page; amongst others you can use the UUAGC nowadays to generate this code form uuagc input. http://www.cs.uu.nl/wiki/bin/view/Center/PhDs Doaitse Swierstra On Jan 26, 2013, at 23:03 , Petr P wrote: > Dear Haskellers, > > I read some stuff about attribute grammars recently [1] and how UUAGC [2] can be used for code generation. I felt like this should be possible inside Haskell too so I did some experiments and I realized that indeed catamorphisms can be represented in such a way that they can be combined together and all run in a single pass over a data structure. In fact, they form an applicative functor. > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > [2] Utrecht University Attribute Grammar Compiler > > To give an example, let's say we want to compute the average value of a binary tree. If we compute a sum first and then count the elements, the whole tree is retained in memory (and moreover, deforestation won't happen). So it's desirable to compute both values at once during a single pass: > > -- Count nodes in a tree. > count' :: (Num i) => CataBase (BinTree a) i > count' = ... > > -- Sums all nodes in a tree. > sum' :: (Num n) => CataBase (BinTree n) n > sum' = ... > > -- Computes the average value of a tree. > avg' :: (Fractional b) => CataBase (BinTree b) b > avg' = (/) <$> sum' <*> count' > > Then we can compute the average in a single pass like > > runHylo avg' treeAnamorphism seed > > My experiments together with the example are available at https://github.com/ppetr/recursion-attributes > > I wonder, is there an existing library that expresses this idea? > > Best regards, > Petr Pudlak > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -------------- next part -------------- An HTML attachment was scrubbed... URL: From roma at ro-che.info Sun Jan 27 21:17:47 2013 From: roma at ro-che.info (Roman Cheplyaka) Date: Sun, 27 Jan 2013 22:17:47 +0200 Subject: [Haskell-cafe] GHC.Generics and newtypes Message-ID: <20130127201747.GA18547@sniper> Hi, Is it possible to generate different instances for newtypes and datatypes using GHC.Generics? Roman From jpm at cs.uu.nl Sun Jan 27 21:29:52 2013 From: jpm at cs.uu.nl (=?ISO-8859-1?Q?Jos=E9_Pedro_Magalh=E3es?=) Date: Sun, 27 Jan 2013 20:29:52 +0000 Subject: [Haskell-cafe] GHC.Generics and newtypes In-Reply-To: <20130127201747.GA18547@sniper> References: <20130127201747.GA18547@sniper> Message-ID: Hi Roman, Yes, the automatic derivation of Generic instances does not see through newtypes. Cheers, Pedro On Sun, Jan 27, 2013 at 8:17 PM, Roman Cheplyaka wrote: > Hi, > > Is it possible to generate different instances for newtypes and > datatypes using GHC.Generics? > > Roman > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From roma at ro-che.info Sun Jan 27 21:34:33 2013 From: roma at ro-che.info (Roman Cheplyaka) Date: Sun, 27 Jan 2013 22:34:33 +0200 Subject: [Haskell-cafe] GHC.Generics and newtypes In-Reply-To: References: <20130127201747.GA18547@sniper> Message-ID: <20130127203433.GA18746@sniper> Sorry, I wasn't clear. What I want is somehow to find out whether the type under consideration is declared using data or newtype. Is it possible? Roman * Jos? Pedro Magalh?es [2013-01-27 20:29:52+0000] > Hi Roman, > > Yes, the automatic derivation of Generic instances does not see through > newtypes. > > > Cheers, > Pedro > > On Sun, Jan 27, 2013 at 8:17 PM, Roman Cheplyaka wrote: > > > Hi, > > > > Is it possible to generate different instances for newtypes and > > datatypes using GHC.Generics? > > > > Roman > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe at haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > From jpm at cs.uu.nl Sun Jan 27 21:36:54 2013 From: jpm at cs.uu.nl (=?ISO-8859-1?Q?Jos=E9_Pedro_Magalh=E3es?=) Date: Sun, 27 Jan 2013 20:36:54 +0000 Subject: [Haskell-cafe] GHC.Generics and newtypes In-Reply-To: <20130127203433.GA18746@sniper> References: <20130127201747.GA18547@sniper> <20130127203433.GA18746@sniper> Message-ID: Ah, no. But that would be easy to add to the Datatype class, I think. Perhaps open a feature request? Cheers, Pedro On Sun, Jan 27, 2013 at 8:34 PM, Roman Cheplyaka wrote: > Sorry, I wasn't clear. What I want is somehow to find out whether the > type under consideration is declared using data or newtype. > > Is it possible? > > Roman > > * Jos? Pedro Magalh?es [2013-01-27 20:29:52+0000] > > Hi Roman, > > > > Yes, the automatic derivation of Generic instances does not see through > > newtypes. > > > > > > Cheers, > > Pedro > > > > On Sun, Jan 27, 2013 at 8:17 PM, Roman Cheplyaka > wrote: > > > > > Hi, > > > > > > Is it possible to generate different instances for newtypes and > > > datatypes using GHC.Generics? > > > > > > Roman > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > Haskell-Cafe at haskell.org > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From roma at ro-che.info Sun Jan 27 21:39:44 2013 From: roma at ro-che.info (Roman Cheplyaka) Date: Sun, 27 Jan 2013 22:39:44 +0200 Subject: [Haskell-cafe] GHC.Generics and newtypes In-Reply-To: References: <20130127201747.GA18547@sniper> <20130127203433.GA18746@sniper> Message-ID: <20130127203944.GA18889@sniper> Sure. Should it go to the GHC bug tracker? Roman * Jos? Pedro Magalh?es [2013-01-27 20:36:54+0000] > Ah, no. But that would be easy to add to the Datatype > class, > I think. Perhaps > open a feature request? > > > Cheers, > Pedro > > On Sun, Jan 27, 2013 at 8:34 PM, Roman Cheplyaka wrote: > > > Sorry, I wasn't clear. What I want is somehow to find out whether the > > type under consideration is declared using data or newtype. > > > > Is it possible? > > > > Roman > > > > * Jos? Pedro Magalh?es [2013-01-27 20:29:52+0000] > > > Hi Roman, > > > > > > Yes, the automatic derivation of Generic instances does not see through > > > newtypes. > > > > > > > > > Cheers, > > > Pedro > > > > > > On Sun, Jan 27, 2013 at 8:17 PM, Roman Cheplyaka > > wrote: > > > > > > > Hi, > > > > > > > > Is it possible to generate different instances for newtypes and > > > > datatypes using GHC.Generics? > > > > > > > > Roman > > > > > > > > _______________________________________________ > > > > Haskell-Cafe mailing list > > > > Haskell-Cafe at haskell.org > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > From tim at dockerz.net Mon Jan 28 01:23:52 2013 From: tim at dockerz.net (Tim Docker) Date: Mon, 28 Jan 2013 11:23:52 +1100 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> Message-ID: <5105C518.40609@dockerz.net> Hi Eric, In a previous project, I chose vty over ncurses: - you can write your own event loop, and hence handle different event sources. - more liberal license (BSD3 versus GPL) Tim On 26/01/13 19:24, Erik de Castro Lopo wrote: > Hi all, > > I am in the process of writing a Haskell program consisting of two > threads, one for performing a calculation and one for an Ncurses UI > (haskell-ncurses). > > The calculation thread needs to feed a stream of numbers back to the > UI thread (about 1 value per second) and the UI needs to take input > from the user and will pass parameter changes to the calculation > thread using an IORef and atomicModifyIORef. > > However, I'm not sure how to hande the UI thread. The UI thread would > normally wait for Ncurses input using getEvent, but I need advice on > how to the the data from the calculation thread. > > Any advice or things to try? > > Cheers, > Erik From hoerdegen at funktional.info Mon Jan 28 09:55:04 2013 From: hoerdegen at funktional.info (=?ISO-8859-15?Q?Heinrich_H=F6rdegen?=) Date: Mon, 28 Jan 2013 09:55:04 +0100 Subject: [Haskell-cafe] Munich Haskell Meeting Message-ID: <51063CE8.6080907@funktional.info> Dear Haskellers, our monthly get-togethers in Munich this year start on Thursday, 31ts of January at 19h30. Our meeting venue will be Cafe Puck once again. If you want to join, please go to http://www.haskell-munich.de/dates and click the button. Of course, you can also invite all those interested in functional programming. Have a successful week, Heinrich From jeanchristophe.mincke at gmail.com Mon Jan 28 10:23:53 2013 From: jeanchristophe.mincke at gmail.com (jean-christophe mincke) Date: Mon, 28 Jan 2013 10:23:53 +0100 Subject: [Haskell-cafe] Mac os x (Intel, 10.6.8) problem compiling yesod-core-1.1.7.1 Message-ID: Hello GHC version : 7.4.2 When I do a cabal-dev instal yesod-core, I get the following error: Loading package blaze-builder-conduit-0.5.0.3 ... linking ... done. Loading package hashable-1.2.0.5 ... linking ... ghc: lookupSymbol failed in resolveImports /Users/V3/windev/Haskell/Z/cabal-dev//lib/HShashable-1.2.0.5.o: unknown symbol `_hashable_siphash24_sse2' ghc: unable to load package `hashable-1.2.0.5' Has anyone encountered the same problem? Thank you J-C -------------- next part -------------- An HTML attachment was scrubbed... URL: From emax at chalmers.se Mon Jan 28 12:05:42 2013 From: emax at chalmers.se (Emil Axelsson) Date: Mon, 28 Jan 2013 12:05:42 +0100 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: References: Message-ID: <51065B86.5070400@chalmers.se> Patrick Bahr does something very similar in Modular Tree Automata [1], also noting the relation to attribute grammars. It's implemented in the compdata package [2]. [1] Patrick Bahr, Modular Tree Automata (MPC 2012), http://dx.doi.org/10.1007/978-3-642-31113-0_14 [2] http://hackage.haskell.org/package/compdata / Emil 2013-01-26 23:03, Petr P skrev: > Dear Haskellers, > > I read some stuff about attribute grammars recently [1] and how UUAGC > [2] can be used for code generation. I felt like this should be possible > inside Haskell too so I did some experiments and I realized that indeed > catamorphisms can be represented in such a way that they can be combined > together and all run in a single pass over a data structure. In fact, > they form an applicative functor. > > [1] http://www.haskell.org/haskellwiki/Attribute_grammar > [2] Utrecht University Attribute Grammar Compiler > > To give an example, let's say we want to compute the average value of a > binary tree. If we compute a sum first and then count the elements, the > whole tree is retained in memory (and moreover, deforestation won't > happen). So it's desirable to compute both values at once during a > single pass: > > -- Count nodes in a tree. > count' :: (Num i) => CataBase (BinTree a) i > count' = ... > > -- Sums all nodes in a tree. > sum' :: (Num n) => CataBase (BinTree n) n > sum' = ... > > -- Computes the average value of a tree. > avg' :: (Fractional b) => CataBase (BinTree b) b > avg' = (/) <$> sum' <*> count' > > Then we can compute the average in a single pass like > > runHylo avg' treeAnamorphism seed > > My experiments together with the example are available > at https://github.com/ppetr/recursion-attributes > > I wonder, is there an existing library that expresses this idea? > > Best regards, > Petr Pudlak > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > From mblazevic at stilo.com Mon Jan 28 17:27:25 2013 From: mblazevic at stilo.com (=?UTF-8?B?TWFyaW8gQmxhxb5ldmnEhw==?=) Date: Mon, 28 Jan 2013 11:27:25 -0500 Subject: [Haskell-cafe] Suggestiong for inter-thread communication In-Reply-To: <20130126212834.e12246f7e15a965567b1f9c8@mega-nerd.com> References: <20130126192433.faf9711076fb6e98312e4de6@mega-nerd.com> <20130126212834.e12246f7e15a965567b1f9c8@mega-nerd.com> Message-ID: <5106A6ED.5030309@stilo.com> On 13-01-26 05:28 AM, Erik de Castro Lopo wrote: > Thiago Negri wrote: > >> Do you need advice on what? I didn't understand your last phrase. > > Well I have data from two sources, stdin and the calculation > thread. If I was doing this in C, I'd probably use a pipe for the > calculation data and then do select on the two file descriptors. > > There is a select package: > > http://hackage.haskell.org/package/select > > but I was wondering if there was a more idiomatic Haskell way > of dealing with inputs from more than one source. There are list arrows, and also coroutines in many guises including pipes, conduits, and iteratees. These are all co-operative concurrency, however, and I can't tell if your problem requires pre-emptive threads. From johan.tibell at gmail.com Mon Jan 28 18:49:23 2013 From: johan.tibell at gmail.com (Johan Tibell) Date: Mon, 28 Jan 2013 09:49:23 -0800 Subject: [Haskell-cafe] Mac os x (Intel, 10.6.8) problem compiling yesod-core-1.1.7.1 In-Reply-To: References: Message-ID: Adding Bryan, who wrote this code. On Mon, Jan 28, 2013 at 1:23 AM, jean-christophe mincke wrote: > Hello > > GHC version : 7.4.2 > > When I do a cabal-dev instal yesod-core, I get the following error: > > Loading package blaze-builder-conduit-0.5.0.3 ... linking ... done. > Loading package hashable-1.2.0.5 ... linking ... ghc: > lookupSymbol failed in resolveImports > /Users/V3/windev/Haskell/Z/cabal-dev//lib/HShashable-1.2.0.5.o: unknown > symbol `_hashable_siphash24_sse2' > ghc: unable to load package `hashable-1.2.0.5' > > Has anyone encountered the same problem? > > Thank you > > J-C > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > From chris at cmears.id.au Tue Jan 29 00:38:55 2013 From: chris at cmears.id.au (Chris Mears) Date: Tue, 29 Jan 2013 10:38:55 +1100 Subject: [Haskell-cafe] Uniplate and rewriting with different types Message-ID: Hi all, I have a question about the Uniplate library, regarding rewriting with transformations that have different types. With the following type, and transformation functions: data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1,t2,t3 :: Even -> Maybe Even t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 (EvenOne (OddZero x)) = Just $ EvenZero (OddOne x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing it is easy to combine the transformations into a single transformation, because they all have the same type. The result can then be passed to the Uniplate's "rewriteBi" function: allts x = t1 x `mplus` t2 x `mplus` t3 x example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBi allts example But if one of the transformations has a different type, you can't do it this way. For instance, redefine t2 to have a different type: t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing and you are stuck because the functions of different types can't be combined into a single transformation. My question is: is there a good way to combine the transformation functions if they have different types? I have come up with a possible solution (see below), but I am not sure that it is the right approach, and it is probably inefficient. Chris Mears {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Control.Monad import Control.Monad.State import Data.Generics import Data.Generics.Uniplate.Data data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing -- The transformers are wrapped in an existential type. data WrappedTransformer from = forall to. (Data to, Biplate from to) => WrappedTransformer (to -> Maybe to) -- Apply a single transformation, and return "Just x" if the -- transformation fired (where x is the result of the rewriting), and -- "Nothing" if no transformation fired. rewriteBiCheck :: Biplate from to => (to -> Maybe to) -> from -> Maybe from rewriteBiCheck f e = case runState (rewriteBiM check e) False of (e', True) -> Just e' (_, False) -> Nothing where check x = case f x of Nothing -> return Nothing Just y -> put True >> return (Just y) -- Apply a list of wrapped transformations until no more -- transformations can be applied. rewriteBiList :: forall from. [WrappedTransformer from] -> from -> from rewriteBiList transformers m = go transformers m where go :: [WrappedTransformer from] -> from -> from go [] m = m go ((WrappedTransformer t):ts) m = case rewriteBiCheck t m of Just m' -> go transformers m' Nothing -> go ts m -- Test case. example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBiList [ WrappedTransformer t1 , WrappedTransformer t2 , WrappedTransformer t3 ] example From artyom.kazak at gmail.com Tue Jan 29 01:27:41 2013 From: artyom.kazak at gmail.com (Artyom Kazak) Date: Tue, 29 Jan 2013 03:27:41 +0300 Subject: [Haskell-cafe] quotRem and divMod Message-ID: Hi! I?ve always thought that `quotRem` is faster than `quot` + `rem`, since both `quot` and `rem` are just "wrappers" that compute both the quotient and the remainder and then just throw one out. However, today I looked into the implementation of `quotRem` for `Int32` and found out that it?s not true: quotRem x@(I32# x#) y@(I32# y#) | y == 0 = divZeroError | x == minBound && y == (-1) = overflowError | otherwise = (I32# (narrow32Int# (x# `quotInt#` y#)), I32# (narrow32Int# (x# `remInt#` y#))) Why? The `DIV` instruction computes both, doesn?t it? And yet it?s being performed twice here. Couldn?t one of the experts clarify this bit? From hutch-lists at recursive.ca Tue Jan 29 02:43:09 2013 From: hutch-lists at recursive.ca (Bob Hutchison) Date: Mon, 28 Jan 2013 20:43:09 -0500 Subject: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache Message-ID: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> Hi, I'm relatively new to Haskell, and consider myself to be towards the beginner side of the scale. Nevertheless, I've got this Haskell program I've been working on that's sitting around 11k lines right now. The pattern has been to let it grow to then knock it back by 'refactoring' or whatever you want to call it? doing it right the second time maybe? or the third time. All I want to get across is that though I consider myself a Haskell beginner I've still managed to produce something that is actually quite complex and of reasonable size in about three months. I'm still getting caught by stuff that I should not be caught by. So. Today I thought it was about time to simplify how new 'things' of a certain kind are added to the system. These things are some a cross between an event and an assertion of a fact in a rule based system. There are many different kinds of these things. I already have more than a dozen commonplace ones, and I expect there's a much larger number of more specialized ones that a user will want to add on their own. While they start out quite differently, they end up satisfying a common interface and follow the identical three or four state lifecycle. This sounded like a type class to me, and in fact, easily implemented as such. Now, this is how I got caught: it seems to be impossible to have collections of things with a common type class if they have different types. How is it that I've written that many lines of code in Haskell and I'm just noticing this now? (If I wasn't so annoyed, I'd look for something clever to reflect how loc count obviously doesn't mean much? but clever seems to be beyond me today). Is this true? Are there any GHC extensions that will let me around this? The immediate problem is mapping an input to the system, some json message containing a reference to the 'thing' (like a key of some kind). I have to take that reference and find the thing and operate on it. All operations are easily accommodated by a type class. However, since I can't have a collection with mixed types even if the types satisfy a type class, I can't quite see how to actually store the things so I can find them. So there are a couple of obvious ways to handle this. I could use an ADT and a sum type of all the known kinds of thing, but I already know that this has to be extended and that's going to be problematic with users doing this on their own. And the type signatures look ugly. So I think that's not the best. I could use an ADT that contains functions that correspond to the functions of the type class, and that close over the 'thing' in question. I think this could be made to work, but I'm concerned with walking into more nasty surprises? If anyone is able to make sense of what I wrote and has any suggestions I'd really appreciate hearing them. Thanks, Bob From t at tmh.cc Tue Jan 29 03:17:12 2013 From: t at tmh.cc (Taylor Hedberg) Date: Mon, 28 Jan 2013 21:17:12 -0500 Subject: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache In-Reply-To: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> References: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> Message-ID: <20130129021712.GC7905@euporie.thebe.ath.cx> If I understand your message well enough, I think you are looking for GHC's `ExistentialQuantification` extension. Building heterogeneous collections is a common example of what existential types are useful for. Take a look at this wiki page [1]; there is an example of how to accomplish this there, along with a handful of other techniques. [1] http://www.haskell.org/haskellwiki/Heterogenous_collections#Existential_types -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 836 bytes Desc: Digital signature URL: From therealkludgy at gmail.com Tue Jan 29 03:35:13 2013 From: therealkludgy at gmail.com (Darren Grant) Date: Mon, 28 Jan 2013 18:35:13 -0800 Subject: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache In-Reply-To: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> References: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> Message-ID: On Mon, Jan 28, 2013 at 5:43 PM, Bob Hutchison wrote: > > Now, this is how I got caught: it seems to be impossible to have > collections of things with a common type class if they have different > types. How is it that I've written that many lines of code in Haskell and > I'm just noticing this now? (If I wasn't so annoyed, I'd look for something > clever to reflect how loc count obviously doesn't mean much? but clever > seems to be beyond me today). > > Is this true? Are there any GHC extensions that will let me around this? > I just encountered this recently myself. There is a GADT extension [1][2] that may help. The greater abstraction appears to lie in existential types [3]. That being said, I'm a beginner as well and haven't yet used these extensions. So far I have found that my code is simplified by redefining heterogeneous types in terms of homogeneous functions. If I have a class that implements common methods, I will reorganize lists by common function types rather than by class. Cheers, Darren --- [1] http://www.haskell.org/haskellwiki/GADT [2] http://www.haskell.org/haskellwiki/GADTs_for_dummies [3] http://www.haskell.org/haskellwiki/Existential_type -------------- next part -------------- An HTML attachment was scrubbed... URL: From shachaf at gmail.com Tue Jan 29 07:09:37 2013 From: shachaf at gmail.com (Shachaf Ben-Kiki) Date: Mon, 28 Jan 2013 22:09:37 -0800 Subject: [Haskell-cafe] quotRem and divMod In-Reply-To: References: Message-ID: On Mon, Jan 28, 2013 at 4:27 PM, Artyom Kazak wrote: > Hi! > > I?ve always thought that `quotRem` is faster than `quot` + `rem`, since both > `quot` and `rem` are just "wrappers" that compute both the quotient and the > remainder and then just throw one out. However, today I looked into the > implementation of `quotRem` for `Int32` and found out that it?s not true: > > quotRem x@(I32# x#) y@(I32# y#) > | y == 0 = divZeroError > | x == minBound && y == (-1) = overflowError > | otherwise = (I32# (narrow32Int# (x# `quotInt#` > y#)), > I32# (narrow32Int# (x# `remInt#` > y#))) > > Why? The `DIV` instruction computes both, doesn?t it? And yet it?s being > performed twice here. Couldn?t one of the experts clarify this bit? > That code is from base 4.5. Here's base 4.6: quotRem x@(I32# x#) y@(I32# y#) | y == 0 = divZeroError -- Note [Order of tests] | y == (-1) && x == minBound = (overflowError, 0) | otherwise = case x# `quotRemInt#` y# of (# q, r #) -> (I32# (narrow32Int# q), I32# (narrow32Int# r)) So it looks like it was improved in GHC 7.6. In particular, by this commit: http://www.haskell.org/pipermail/cvs-libraries/2012-February/014880.html Shachaf From caseybasichis at gmail.com Tue Jan 29 08:21:19 2013 From: caseybasichis at gmail.com (Casey Basichis) Date: Mon, 28 Jan 2013 23:21:19 -0800 Subject: [Haskell-cafe] Most used functions in hackage Message-ID: Hi, I'm guessing this is a long shot, but Is there any link that counts the use of all functions in all packages in Hackage and lists them by frequency or by other stats? I'm still new to haskell but I've been working my way through tons and tons of tutorials and books. It would be very helpful to target in on the current reality of the most critical functions. Anything like this out there? Thanks, Casey -------------- next part -------------- An HTML attachment was scrubbed... URL: From akamaus at gmail.com Tue Jan 29 08:57:18 2013 From: akamaus at gmail.com (Dmitry Vyal) Date: Tue, 29 Jan 2013 11:57:18 +0400 Subject: [Haskell-cafe] Most used functions in hackage In-Reply-To: References: Message-ID: <510780DE.5060209@gmail.com> On 01/29/2013 11:21 AM, Casey Basichis wrote: > > Is there any link that counts the use of all functions in all packages > in Hackage and lists them by frequency or by other stats? > > I'm still new to haskell but I've been working my way through tons and > tons of tutorials and books. It would be very helpful to target in on > the current reality of the most critical functions. > Hello Casey, You can use Hoogle http://www.haskell.org/hoogle/ to get information about a particular function or to find a function by a part of it's signature. While it's helpful to carefully study some basic modules like Prelude function by function, I don't think it's a good approach in general. I suggest you to look for reviews of popular modules. Personally, I found "24 days of hackage" http://ocharles.org.uk/blog/ to be quite informative. I wouldn't argue it's a best source for a beginner, but at least it gives quite a broad perspective. Best regards, Dmitry From caseybasichis at gmail.com Tue Jan 29 09:23:34 2013 From: caseybasichis at gmail.com (Casey Basichis) Date: Tue, 29 Jan 2013 00:23:34 -0800 Subject: [Haskell-cafe] Most used functions in hackage In-Reply-To: <510780DE.5060209@gmail.com> References: <510780DE.5060209@gmail.com> Message-ID: Hi Dmitry, Thanks for the links. I've been through the 24 Days of Hackage, but I think its time to run through them again now that I'm a little more familiar with everything. Why do you think browsing function by function is a bad idea? It seems that knowing exactly what the most used functions are would be an extremely effective way of finding both which parts of the Prelude and Hackage are most broadly useful (instead of browsing them like a phonebook) and also finding support from the community as the most commonly used functions would likely be the easiest to find support for. I guess what I'm looking for doesn't exist, which is what it is. I'm just interested in why it's not an ideal way to take in Haskell, starting with the common and moving to the to rare. Thanks, Casey On Mon, Jan 28, 2013 at 11:57 PM, Dmitry Vyal wrote: > On 01/29/2013 11:21 AM, Casey Basichis wrote: > >> >> Is there any link that counts the use of all functions in all packages in >> Hackage and lists them by frequency or by other stats? >> >> I'm still new to haskell but I've been working my way through tons and >> tons of tutorials and books. It would be very helpful to target in on the >> current reality of the most critical functions. >> >> Hello Casey, > > You can use Hoogle http://www.haskell.org/hoogle/ to get information > about a particular function or to find a function by a part of it's > signature. > > While it's helpful to carefully study some basic modules like Prelude > function by function, I don't think it's a good approach in general. I > suggest you to look for reviews of popular modules. Personally, I found "24 > days of hackage" http://ocharles.org.uk/blog/ to be quite informative. I > wouldn't argue it's a best source for a beginner, but at least it gives > quite a broad perspective. > > Best regards, > Dmitry > > -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis at gmail.com 310.387.7540 -------------- next part -------------- An HTML attachment was scrubbed... URL: From efiish at gmail.com Tue Jan 29 10:25:49 2013 From: efiish at gmail.com (Junior White) Date: Tue, 29 Jan 2013 17:25:49 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different Message-ID: Hi Cafe, I have two programs for the same problem "Eight queens problem", the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. My two grograms only has little difference, but the performance, this is my solution: -- solution 1------------------------------------------------------------ queens1 :: Int -> [[Int]] queens1 n = map reverse $ queens' n where queens' 0 = [[]] queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), isSafe q qs] isSafe try qs = not (try `elem` qs || sameDiag try qs) sameDiag try qs = any (?(colDist, q) -> abs (try - q) == colDist) $ zip [1..] qs -- solution 2-------------------------------------------------------------- queens2 :: Int -> [[Int]] queens2 n = map reverse $ queens' n where queens' 0 = [[]] queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], isSafe q qs] isSafe try qs = not (try `elem` qs || sameDiag try qs) sameDiag try qs = any (?(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs the performance difference is: (set :set +s in ghci) *Main> length (queens1 8) 92 (287.85 secs, 66177031160 bytes) *Main> length (queens2 8) 92 (0.07 secs, 17047968 bytes) *Main> The only different in the two program is in the first is "q <- [1..n], qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". Does sequence in list comprehansion matter? And why? -------------- next part -------------- An HTML attachment was scrubbed... URL: From artyom.kazak at gmail.com Tue Jan 29 10:31:02 2013 From: artyom.kazak at gmail.com (Artyom Kazak) Date: Tue, 29 Jan 2013 12:31:02 +0300 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: Message-ID: Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 12:25:49 +0300: > The only different in the two program is in the first is "q <- [1..n], qs > <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". In the first case `queens' (k-1)` is being recomputed for every q (that is, n times). Of course it would matter :) From efiish at gmail.com Tue Jan 29 10:40:08 2013 From: efiish at gmail.com (Junior White) Date: Tue, 29 Jan 2013 17:40:08 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: Hi Artyom, Thanks! But I don't understand why in the first case "queens' (k-1)" is being recomputed n times? On Tue, Jan 29, 2013 at 5:31 PM, Artyom Kazak wrote: > Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 > 12:25:49 +0300: > > > The only different in the two program is in the first is "q <- [1..n], qs >> <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". >> > > In the first case `queens' (k-1)` is being recomputed for every q (that > is, n times). Of course it would matter :) > > ______________________________**_________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/**mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From artyom.kazak at gmail.com Tue Jan 29 10:48:31 2013 From: artyom.kazak at gmail.com (Artyom Kazak) Date: Tue, 29 Jan 2013 12:48:31 +0300 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 12:40:08 +0300: > Hi Artyom, > Thanks! But I don't understand why in the first case "queens' (k-1)" > is > being recomputed n times? Because your list comprehension is just a syntactic sugar for concatMap (\q -> concatMap (\qs -> if isSafe q qs then [q:qs] else []) (queens' (k-1))) [1..n] Here `queens' (k-1)` does not depend on `qs`, and therefore it *could* be floated out of the lambda: let queens = queens' (k-1) in concatMap (\q -> concatMap (\qs -> if isSafe q qs then [q:qs] else []) queens) [1..n] But it is an unsafe optimisation. Suppose that the `queens` list is very big. If we apply this optimisation, it will be retained in memory during the whole evaluation, which may be not desirable. That?s why GHC leaves this to you. From efiish at gmail.com Tue Jan 29 10:54:28 2013 From: efiish at gmail.com (Junior White) Date: Tue, 29 Jan 2013 17:54:28 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: Thanks again! I understand now. I'll be careful when the next time I use list comprehension. On Tue, Jan 29, 2013 at 5:48 PM, Artyom Kazak wrote: > Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 > 12:40:08 +0300: > > Hi Artyom, >> Thanks! But I don't understand why in the first case "queens' (k-1)" is >> being recomputed n times? >> > > Because your list comprehension is just a syntactic sugar for > > concatMap (\q -> > concatMap (\qs -> if isSafe q qs then [q:qs] else []) > (queens' (k-1))) > [1..n] > > Here `queens' (k-1)` does not depend on `qs`, and therefore it *could* be > floated out of the lambda: > > let queens = queens' (k-1) > in > concatMap (\q -> > concatMap (\qs -> if isSafe q qs then [q:qs] else []) > queens) > [1..n] > > But it is an unsafe optimisation. Suppose that the `queens` list is very > big. If we apply this optimisation, it will be retained in memory during > the whole evaluation, which may be not desirable. That's why GHC leaves > this to you. > -------------- next part -------------- An HTML attachment was scrubbed... URL: From efiish at gmail.com Tue Jan 29 10:59:31 2013 From: efiish at gmail.com (Junior White) Date: Tue, 29 Jan 2013 17:59:31 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: So this is a problem in lazy evaluation language, it will not appear in python or erlang, am i right? On Tue, Jan 29, 2013 at 5:54 PM, Junior White wrote: > Thanks again! I understand now. I'll be careful when the next time I use > list comprehension. > > > On Tue, Jan 29, 2013 at 5:48 PM, Artyom Kazak wrote: > >> Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 >> 12:40:08 +0300: >> >> Hi Artyom, >>> Thanks! But I don't understand why in the first case "queens' (k-1)" >>> is >>> being recomputed n times? >>> >> >> Because your list comprehension is just a syntactic sugar for >> >> concatMap (\q -> >> concatMap (\qs -> if isSafe q qs then [q:qs] else []) >> (queens' (k-1))) >> [1..n] >> >> Here `queens' (k-1)` does not depend on `qs`, and therefore it *could* be >> floated out of the lambda: >> >> let queens = queens' (k-1) >> in >> concatMap (\q -> >> concatMap (\qs -> if isSafe q qs then [q:qs] else []) >> queens) >> [1..n] >> >> But it is an unsafe optimisation. Suppose that the `queens` list is very >> big. If we apply this optimisation, it will be retained in memory during >> the whole evaluation, which may be not desirable. That's why GHC leaves >> this to you. >> > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From hjgtuyl at chello.nl Tue Jan 29 11:17:49 2013 From: hjgtuyl at chello.nl (Henk-Jan van Tuyl) Date: Tue, 29 Jan 2013 11:17:49 +0100 Subject: [Haskell-cafe] Most used functions in hackage In-Reply-To: References: <510780DE.5060209@gmail.com> Message-ID: On Tue, 29 Jan 2013 09:23:34 +0100, Casey Basichis wrote: > I guess what I'm looking for doesn't exist, which is what it is. I'm > just > interested in why it's not an ideal way to take in Haskell, starting with > the common and moving to the to rare. > It is worth while to study the Prelude functions: A Tour of the Haskell Prelude http://undergraduate.csse.uwa.edu.au/units/CITS1211/Documentation/tourofprelude.html and: A tour of the Haskell Monad functions http://members.chello.nl/hjgtuyl/tourdemonad.html Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming -- From artyom.kazak at gmail.com Tue Jan 29 11:33:58 2013 From: artyom.kazak at gmail.com (Artyom Kazak) Date: Tue, 29 Jan 2013 13:33:58 +0300 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: Junior White ?????(?) ? ????? ?????? Tue, 29 Jan 2013 12:59:31 +0300: > So this is a problem in lazy evaluation language, it will not appear in > python or erlang, am i right? Not quite. Compilers of imperative languages don?t perform CSE (common subexpression elimination) either; `queens' (k-1)` could have some side effects, after all, and performing a side effect only once instead of n times is a definite bug. From daniel.is.fischer at googlemail.com Tue Jan 29 12:39:37 2013 From: daniel.is.fischer at googlemail.com (Daniel Fischer) Date: Tue, 29 Jan 2013 12:39:37 +0100 Subject: [Haskell-cafe] quotRem and divMod In-Reply-To: References: Message-ID: <2168415.Q9FYo6iBum@linux-v7dw.site> On Tuesday 29 January 2013, 03:27:41, Artyom Kazak wrote: > Hi! > > I?ve always thought that `quotRem` is faster than `quot` + `rem`, since > both `quot` and `rem` are just "wrappers" that compute both the quotient > and the remainder and then just throw one out. However, today I looked > into the implementation of `quotRem` for `Int32` and found out that it?s > not true: > > quotRem x@(I32# x#) y@(I32# y#) > > | y == 0 = divZeroError > | x == minBound && y == (-1) = overflowError > | otherwise = (I32# (narrow32Int# (x# `quotInt#` > > y#)), > I32# (narrow32Int# (x# `remInt#` > y#))) > > Why? The `DIV` instruction computes both, doesn?t it? And yet it?s being > performed twice here. Couldn?t one of the experts clarify this bit? It's not necessarily performed twice. func a b = case a `quotRem` b of (q, r) -> q+r produces idivq 8(%rbp) movq %rax,%rbx movq $GHC.Int.I32#_con_info,-8(%r12) movslq %edx,%rax movslq %ebx,%rbx addq %rax,%rbx as the relevant part of the assembly, with only one idivq instruction. I don't know whether you can rely on the code generator emitting only one division instruction in all cases, but in simple cases, it does (on x86_64, at least). Cheers, Daniel From artyom.kazak at gmail.com Tue Jan 29 13:26:02 2013 From: artyom.kazak at gmail.com (Artyom Kazak) Date: Tue, 29 Jan 2013 15:26:02 +0300 Subject: [Haskell-cafe] quotRem and divMod In-Reply-To: References:

Message-ID: Shachaf Ben-Kiki ?????(?) ? ????? ?????? Tue, 29 Jan 2013 09:09:37 +0300: > That code is from base 4.5. Here's base 4.6: > > quotRem x@(I32# x#) y@(I32# y#) > | y == 0 = divZeroError > -- Note [Order of tests] > | y == (-1) && x == minBound = (overflowError, 0) > | otherwise = case x# `quotRemInt#` y# of > (# q, r #) -> > (I32# (narrow32Int# q), > I32# (narrow32Int# r)) > > So it looks like it was improved in GHC 7.6. In particular, by this > commit: > http://www.haskell.org/pipermail/cvs-libraries/2012-February/014880.html > > Shacha Well, I?m glad to know that it has been fixed in the newer GHC (I?m still on 7.4). Thanks! From ross at soi.city.ac.uk Tue Jan 29 16:23:43 2013 From: ross at soi.city.ac.uk (Ross Paterson) Date: Tue, 29 Jan 2013 15:23:43 +0000 Subject: [Haskell-cafe] catamorphisms and attribute grammars In-Reply-To: <20130127002025.GA4119@sniper> References: <20130127002025.GA4119@sniper> Message-ID: <20130129152343.GA26479@soi.city.ac.uk> On Sun, Jan 27, 2013 at 12:20:25AM +0000, Roman Cheplyaka wrote: > Very nice! This can be generalized to arbitrary arrows: > > {-# LANGUAGE ExistentialQuantification #-} > > import Prelude hiding (id) > import Control.Arrow > import Control.Applicative > import Control.Category > > data F from to b c = forall d . F (from b d) (to d c) > > instance (Arrow from, Arrow to) => Functor (F from to b) where > fmap f x = pure f <*> x > > instance (Arrow from, Arrow to) => Applicative (F from to b) where > pure x = F (arr $ const x) id > F from1 to1 <*> F from2 to2 = > F (from1 &&& from2) (to1 *** to2 >>> arr (uncurry id)) You only require that from b is Applicative, so that in turn can be generalized: data F g to c = forall d . F (g d) (to d c) instance (Applicative g, Arrow to) => Functor (F g to) where fmap f x = pure f <*> x instance (Applicative g, Arrow to) => Applicative (F g to) where pure x = F (pure x) id F from1 to1 <*> F from2 to2 = F ((,) <$> from1 <*> from2) (to1 *** to2 >>> arr (uncurry id)) > I wonder what's a categorical interpretation of F itself. It's a variety of left Kan extension (cf section 5 of "Constructing Applicative Functors" at MPC'2012). From qdunkan at gmail.com Tue Jan 29 21:11:31 2013 From: qdunkan at gmail.com (Evan Laforge) Date: Tue, 29 Jan 2013 12:11:31 -0800 Subject: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache In-Reply-To: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> References: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> Message-ID: > Today I thought it was about time to simplify how new 'things' of a certain kind are added to the system. These things are some a cross between an event and an assertion of a fact in a rule based system. There are many different kinds of these things. I already have more than a dozen commonplace ones, and I expect there's a much larger number of more specialized ones that a user will want to add on their own. While they start out quite differently, they end up satisfying a common interface and follow the identical three or four state lifecycle. This sounded like a type class to me, and in fact, easily implemented as such. I hardly ever use typeclasses, I've never used existential types or GADTs, and it's worked fine for me for many years. Maybe just a difference in programming style, or the sorts of things I write, but implies at least that you can get very far not using any of that stuff. If each of your things have the same 3 or 4 states, can you make a state into a value, and compose them? E.g. 'thing1 = state1 <> state2 <> thing1state where thing1state = ...' and state1 and state2 are defined in a library. If you have lots of different ways to take A to B and want to let the caller configure it, then just pass an A->B function. If you want to configure an unpredictable subset of things, then maybe make a default record and pass 'default { aToB = customVersion }'. If each function depends on a configuration environment that you want to inherit from callers, then maybe put that record into a Reader. In my case, the main design decision winds up being the balance of data (i.e. records with values or functions) and code (i.e. functions that do parts of what you want and can be composed together in various ways). Code is extensible and flexible but can't be manipulated, data is inflexible (in that you have to hardcode some kind of "schema"), but that means you can write functions to transform it. From ndmitchell at gmail.com Tue Jan 29 22:00:13 2013 From: ndmitchell at gmail.com (Neil Mitchell) Date: Tue, 29 Jan 2013 21:00:13 +0000 Subject: [Haskell-cafe] Uniplate and rewriting with different types In-Reply-To: References: Message-ID: Hi Chris, > With the following type, and transformation functions: > > data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) > data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) > > t1,t2,t3 :: Even -> Maybe Even > But if one of the transformations has a different type, you can't do > it this way. For instance, redefine t2 to have a different type: > > t2 :: Odd -> Maybe Odd > t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) > t2 x = Nothing > > and you are stuck because the functions of different types can't be > combined into a single transformation. > > My question is: is there a good way to combine the transformation > functions if they have different types? Currently, no. Although there is something definitely related, with transformBis: http://hackage.haskell.org/packages/archive/uniplate/1.6.10/doc/html/Data-Generics-Uniplate-Data.html#v:transformBis That takes a list of transformation functions of different types and acts as though you did transform on each one in turn. You could certainly imagine adding rewriteBis in the same style, and with your version you almost have. The transformBis function is particularly efficient because it "knows" which traversals or parts of traversals can be fused without changing the semantics. rewriteBis could certainly do the same trick. If you provided a patch for rewriteBis I'd certainly apply it! Thanks, Neil From ganesh at earth.li Tue Jan 29 23:15:19 2013 From: ganesh at earth.li (Ganesh Sittampalam) Date: Tue, 29 Jan 2013 22:15:19 +0000 Subject: [Haskell-cafe] Heads up: planned removal of String instances from HTTP package Message-ID: <510849F7.7020107@earth.li> Hi, tl;dr: I'm planning on removing the String instances from the HTTP package. This is likely to break code. Obviously it will involve a major version bump. The basic reason is that this instance is rather broken in itself. A String ought to represent Unicode data, but the HTTP wire format is bytes, and HTTP makes no attempt to handle encoding. This was discussed on the libraries@ list a while back, but I'm happy to discuss further if there's a general feeling that this is a bad thing to do: http://www.haskell.org/pipermail/libraries/2012-September/018426.html I will probably upload the new version in a week or two. Cheers, Ganesh From johan.tibell at gmail.com Tue Jan 29 23:46:17 2013 From: johan.tibell at gmail.com (Johan Tibell) Date: Tue, 29 Jan 2013 14:46:17 -0800 Subject: [Haskell-cafe] Heads up: planned removal of String instances from HTTP package In-Reply-To: <510849F7.7020107@earth.li> References: <510849F7.7020107@earth.li> Message-ID: On Tue, Jan 29, 2013 at 2:15 PM, Ganesh Sittampalam wrote: > tl;dr: I'm planning on removing the String instances from the HTTP > package. This is likely to break code. Obviously it will involve a major > version bump. > > The basic reason is that this instance is rather broken in itself. A > String ought to represent Unicode data, but the HTTP wire format is > bytes, and HTTP makes no attempt to handle encoding. > > This was discussed on the libraries@ list a while back, but I'm happy to > discuss further if there's a general feeling that this is a bad thing to do: > > http://www.haskell.org/pipermail/libraries/2012-September/018426.html > > I will probably upload the new version in a week or two. I think it's the right thing to do. Providing a little upgrade guide should help things to go smoother. From evohunz at gmail.com Tue Jan 29 23:59:14 2013 From: evohunz at gmail.com (Thiago Negri) Date: Tue, 29 Jan 2013 20:59:14 -0200 Subject: [Haskell-cafe] Handling exceptions or gracefully releasing resources Message-ID: `Control.Exception.bracket` is a nice function to acquire and release a resource in a small context. But, how should I handle resources that are hold for a long time? Should I put `Control.Exception.finally` on every single line of my finalizers? What exceptions may occur on an IO operation? Every IO function has the risk of throwing an exception? Thanks, Thiago. -------------- next part -------------- An HTML attachment was scrubbed... URL: From johan.tibell at gmail.com Wed Jan 30 00:26:57 2013 From: johan.tibell at gmail.com (Johan Tibell) Date: Tue, 29 Jan 2013 15:26:57 -0800 Subject: [Haskell-cafe] Handling exceptions or gracefully releasing resources In-Reply-To: References: Message-ID: Hi, The pattern is essentially the same as in imperative languages; every allocation should involve a finally clause that deallocates the resource. On Tue, Jan 29, 2013 at 2:59 PM, Thiago Negri wrote: > Should I put `Control.Exception.finally` on every single line of my > finalizers? I'm not sure what you're asking here. If your finally clause tries to call close, you don't have to catch exceptions raise by close (what would you do with them anyway). From ok at cs.otago.ac.nz Wed Jan 30 01:20:10 2013 From: ok at cs.otago.ac.nz (Richard O'Keefe) Date: Wed, 30 Jan 2013 13:20:10 +1300 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References:

Message-ID: On 29/01/2013, at 10:59 PM, Junior White wrote: > So this is a problem in lazy evaluation language, it will not appear in python or erlang, am i right? Wrong. Let's take Erlang: [f(X, Y) || X <- g(), Y <- h()] Does the order of the generators matter here? You _bet_ it does. First off, in all of these languages, it affects the order of the results. Let's take a toy case: g() -> [1,2]. h() -> [a,b]. % constants f(X, Y) -> {X,Y}. % a pair [f(X, Y) || X <- g(), Y <- h()] yields [{1,a},{1,b},{2,a},{2,b}] [f(X, Y) || Y <- h(), X <- g()] yields [{1,a},{2,a},{1,b},{2,b}] Now let's change it by giving g/0 and h/0 (benign) side effects. g() -> io:write('g called'), io:nl(), [1,2]. h() -> io:write('h called'), io:nl(), [a,b]. Generating X before Y yields 'g called' 'h called' 'h called' [{1,a},{1,b},{2,a},{2,b}] Generating Y before X yields 'h called' 'g called' 'g called' [{1,a},{2,a},{1,b},{2,b}] If a function call may yield side effects, then the compiler must not re-order or coalesce calls to that function. This applies to both Erlang and Python (and to SETL, which had set and tuple comprehensions before Erlang, Python, or Haskell were conceived). From wren at freegeek.org Wed Jan 30 06:56:10 2013 From: wren at freegeek.org (wren ng thornton) Date: Wed, 30 Jan 2013 00:56:10 -0500 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: Message-ID: <5108B5FA.9030703@freegeek.org> On 1/29/13 4:25 AM, Junior White wrote: > Hi Cafe, > I have two programs for the same problem "Eight queens problem", > the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. > My two grograms only has little difference, but the performance, this is > my solution: The difference is what's called "dynamic programming" (an utterly non-intuitive an un-insightful name). When we have the program: [ f x xs | xs <- g, x <- h ] we're saying, first get me a partial solution (xs), and then try every possible way of extending that to a larger solution (x). It should be obvious from this description that the computation of each partial solution xs will be shared among all candidates x, but that the computation of x will not be shared between each xs. On the other hand, when we have the program: [ f x xs | x <- h, xs <- g ] we're saying, first get me all ways to start a solution (x), and then try to solve the rest of the problem (xs). It should be obvious from this description that the computation of each x will be shared, but the computation of each xs will not. Imperatively, this is exactly the same distinction as between the following programs: for xs in g: for x in h: yield f(x,xs) for x in h: for xs in g: yield f(x,xs) This difference in sharing can, as you've seen, cause huge differences in runtime. Usually it's the difference between a polytime algorithm and some exptime algorithm. To see why, just think about the call graph. It may be more helpful here to think about something like Fibbonaci numbers. In the memoizing version, you're storing the work from solving smaller problems and sharing that among the different ways of extending the solution; whereas in the naive version, you're recomputing the same thing over and over. The call graph for the former is a DAG (or more generally, a packed forest) whereas the call graph for the latter is the tree you get by unfurling all the shared structure in the DAG. This distinction has nothing whatsoever to do with Haskell, and has everything to do with Intro Algorithms. Loop ordering matters in every language with loops, from Haskell to C to Python to Prolog. -- Live well, ~wren From ganesh at earth.li Wed Jan 30 08:00:37 2013 From: ganesh at earth.li (Ganesh Sittampalam) Date: Wed, 30 Jan 2013 07:00:37 +0000 Subject: [Haskell-cafe] Heads up: planned removal of String instances from HTTP package In-Reply-To: References: <510849F7.7020107@earth.li> Message-ID: <5108C515.9060402@earth.li> On 29/01/2013 22:46, Johan Tibell wrote: > On Tue, Jan 29, 2013 at 2:15 PM, Ganesh Sittampalam wrote: >> tl;dr: I'm planning on removing the String instances from the HTTP >> package. This is likely to break code. Obviously it will involve a major >> version bump. > > I think it's the right thing to do. Providing a little upgrade guide > should help things to go smoother. One obvious cheap-and-dirty migration is via a newtype wrapper for String that embeds the old broken behaviour (char8 encoding). Perhaps the package should provide that? (with appropriate warnings etc) Ganesh From alfredo.dinapoli at gmail.com Wed Jan 30 09:01:34 2013 From: alfredo.dinapoli at gmail.com (Alfredo Di Napoli) Date: Wed, 30 Jan 2013 08:01:34 +0000 Subject: [Haskell-cafe] Heads up: planned removal of String instances from HTTP package In-Reply-To: <5108C515.9060402@earth.li> References: <510849F7.7020107@earth.li> <5108C515.9060402@earth.li> Message-ID: Maybe (just my 2 cents!) since you are going to broke the API anyway, go for it and seize the occasion to really clean up :) Obviously I'm saying this from a non-http-user point of view, maybe if I had some code in production affected by this, my suggestion would have been different :P Regards, A. On 30 January 2013 07:00, Ganesh Sittampalam wrote: > On 29/01/2013 22:46, Johan Tibell wrote: > > On Tue, Jan 29, 2013 at 2:15 PM, Ganesh Sittampalam > wrote: > >> tl;dr: I'm planning on removing the String instances from the HTTP > >> package. This is likely to break code. Obviously it will involve a major > >> version bump. > > > > I think it's the right thing to do. Providing a little upgrade guide > > should help things to go smoother. > > One obvious cheap-and-dirty migration is via a newtype wrapper for > String that embeds the old broken behaviour (char8 encoding). Perhaps > the package should provide that? (with appropriate warnings etc) > > Ganesh > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From ok at cs.otago.ac.nz Wed Jan 30 09:12:14 2013 From: ok at cs.otago.ac.nz (ok at cs.otago.ac.nz) Date: Wed, 30 Jan 2013 21:12:14 +1300 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: <5108B5FA.9030703@freegeek.org> References: <5108B5FA.9030703@freegeek.org> Message-ID: <00248cd3a668ecc447353c370254e0ae.squirrel@chasm.otago.ac.nz> > The difference is what's called "dynamic programming" (an utterly > non-intuitive and un-insightful name). It was meant to be. The name was chosen to be truthful while not revealing too much to a US Secretary of Defense of whom Bellman wrote: "His face would suffuse, he would turn red, and he would get violent if people used the term, research, in his presence. You can imagine how he felt, then, about the term, mathematical." (http://en.wikipedia.org/wiki/Dynamic_programming) Every time I try to imagine this guy having Haskell explained to him my brain refuses to co-operate. The word "programming" here is used in the same sense as in "linear programming" and "quadratic programming", that is, "optimisation". "Dynamic" does hint at the multistage decision process idea involved. From tim at dockerz.net Wed Jan 30 10:23:33 2013 From: tim at dockerz.net (Tim Docker) Date: Wed, 30 Jan 2013 20:23:33 +1100 Subject: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache In-Reply-To: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> References: <1734C212-50C4-457F-AAAE-7FE29B686BC6@recursive.ca> Message-ID: On 29/01/2013, at 12:43 PM, Bob Hutchison wrote: > > The immediate problem is mapping an input to the system, some json message containing a reference to the 'thing' (like a key of some kind). I have to take that reference and find the thing and operate on it. All operations are easily accommodated by a type class. However, since I can't have a collection with mixed types even if the types satisfy a type class, I can't quite see how to actually store the things so I can find them. > > So there are a couple of obvious ways to handle this. > > I could use an ADT and a sum type of all the known kinds of thing, but I already know that this has to be extended and that's going to be problematic with users doing this on their own. And the type signatures look ugly. So I think that's not the best. > > I could use an ADT that contains functions that correspond to the functions of the type class, and that close over the 'thing' in question. I think this could be made to work, but I'm concerned with walking into more nasty surprises? > My advice is to go for the latter option. I'm not sure what nasty surprises you are expecting, but this "record of functions" approach is the one that I normally take when I am building a system that needs new types added without requiring global changes. I know that existentials and GADTs are possible solutions, but I've not needed the extra complexity here. Cheers, Tim From doaitse at swierstra.net Wed Jan 30 10:51:41 2013 From: doaitse at swierstra.net (Doaitse Swierstra) Date: Wed, 30 Jan 2013 10:51:41 +0100 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: Message-ID: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> From the conclusion that both programs compute the same result it can be concluded that the fact that you have made use of a list comprehension has forced you to make a choice which should not matter, i.e. the order in which to place the generators. This should be apparent from your code. My approach is such a situation is to "define your own generator" (assuming here that isSafe needs both its parameters): pl `x` ql = [ (p,q) | p <-pl, q <- ql] queens3 n = map reverse $ queens' n where queens' 0 = [[]] queens' k = [q:qs | (qs, q) <- queens' (k-1) `x` [1..n], isSafe q qs] isSafe try qs = not (try `elem` qs || sameDiag try qs) sameDiag try qs = any (\(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs Of course you can make more refined versions of `x`, which perform all kinds of fair enumeration, but that is not the main point here. It is the fact that the parameters to `x` are only evaluated once which matters here. Doaitse On Jan 29, 2013, at 10:25 , Junior White wrote: > Hi Cafe, > I have two programs for the same problem "Eight queens problem", > the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. > My two grograms only has little difference, but the performance, this is my solution: > > -- solution 1------------------------------------------------------------ > queens1 :: Int -> [[Int]] > queens1 n = map reverse $ queens' n > where queens' 0 = [[]] > queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > sameDiag try qs = any (?(colDist, q) -> abs (try - q) == colDist) $ zip [1..] qs > > -- solution 2-------------------------------------------------------------- > queens2 :: Int -> [[Int]] > queens2 n = map reverse $ queens' n > where queens' 0 = [[]] > queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > sameDiag try qs = any (?(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs > > the performance difference is: (set :set +s in ghci) > *Main> length (queens1 8) > 92 > (287.85 secs, 66177031160 bytes) > *Main> length (queens2 8) > 92 > (0.07 secs, 17047968 bytes) > *Main> > > The only different in the two program is in the first is "q <- [1..n], qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". > > Does sequence in list comprehansion matter? And why? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -------------- next part -------------- An HTML attachment was scrubbed... URL: From arkeet at gmail.com Wed Jan 30 12:54:11 2013 From: arkeet at gmail.com (Adrian Keet) Date: Wed, 30 Jan 2013 03:54:11 -0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> Message-ID: <510909E3.10003@gmail.com> The whole point here is to evaluate both lists inside the list comprehension only once. There is a very simple way to accomplish this: [q:qs | let qss = queens' (k-1), q <- [1..n], qs <- qss] Here, queens' (k-1) is only evaluated once, and is shared for all q. (Note: If queens' (k-1) is polymorphic (which it is) and you use -XNoMonomorphismRestriction, then you better add a type annotation to qss to ensure sharing.) Adrian On 2013/01/30 1:51, Doaitse Swierstra wrote: > From the conclusion that both programs compute the same result it can > be concluded that the fact that you have made use of a list > comprehension has forced you to make a choice which should not > matter, i.e. the order in which to place the generators. This should > be apparent from your code. > > My approach is such a situation is to "define your own generator" > (assuming here that isSafe needs both its parameters): > > pl `x` ql = [ (p,q) | p <-pl, q <- ql] > > queens3 n = map reverse $ queens' n > where queens' 0 = [[]] > queens' k = [q:qs | (qs, q) <- queens' (k-1) `x` > [1..n], isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > sameDiag try qs = any (\(colDist,q) -> abs (try - q) == > colDist) $ zip [1..] qs > > Of course you can make more refined versions of `x`, which perform all > kinds of fair enumeration, but that is not the main point here. It is > the fact that the parameters to `x` are only evaluated once which > matters here. > > Doaitse > > On Jan 29, 2013, at 10:25 , Junior White > wrote: > >> Hi Cafe, >> I have two programs for the same problem "Eight queens problem", >> the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. >> My two grograms only has little difference, but the performance, >> this is my solution: >> >> -- solution 1------------------------------------------------------------ >> queens1 :: Int -> [[Int]] >> queens1 n = map reverse $ queens' n >> where queens' 0 = [[]] >> queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), >> isSafe q qs] >> isSafe try qs = not (try `elem` qs || sameDiag try qs) >> sameDiag try qs = any (?(colDist, q) -> abs (try - q) == >> colDist) $ zip [1..] qs >> >> -- solution >> 2-------------------------------------------------------------- >> queens2 :: Int -> [[Int]] >> queens2 n = map reverse $ queens' n >> where queens' 0 = [[]] >> queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], >> isSafe q qs] >> isSafe try qs = not (try `elem` qs || sameDiag try qs) >> sameDiag try qs = any (?(colDist,q) -> abs (try - q) == >> colDist) $ zip [1..] qs >> >> the performance difference is: (set :set +s in ghci) >> *Main> length (queens1 8) >> 92 >> (287.85 secs, 66177031160 bytes) >> *Main> length (queens2 8) >> 92 >> (0.07 secs, 17047968 bytes) >> *Main> >> >> The only different in the two program is in the first is "q <- >> [1..n], qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), >> q <- [1..n]". >> >> Does sequence in list comprehansion matter? And why? >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe at haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -------------- next part -------------- An HTML attachment was scrubbed... URL: From efiish at gmail.com Wed Jan 30 13:02:15 2013 From: efiish at gmail.com (Junior White) Date: Wed, 30 Jan 2013 20:02:15 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> Message-ID: On Wed, Jan 30, 2013 at 5:51 PM, Doaitse Swierstra wrote: > From the conclusion that both programs compute the same result it can be > concluded that the fact that you have made use of a list comprehension has > forced you to make a choice which should not matter, i.e. the order in > which to place the generators. This should be apparent from your code. > > My approach is such a situation is to "define your own generator" > (assuming here that isSafe needs both its parameters): > > pl `x` ql = [ (p,q) | p <-pl, q <- ql] > > queens3 n = map reverse $ queens' n > where queens' 0 = [[]] > > queens' k = [q:qs | (qs, q) <- queens' (k-1) `x` [1..n], > isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > > sameDiag try qs = any (\(colDist,q) -> abs (try - q) == colDist) > $ zip [1..] qs > > Of course you can make more refined versions of `x`, which perform all > kinds of fair enumeration, but that is not the main point here. It is the > fact that the parameters to `x` are only evaluated once which matters here. > Thanks for your reply! I must learn more to fully understand what's going on inside the list comprehension. But when I frist learn Haskell, it says sequence doesn't matter, but now it is a big matter, can compiler do some thing for us? I think this behavior is not friendly to newbies like me, I will take a very long time to work through it. -------------- next part -------------- An HTML attachment was scrubbed... URL: From efiish at gmail.com Wed Jan 30 13:05:44 2013 From: efiish at gmail.com (Junior White) Date: Wed, 30 Jan 2013 20:05:44 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: <510909E3.10003@gmail.com> References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> <510909E3.10003@gmail.com> Message-ID: Thinks! I think compiler should do this for us, isn't it? On Wed, Jan 30, 2013 at 7:54 PM, Adrian Keet wrote: > The whole point here is to evaluate both lists inside the list > comprehension only once. There is a very simple way to accomplish this: > > [q:qs | let qss = queens' (k-1), q <- [1..n], qs <- qss] > > Here, queens' (k-1) is only evaluated once, and is shared for all q. > > (Note: If queens' (k-1) is polymorphic (which it is) and you use > -XNoMonomorphismRestriction, then you better add a type annotation to qss > to ensure sharing.) > > Adrian > > > On 2013/01/30 1:51, Doaitse Swierstra wrote: > > From the conclusion that both programs compute the same result it can be > concluded that the fact that you have made use of a list comprehension has > forced you to make a choice which should not matter, i.e. the order in > which to place the generators. This should be apparent from your code. > > My approach is such a situation is to "define your own generator" > (assuming here that isSafe needs both its parameters): > > pl `x` ql = [ (p,q) | p <-pl, q <- ql] > > queens3 n = map reverse $ queens' n > where queens' 0 = [[]] > > queens' k = [q:qs | (qs, q) <- queens' (k-1) `x` [1..n], > isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > > sameDiag try qs = any (\(colDist,q) -> abs (try - q) == colDist) > $ zip [1..] qs > > Of course you can make more refined versions of `x`, which perform all > kinds of fair enumeration, but that is not the main point here. It is the > fact that the parameters to `x` are only evaluated once which matters here. > > Doaitse > > On Jan 29, 2013, at 10:25 , Junior White wrote: > > Hi Cafe, > I have two programs for the same problem "Eight queens problem", > the link is http://www.haskell.org/haskellwiki/99_questions/90_to_94. > My two grograms only has little difference, but the performance, this > is my solution: > > -- solution 1------------------------------------------------------------ > queens1 :: Int -> [[Int]] > > queens1 n = map reverse $ queens' n > > where queens' 0 = [[]] > > queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), > isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > > sameDiag try qs = any (?(colDist, q) -> abs (try - q) == > colDist) $ zip [1..] qs > > -- solution > 2-------------------------------------------------------------- > queens2 :: Int -> [[Int]] > > queens2 n = map reverse $ queens' n > > where queens' 0 = [[]] > > queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], > isSafe q qs] > isSafe try qs = not (try `elem` qs || sameDiag try qs) > > sameDiag try qs = any (?(colDist,q) -> abs (try - q) == colDist) > $ zip [1..] qs > > the performance difference is: (set :set +s in ghci) > *Main> length (queens1 8) > 92 > (287.85 secs, 66177031160 bytes) > *Main> length (queens2 8) > 92 > (0.07 secs, 17047968 bytes) > *Main> > > The only different in the two program is in the first is "q <- [1..n], > qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]". > > Does sequence in list comprehansion matter? And why? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing listHaskell-Cafe at haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From jan.stolarek at p.lodz.pl Wed Jan 30 13:21:18 2013 From: jan.stolarek at p.lodz.pl (Jan Stolarek) Date: Wed, 30 Jan 2013 13:21:18 +0100 Subject: [Haskell-cafe] Monadic parser vs. combinator parser Message-ID: <201301301321.18592.jan.stolarek@p.lodz.pl> I will be writing a parser in Haskell and I wonder how to approach the problem. My first thought was to use monadic parser, e.g. like the one described by Hutton and Meijer in "Monadic Parsing in Haskell" functional pearl. But then I stumbled upon this: https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser Monadic parser seems extremely verbose and not very straightforward compared to this one. I started to wonder whether I should use monadic parser for the sake of it being monadic or should I just go with the combinator approach? Any thoughts will be appreciated before I shoot myself in the foot :) Janek From es at ertes.de Wed Jan 30 13:38:47 2013 From: es at ertes.de (Ertugrul =?UTF-8?B?U8O2eWxlbWV6?=) Date: Wed, 30 Jan 2013 13:38:47 +0100 Subject: [Haskell-cafe] Monadic parser vs. combinator parser References: <201301301321.18592.jan.stolarek@p.lodz.pl> Message-ID: <20130130133847.0420ab87@tritium.streitmacht.eu> Jan Stolarek wrote: > I will be writing a parser in Haskell and I wonder how to approach the > problem. My first thought was to use monadic parser, e.g. like the one > described by Hutton and Meijer in "Monadic Parsing in Haskell" > functional pearl. But then I stumbled upon this: > > https://github.com/alephnullplex/cradle/tree/master/code/src/Lbach/Parser > > Monadic parser seems extremely verbose and not very straightforward > compared to this one. I started to wonder whether I should use monadic > parser for the sake of it being monadic or should I just go with the > combinator approach? Any thoughts will be appreciated before I shoot > myself in the foot :) A monadic parser /is/ a combinator parser. The code you linked just doesn't go as far as wrapping it up with a newtype and providing a monad instance. Monadic parsers aren't verbose, because there is the applicative style. Let's rewrite this noisy example (assuming automatic backtracking): inParens c = do char '(' x <- c char ')' return x All monads are also applicative functors, which means that you can use applicative style to write this one more nicely: inParens c = char '(' *> c <* char ')' If your parser is also an IsString you could even write: inParens c = "(" *> c <* ")" If that's not nice and concise I don't know what is. =) Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 836 bytes Desc: not available URL: From felipe.lessa at gmail.com Wed Jan 30 13:47:34 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Wed, 30 Jan 2013 10:47:34 -0200 Subject: [Haskell-cafe] Handling exceptions or gracefully releasing resources In-Reply-To: References: Message-ID: Everything that Johan Tibell said + you may be interested in the resourcet package [1] (which is used by conduit to handle resources). Cheers, [1] http://hackage.haskell.org/package/resourcet On Tue, Jan 29, 2013 at 8:59 PM, Thiago Negri wrote: > `Control.Exception.bracket` is a nice function to acquire and release a > resource in a small context. > > But, how should I handle resources that are hold for a long time? > > Should I put `Control.Exception.finally` on every single line of my > finalizers? > What exceptions may occur on an IO operation? > Every IO function has the risk of throwing an exception? > > Thanks, > Thiago. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Felipe. From evohunz at gmail.com Wed Jan 30 14:15:53 2013 From: evohunz at gmail.com (Thiago Negri) Date: Wed, 30 Jan 2013 11:15:53 -0200 Subject: [Haskell-cafe] Handling exceptions or gracefully releasing resources In-Reply-To: References:

Message-ID: Felipe, I'm trying to use your Hipmunk package. :) The resources I need to keep around are the objects used for the simulation. Do you recomend using "resourcet" to handle this or something else? Thanks. 2013/1/30 Felipe Almeida Lessa > Everything that Johan Tibell said + you may be interested in the > resourcet package [1] (which is used by conduit to handle resources). > > Cheers, > > [1] http://hackage.haskell.org/package/resourcet > > On Tue, Jan 29, 2013 at 8:59 PM, Thiago Negri wrote: > > `Control.Exception.bracket` is a nice function to acquire and release a > > resource in a small context. > > > > But, how should I handle resources that are hold for a long time? > > > > Should I put `Control.Exception.finally` on every single line of my > > finalizers? > > What exceptions may occur on an IO operation? > > Every IO function has the risk of throwing an exception? > > > > Thanks, > > Thiago. > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe at haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > -- > Felipe. > -------------- next part -------------- An HTML attachment was scrubbed... URL: From felipe.lessa at gmail.com Wed Jan 30 14:52:58 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Wed, 30 Jan 2013 11:52:58 -0200 Subject: [Haskell-cafe] Handling exceptions or gracefully releasing resources In-Reply-To: References:

Message-ID: AFAIR, the only object that you need to be careful with is the Space [1], everything else is garbage collected. You could put the Space in a ResourceT, but only if it ran on its own thread (as soon as a block of ResourceT finishes, everything is deallocated, so you wouldn't be able to follow the approach taken by HipmunkPlayground). Cheers, [1] http://hackage.haskell.org/packages/archive/Hipmunk/5.2.0.10/doc/html/Physics-Hipmunk-Space.html#v:freeSpace On Wed, Jan 30, 2013 at 11:15 AM, Thiago Negri wrote: > Felipe, I'm trying to use your Hipmunk package. :) > The resources I need to keep around are the objects used for the simulation. > Do you recomend using "resourcet" to handle this or something else? > > Thanks. > > > 2013/1/30 Felipe Almeida Lessa >> >> Everything that Johan Tibell said + you may be interested in the >> resourcet package [1] (which is used by conduit to handle resources). >> >> Cheers, >> >> [1] http://hackage.haskell.org/package/resourcet >> >> On Tue, Jan 29, 2013 at 8:59 PM, Thiago Negri wrote: >> > `Control.Exception.bracket` is a nice function to acquire and release a >> > resource in a small context. >> > >> > But, how should I handle resources that are hold for a long time? >> > >> > Should I put `Control.Exception.finally` on every single line of my >> > finalizers? >> > What exceptions may occur on an IO operation? >> > Every IO function has the risk of throwing an exception? >> > >> > Thanks, >> > Thiago. >> > >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe at haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >> >> >> -- >> Felipe. > > -- Felipe. From guillaumh at gmail.com Wed Jan 30 15:47:31 2013 From: guillaumh at gmail.com (Guillaume Hoffmann) Date: Wed, 30 Jan 2013 15:47:31 +0100 Subject: [Haskell-cafe] Darcs Hacking Sprint #8 (February 15-17, Paris) Message-ID: Dear hackers, the Darcs project is glad to announce that the next Hacking Sprint will take place in Paris on February 15-17, at the IRILL (near Place d'Italie). Please check out the details at: http://darcs.net/Sprints/2013-02 Here are three things to know 1. Everybody is welcome to join us. We'd love to have you, whatever your Haskell or Darcs hacking experience. Also, if you've got a wacky idea for the future of version control, or a cool use for the Darcs library, you should join us too :-) 2. Please let us know if you're attending. Just add your name to http://wiki.darcs.net/Sprints/2013-02 and you're good to go. You can also send us an email. 3. We can reimburse travel costs (within reason!). Let us know if you'd like a reimbursement, and save your receipts. Many thanks to everybody who participated in our fundraising drives or who gave money on the side. Thanks also to the Software Freedom Conservancy for making fundraising and reimbursements so painless! If you can't join us in person, but you'd like to cheer us on, say hello at http://darcs.net/Donations ! Looking forward to seeing you! Guillaume From allbery.b at gmail.com Wed Jan 30 16:07:44 2013 From: allbery.b at gmail.com (Brandon Allbery) Date: Wed, 30 Jan 2013 10:07:44 -0500 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> Message-ID: On Wed, Jan 30, 2013 at 7:02 AM, Junior White wrote: > Thanks for your reply! I must learn more to fully understand what's going > on inside the list comprehension. > But when I frist learn Haskell, it says sequence doesn't matter, but now > it is a big matter, can compiler do some thing for us? I think this > behavior is not friendly to newbies like me, I will take a very long time > to work through it. > No, the compiler can't help you here. The compiler is not an oracle; even if it could invert your calculation (effectively swapping the loops around), it can't know which one is more appropriate. As to sequences: sequence doesn't matter indeed; data dependencies matter, and loop ordering imposes a data dependency because loops in Haskell are encoded as data structures (lists). -- brandon s allbery kf8nh sine nomine associates allbery.b at gmail.com ballbery at sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net -------------- next part -------------- An HTML attachment was scrubbed... URL: From rustompmody at gmail.com Wed Jan 30 17:41:14 2013 From: rustompmody at gmail.com (Rustom Mody) Date: Wed, 30 Jan 2013 22:11:14 +0530 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> Message-ID: On Wed, Jan 30, 2013 at 5:32 PM, Junior White wrote: > > > Thanks for your reply! I must learn more to fully understand what's going > on inside the list comprehension. > But when I frist learn Haskell, it says sequence doesn't matter, but now > it is a big matter, can compiler do some thing for us? I think this > behavior is not friendly to newbies like me, I will take a very long time > to work through it. > Good point. Being a programmer means having to juggle many hats -- two important ones being the mathematician-hat and the machine-hat, also called declaration and 'imperation' Get only the first and your programs will run very inefficiently. Get only the second and your program will have bugs. Specifically in the case of list comprehensions the newbie needs - to practice thinking of the comprehension like a set comprehension and ignoring computation sequences - to practice thinking of comprehension in terms of map/filter etc ie operationally Both views are needed. Rusi -- http://www.the-magus.in http://blog.languager.org -------------- next part -------------- An HTML attachment was scrubbed... URL: From stephen.tetley at gmail.com Wed Jan 30 18:51:03 2013 From: stephen.tetley at gmail.com (Stephen Tetley) Date: Wed, 30 Jan 2013 17:51:03 +0000 Subject: [Haskell-cafe] Monadic parser vs. combinator parser In-Reply-To: <20130130133847.0420ab87@tritium.streitmacht.eu> References: <201301301321.18592.jan.stolarek@p.lodz.pl> <20130130133847.0420ab87@tritium.streitmacht.eu> Message-ID: On 30 January 2013 12:38, Ertugrul S?ylemez wrote: > > A monadic parser /is/ a combinator parser. The code you linked just > doesn't go as far as wrapping it up with a newtype and providing a monad > instance. Further, (+>) in the linked example is monadic bind and `result` is `return`. The code looks more succinct than early Parser combinator libraries (like Hutton / Meijer) because it defines quite a few more combinators. Equivalents are available if you use say Parsec plus the usual applicative combinators. From ganesh at earth.li Wed Jan 30 20:12:19 2013 From: ganesh at earth.li (Ganesh Sittampalam) Date: Wed, 30 Jan 2013 19:12:19 +0000 Subject: [Haskell-cafe] Heads up: planned removal of String instances from HTTP package In-Reply-To: References: <510849F7.7020107@earth.li> <5108C515.9060402@earth.li> Message-ID: <51097093.5090605@earth.li> Unfortunately I lack the time for a wholesale cleanup. If others have proposals and are willing to supply the patches that's a different matter! On 30/01/2013 08:01, Alfredo Di Napoli wrote: > Maybe (just my 2 cents!) since you are going to broke the API anyway, go > for it > and seize the occasion to really clean up :) > Obviously I'm saying this from a non-http-user point of view, maybe if I had > some code in production affected by this, my suggestion would have been > different :P > > Regards, > A. > > On 30 January 2013 07:00, Ganesh Sittampalam > wrote: > > On 29/01/2013 22:46, Johan Tibell wrote: > > On Tue, Jan 29, 2013 at 2:15 PM, Ganesh Sittampalam > > wrote: > >> tl;dr: I'm planning on removing the String instances from the HTTP > >> package. This is likely to break code. Obviously it will involve > a major > >> version bump. > > > > I think it's the right thing to do. Providing a little upgrade guide > > should help things to go smoother. > > One obvious cheap-and-dirty migration is via a newtype wrapper for > String that embeds the old broken behaviour (char8 encoding). Perhaps > the package should provide that? (with appropriate warnings etc) > > Ganesh > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > From ezyang at MIT.EDU Wed Jan 30 20:27:32 2013 From: ezyang at MIT.EDU (Edward Z. Yang) Date: Wed, 30 Jan 2013 11:27:32 -0800 Subject: [Haskell-cafe] Ticking time bomb Message-ID: <1359574031-sup-1195@javelin> https://status.heroku.com/incidents/489 Unsigned Hackage packages are a ticking time bomb. Cheers, Edward From mail at nh2.me Wed Jan 30 20:44:27 2013 From: mail at nh2.me (=?UTF-8?B?TmlrbGFzIEhhbWLDvGNoZW4=?=) Date: Wed, 30 Jan 2013 19:44:27 +0000 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359574031-sup-1195@javelin> References: <1359574031-sup-1195@javelin> Message-ID: <5109781B.8010304@nh2.me> As long as we upload packages via plain HTTP, signing won't help though. On Wed 30 Jan 2013 19:27:32 GMT, Edward Z. Yang wrote: > https://status.heroku.com/incidents/489 > > Unsigned Hackage packages are a ticking time bomb. > > Cheers, > Edward > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe From ezyang at MIT.EDU Wed Jan 30 20:47:00 2013 From: ezyang at MIT.EDU (Edward Z. Yang) Date: Wed, 30 Jan 2013 11:47:00 -0800 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <5109781B.8010304@nh2.me> References: <1359574031-sup-1195@javelin> <5109781B.8010304@nh2.me> Message-ID: <1359575164-sup-2642@javelin> > As long as we upload packages via plain HTTP, signing won't help though. I don't think that's true? If the package is tampered with, then the signature will be invalid; if the signature is also forged, then the private key is compromised and we can blacklist it. We care only about integrity, not secrecy. Edward From bob at redivi.com Wed Jan 30 20:58:09 2013 From: bob at redivi.com (Bob Ippolito) Date: Wed, 30 Jan 2013 11:58:09 -0800 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <5109781B.8010304@nh2.me> References: <1359574031-sup-1195@javelin> <5109781B.8010304@nh2.me> Message-ID: HTTPS doesn't really change anything if the server is compromised, it only prevents bad things from happening in transit. Sign the packages with GPG (or equivalent) before upload. The server never sees the package author's private key, only the public key. Server and/or client can warn or fail if the public key doesn't match their previous credentials or the signature verification fails. On Wed, Jan 30, 2013 at 11:44 AM, Niklas Hamb?chen wrote: > As long as we upload packages via plain HTTP, signing won't help though. > > On Wed 30 Jan 2013 19:27:32 GMT, Edward Z. Yang wrote: > > https://status.heroku.com/incidents/489 > > > > Unsigned Hackage packages are a ticking time bomb. > > > > Cheers, > > Edward > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe at haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From nomeata at debian.org Wed Jan 30 21:59:48 2013 From: nomeata at debian.org (Joachim Breitner) Date: Wed, 30 Jan 2013 21:59:48 +0100 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359574031-sup-1195@javelin> References: <1359574031-sup-1195@javelin> Message-ID: <1359579588.5131.4.camel@kirk> Hi, Am Mittwoch, den 30.01.2013, 11:27 -0800 schrieb Edward Z. Yang: > https://status.heroku.com/incidents/489 > > Unsigned Hackage packages are a ticking time bomb. another reason why Cabal is no package manager?. (Ok, I admit that I don?t review every line of diff between the Haskell packages I uploads. But thanks to http://hdiff.luite.com/ I at least glance over them most of the time ? a hurdle that malicious code would have to take. And once a package has entered a distribution like Debian (which it only can with a valid cryptographic signatures), checksums and signatures are used in many places to (mostly) guarantee that the package reaches the user unmodified.) Greetings, Joachim ? http://ivanmiljenovic.wordpress.com/2010/03/15/repeat-after-me-cabal-is-not-a-package-manager/ -- Joachim "nomeata" Breitner Debian Developer nomeata at debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata at joachim-breitner.de | http://people.debian.org/~nomeata -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 198 bytes Desc: This is a digitally signed message part URL: From arnaud.oqube at gmail.com Wed Jan 30 22:29:23 2013 From: arnaud.oqube at gmail.com (Arnaud Bailly) Date: Wed, 30 Jan 2013 22:29:23 +0100 Subject: [Haskell-cafe] Package conflicts using cabal-dev Message-ID: Hello, I am running into a strange issue that reminds me of Java's classloader black magic havoc :-) My code compiles fine using cabal-dev install, but when I try to compile an individual file with ghc, I got the following error: $ ghc -package-conf cabal-dev/packages-7.4.1.conf YakGraph.hs [1 of 1] Compiling YakGraph ( YakGraph.hs, YakGraph.o ) YakGraph.hs:13:30: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `s' In the expression: unpack s In an equation for `fromGraphId': fromGraphId (Str s) = unpack s YakGraph.hs:24:57: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `name' In the expression: unpack name In a case alternative: (Just (Str name)) : _ -> unpack name The code leading to this error is here: https://gist.github.com/4677191 Any insights on this would be appreciated. Arnaud -------------- next part -------------- An HTML attachment was scrubbed... URL: From felipe.lessa at gmail.com Wed Jan 30 23:31:51 2013 From: felipe.lessa at gmail.com (Felipe Almeida Lessa) Date: Wed, 30 Jan 2013 20:31:51 -0200 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359579588.5131.4.camel@kirk> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> Message-ID: IMHO Hackage and Cabal should support package signing even if they aren't package managers. On Wed, Jan 30, 2013 at 6:59 PM, Joachim Breitner wrote: > Hi, > > Am Mittwoch, den 30.01.2013, 11:27 -0800 schrieb Edward Z. Yang: >> https://status.heroku.com/incidents/489 >> >> Unsigned Hackage packages are a ticking time bomb. > > another reason why Cabal is no package manager?. > > (Ok, I admit that I don?t review every line of diff between the Haskell > packages I uploads. But thanks to http://hdiff.luite.com/ I at least > glance over them most of the time ? a hurdle that malicious code would > have to take. And once a package has entered a distribution like Debian > (which it only can with a valid cryptographic signatures), checksums and > signatures are used in many places to (mostly) guarantee that the > package reaches the user unmodified.) > > Greetings, > Joachim > > ? http://ivanmiljenovic.wordpress.com/2010/03/15/repeat-after-me-cabal-is-not-a-package-manager/ > > -- > Joachim "nomeata" Breitner > Debian Developer > nomeata at debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C > JID: nomeata at joachim-breitner.de | http://people.debian.org/~nomeata > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Felipe. From daniel.is.fischer at googlemail.com Wed Jan 30 23:40:20 2013 From: daniel.is.fischer at googlemail.com (Daniel Fischer) Date: Wed, 30 Jan 2013 23:40:20 +0100 Subject: [Haskell-cafe] Package conflicts using cabal-dev In-Reply-To: References: Message-ID: <18135774.oWG5nK6Icc@linux-v7dw.site> On Wednesday 30 January 2013, 22:29:23, Arnaud Bailly wrote: YakGraph.hs:13:30: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' one package (at least) you use - probably graphviz - was compiled with a version of `text` that is not the newest you have installed. GHC selects the newest installed version of a package by default, cabal and cabal-dev select the version that the other used packages require. Add a -package text-0.11.2.0 flag to the command line. -------------- next part -------------- An HTML attachment was scrubbed... URL: From ezyang at MIT.EDU Wed Jan 30 23:43:42 2013 From: ezyang at MIT.EDU (Edward Z. Yang) Date: Wed, 30 Jan 2013 14:43:42 -0800 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359579588.5131.4.camel@kirk> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> Message-ID: <1359585638-sup-7671@javelin> Excerpts from Joachim Breitner's message of Wed Jan 30 12:59:48 -0800 2013: > another reason why Cabal is no package manager?. Based on the linked post, it seems that you are arguing that cabal-install is not a package manager, and thus it is not necessary for it to duplicate the work that real package managers e.g. Debian or Ubuntu put into vetting, signing and releasing software. (Though I am not sure, so please correct me if I am wrong.) This argument seems specious. Whether or not cabal-install is or not intended to be a package manager, users expect it to act like one (as users expect rubygems to be a package manager), and, at the end of the day, that is what matters. Edward From Ramana.Kumar at cl.cam.ac.uk Wed Jan 30 23:46:26 2013 From: Ramana.Kumar at cl.cam.ac.uk (Ramana Kumar) Date: Wed, 30 Jan 2013 22:46:26 +0000 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359585638-sup-7671@javelin> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> Message-ID: On Wed, Jan 30, 2013 at 10:43 PM, Edward Z. Yang wrote: > This argument seems specious. Whether or not cabal-install is or not > intended to be a package manager, users expect it to act like one (as > users expect rubygems to be a package manager), and, at the end of the > day, that is what matters. > But playing along with their delusion might make it harder to change their minds. > > Edward > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe at haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -------------- next part -------------- An HTML attachment was scrubbed... URL: From mail at nh2.me Wed Jan 30 23:48:20 2013 From: mail at nh2.me (=?UTF-8?B?TmlrbGFzIEhhbWLDvGNoZW4=?=) Date: Wed, 30 Jan 2013 22:48:20 +0000 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359575164-sup-2642@javelin> References: <1359574031-sup-1195@javelin> <5109781B.8010304@nh2.me> <1359575164-sup-2642@javelin> Message-ID: <5109A334.9030209@nh2.me> You are right, I skipped over that this was actually a server-side exploit - sure, end-to-end signing will help here. On 30/01/13 19:47, Edward Z. Yang wrote: >> As long as we upload packages via plain HTTP, signing won't help though. From ezyang at MIT.EDU Wed Jan 30 23:48:27 2013 From: ezyang at MIT.EDU (Edward Z. Yang) Date: Wed, 30 Jan 2013 14:48:27 -0800 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> Message-ID: <1359586077-sup-6738@javelin> Excerpts from Ramana Kumar's message of Wed Jan 30 14:46:26 -0800 2013: > > This argument seems specious. Whether or not cabal-install is or not > > intended to be a package manager, users expect it to act like one (as > > users expect rubygems to be a package manager), and, at the end of the > > day, that is what matters. > > > > But playing along with their delusion might make it harder to change their > minds. Looking at the library ecosystems of the most popular programming languages, I think this ship has already sailed. Edward From Ramana.Kumar at cl.cam.ac.uk Wed Jan 30 23:54:23 2013 From: Ramana.Kumar at cl.cam.ac.uk (Ramana Kumar) Date: Wed, 30 Jan 2013 22:54:23 +0000 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359586077-sup-6738@javelin> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> <1359586077-sup-6738@javelin> Message-ID: On Wed, Jan 30, 2013 at 10:48 PM, Edward Z. Yang wrote: > Excerpts from Ramana Kumar's message of Wed Jan 30 14:46:26 -0800 2013: > > > This argument seems specious. Whether or not cabal-install is or not > > > intended to be a package manager, users expect it to act like one (as > > > users expect rubygems to be a package manager), and, at the end of the > > > day, that is what matters. > > > > > > > But playing along with their delusion might make it harder to change > their > > minds. > > Looking at the library ecosystems of the most popular programming > languages, > I think this ship has already sailed. > I was talking only about Haskell and cabal. There is a viable alternative to using cabal as a package manager on Arch Linux (the Arch-Haskell package repositories). There is also the Stackage project that might make this possible on more distributions with real package managers. But if you keep calling cabal a package manager, eventually you'll have to write the patches to make it one. > > Edward > -------------- next part -------------- An HTML attachment was scrubbed... URL: From nomeata at debian.org Wed Jan 30 23:57:28 2013 From: nomeata at debian.org (Joachim Breitner) Date: Wed, 30 Jan 2013 23:57:28 +0100 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359585638-sup-7671@javelin> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> Message-ID: <1359586648.7665.4.camel@kirk> Hi, Am Mittwoch, den 30.01.2013, 14:43 -0800 schrieb Edward Z. Yang: > Excerpts from Joachim Breitner's message of Wed Jan 30 12:59:48 -0800 2013: > > another reason why Cabal is no package manager?. > > Based on the linked post, it seems that you are arguing that cabal-install is > not a package manager, and thus it is not necessary for it to duplicate > the work that real package managers e.g. Debian or Ubuntu put into > vetting, signing and releasing software. (Though I am not sure, so please > correct me if I am wrong.) I?m not against cryptographically signed packages on hackage. In fact, I would whole-heatedly appreciate it, as it would make my work as a package maintainer easier. I was taking the opportunity to point out an advantage of established package management systems, to shamelessly advertise my work there, as not everyone sees distro-packaged libraries as a useful thing. Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata at debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata at joachim-breitner.de | http://people.debian.org/~nomeata -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 198 bytes Desc: This is a digitally signed message part URL: From ezyang at MIT.EDU Thu Jan 31 00:07:09 2013 From: ezyang at MIT.EDU (Edward Z. Yang) Date: Wed, 30 Jan 2013 15:07:09 -0800 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359586648.7665.4.camel@kirk> References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> <1359586648.7665.4.camel@kirk> Message-ID: <1359586786-sup-5614@javelin> Excerpts from Joachim Breitner's message of Wed Jan 30 14:57:28 -0800 2013: > I?m not against cryptographically signed packages on hackage. In fact, I > would whole-heatedly appreciate it, as it would make my work as a > package maintainer easier. > > I was taking the opportunity to point out an advantage of established > package management systems, to shamelessly advertise my work there, as > not everyone sees distro-packaged libraries as a useful thing. Yes. In fact, I am a sysadmin for a large shared hosting environment, and the fact that programming language libraries tend not to be distro-packaged is an endless headache for us. We would like it if everything were just packaged properly! On the other hand, working in these circumstances has made me realize that there is a huge tension between the goals of package library authors and distribution managers (a package library author is desires ease of installation of their packages, keeping everyone up-to-date as possible and tends to be selfish when it comes to the rest of the ecosystem, whereas the distribution manager values stability, security, and global consistency of the ecosystem.) So there is a lot of work to be done here. Nevertheless, I believe we are in violent agreement that cryptographically signed Hackage packages should happen as soon as possible! Edward From trebla at vex.net Thu Jan 31 02:19:26 2013 From: trebla at vex.net (Albert Y. C. Lai) Date: Wed, 30 Jan 2013 20:19:26 -0500 Subject: [Haskell-cafe] TIPS: To Insure Package Sanity Message-ID: <5109C69E.7030003@vex.net> If you possess multiple instances (may be different versions, may be same version different builds) of a package, life can be hard and confusing. The problems are explained in my http://www.vex.net/~trebla/haskell/sicp.xhtml and faced by many people regularly. (Just read this mailing list.) cabal-install has a mechanism for not adding more instances. It is available since version 0.14.0, or earlier. It is just little known. It is also a bit manual. You have to give 1 instruction for each package. If you want to say it for n packages, you have to give n instructions. Also, if you give such an instruction for a package you do not already have, there is a problem: now you can't install that package. To insure package sanity, add these lines to your $HOME/.cabal/config, one line per package you want to protect. (Cannot merge into one line.) constraint: array installed constraint: bytestring installed constraint: Cabal installed constraint: containers installed ... Generally, do it for every package that comes with GHC, and every package that comes with the Haskell Platform if you have it, plus every package that you want stable. (You may omit packages that cannot possibly be on hackage, e.g., integer-gmp.) Remember to revise those lines whenever you switch GHC versions or Haskell Platform versions, since their package lists change. Alternatively, if you want even more manual work (in exchange for more fine-tuning perhaps?): every time you use "cabal install": cabal install --constraint="array installed" --constraint="bytestring installed" --constraint="Cabal installed" --constraint="containers installed" ... From dstcruz at gmail.com Thu Jan 31 03:39:54 2013 From: dstcruz at gmail.com (Daniel Santa Cruz) Date: Wed, 30 Jan 2013 21:39:54 -0500 Subject: [Haskell-cafe] Haskell Weekly News: Issue 256 Message-ID: Welcome to issue 256 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of January 20 to 26, 2013. Quotes of the Week * elliott: cmccann: unfortunately it is too perfect an abstraction to be useful. * SamanthaD: shachaf: you're one of those dirty imperative communists who want the state to dictate everything! * monochrom: I refuse camel case and mark zuckerberg. same level. not negotiable. * mauke: a newtype is like an existing type but wearing glasses and a fake mustache and a sign saying "you've never seen me before" Top Reddit Stories * Taking magic out of GHC or: Tracing compilation by transformation (intro to Core transformations, inlining,.. Domain: ics.p.lodz.pl, Score: 59, Comments: 2 On Reddit: [1] http://goo.gl/lJmsb Original: [2] http://goo.gl/IbJ5O * Introduction to Haskell IO Domain: haskellforall.com, Score: 57, Comments: 26 On Reddit: [3] http://goo.gl/fPky4 Original: [4] http://goo.gl/Xymbr * The longest type signature I have ever seen (package Nussinov78) Domain: i.imgur.com, Score: 48, Comments: 26 On Reddit: [5] http://goo.gl/ePwNs Original: [6] http://goo.gl/B74yW * There is now a patch for the GHC runtime system which implements a scheduler which can handle threads with priorities. Domain: hackage.haskell.org, Score: 47, Comments: 12 On Reddit: [7] http://goo.gl/22XgH Original: [8] http://goo.gl/8iC8w * Success story: novice haskeller moves console cursor with ?netwire? FRP library Domain: danbst.wordpress.com, Score: 43, Comments: 8 On Reddit: [9] http://goo.gl/dm8ga Original: [10] http://goo.gl/Ue7pK * Haskell as pseudo code Domain: arxiv.org, Score: 33, Comments: 2 On Reddit: [11] http://goo.gl/JL8Mf Original: [12] http://goo.gl/5D4zV * Generalizing do notation Domain: self.haskell, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/teYAD Original: [14] http://goo.gl/teYAD * Tutorial: Up-front Unit Testing in Haskell Domain: github.com, Score: 26, Comments: 21 On Reddit: [15] http://goo.gl/kjzYJ Original: [16] http://goo.gl/oB2jg * The Design of a Practical Proof Checker for a Lazy Functional Language [PDF] Domain: people.cs.missouri.edu, Score: 26, Comments: On Reddit: [17] http://goo.gl/sj5Aj Original: [18] http://goo.gl/eEUOj * Introduction to Haskell, Lecture 2 is Live Domain: shuklan.com, Score: 24, Comments: 4 On Reddit: [19] http://goo.gl/1nUVv Original: [20] http://goo.gl/rCbfx * Hugs in your browser Domain: tryhaskell.org, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/zNtex Original: [22] http://goo.gl/wMPPW * Free functors - the reason Free and operational are not equivalent Domain: self.haskell, Score: 24, Comments: 12 On Reddit: [23] http://goo.gl/E7qe7 Original: [24] http://goo.gl/E7qe7 * Scribbling, smudging and smearing with Haskell Domain: self.haskell, Score: 22, Comments: 108 On Reddit: [25] http://goo.gl/KBw0V Original: [26] http://goo.gl/KBw0V * Touch and Keyboard API (announcing Elm 0.7.1) Domain: elm-lang.org, Score: 20, Comments: 3 On Reddit: [27] http://goo.gl/OhFaH Original: [28] http://goo.gl/rV9bU * (re)implementing the generalised Functor strength function using lenses. Domain: statusfailed.com, Score: 20, Comments: 3 On Reddit: [29] http://goo.gl/gEYHr Original: [30] http://goo.gl/TDX2A Top StackOverflow Questions * Can using UndecidableInstances pragma locally have global consequences on compilation termination? votes: 20, answers: 1 Read on SO: [31] http://goo.gl/PcQF1 * Why doesn't GHC Haskell support overloaded record parameter names? votes: 14, answers: 1 Read on SO: [32] http://goo.gl/BJ71T * How much does it cost for Haskell FFI to go into C and back? votes: 14, answers: 2 Read on SO: [33] http://goo.gl/kUCKn * Compile unsafe Haskell votes: 13, answers: 1 Read on SO: [34] http://goo.gl/nGaCX * Why GADT/existential data constructors cannot be used in lazy patterns? votes: 12, answers: 2 Read on SO: [35] http://goo.gl/zhpCh * Correct design for Haskell exception handling votes: 11, answers: 2 Read on SO: [36] http://goo.gl/zeWoH * How to use phase control of inlining in haskell? votes: 10, answers: 0 Read on SO: [37] http://goo.gl/dyuGI * Looking for resources that help testing a Haskell implementation for standard conformance votes: 9, answers: 1 Read on SO: [38] http://goo.gl/DL7vY * Generate a C struct based on a complex Haskell type votes: 8, answers: 1 Read on SO: [39] http://goo.gl/7E8OT * How to structure a Haskell project? votes: 7, answers: 1 Read on SO: [40] http://goo.gl/J4GuA * Difference between two functions creating a singleton list votes: 7, answers: 3 Read on SO: [41] http://goo.gl/ZXzw0 Until next time, Daniel Santa Cruz References 1. http://ics.p.lodz.pl/~stolarek/blog/2013/01/taking-magic-out-of-ghc-or-tracing-compilation-by-transformation/ 2. http://www.reddit.com/r/haskell/comments/17bu54/taking_magic_out_of_ghc_or_tracing_compilation_by/ 3. http://www.haskellforall.com/2013/01/introduction-to-haskell-io.html 4. http://www.reddit.com/r/haskell/comments/17712e/introduction_to_haskell_io/ 5. http://i.imgur.com/Zv9JH9A.png 6. http://www.reddit.com/r/haskell/comments/178jwh/the_longest_type_signature_i_have_ever_seen/ 7. http://hackage.haskell.org/trac/ghc/ticket/7606 8. http://www.reddit.com/r/haskell/comments/1725g0/there_is_now_a_patch_for_the_ghc_runtime_system/ 9. http://danbst.wordpress.com/2013/01/23/novice-netwire-user/ 10. http://www.reddit.com/r/haskell/comments/173cxi/success_story_novice_haskeller_moves_console/ 11. http://arxiv.org/abs/1301.5076 12. http://www.reddit.com/r/haskell/comments/176xk0/haskell_as_pseudo_code/ 13. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 14. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 15. https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/tutorial.md 16. http://www.reddit.com/r/haskell/comments/1750i9/tutorial_upfront_unit_testing_in_haskell/ 17. http://people.cs.missouri.edu/~harrisonwl/papers/tfp2012.pdf 18. http://www.reddit.com/r/haskell/comments/179367/the_design_of_a_practical_proof_checker_for_a/ 19. http://shuklan.com/haskell 20. http://www.reddit.com/r/haskell/comments/172h5q/introduction_to_haskell_lecture_2_is_live/ 21. http://tryhaskell.org/hugs/ 22. http://www.reddit.com/r/haskell/comments/177a56/hugs_in_your_browser/ 23. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_free_and_operational_are/ 24. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_free_and_operational_are/ 25. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_smearing_with_haskell/ 26. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_smearing_with_haskell/ 27. http://elm-lang.org/blog/announce/version-0.7.1.elm 28. http://www.reddit.com/r/haskell/comments/179nwl/touch_and_keyboard_api_announcing_elm_071/ 29. http://statusfailed.com/blog/2013/01/26/haskells-strength-generalising-with-lenses.html 30. http://www.reddit.com/r/haskell/comments/17b5ak/reimplementing_the_generalised_functor_strength/ 31. http://stackoverflow.com/questions/14476230/can-using-undecidableinstances-pragma-locally-have-global-consequences-on-compil 32. http://stackoverflow.com/questions/14422169/why-doesnt-ghc-haskell-support-overloaded-record-parameter-names 33. http://stackoverflow.com/questions/14519905/how-much-does-it-cost-for-haskell-ffi-to-go-into-c-and-back 34. http://stackoverflow.com/questions/14429966/compile-unsafe-haskell 35. http://stackoverflow.com/questions/14538255/why-gadt-existential-data-constructors-cannot-be-used-in-lazy-patterns 36. http://stackoverflow.com/questions/14538294/correct-design-for-haskell-exception-handling 37. http://stackoverflow.com/questions/14446368/how-to-use-phase-control-of-inlining-in-haskell 38. http://stackoverflow.com/questions/14484569/looking-for-resources-that-help-testing-a-haskell-implementation-for-standard-co 39. http://stackoverflow.com/questions/14500582/generate-a-c-struct-based-on-a-complex-haskell-type 40. http://stackoverflow.com/questions/14421178/how-to-structure-a-haskell-project 41. http://stackoverflow.com/questions/14530555/difference-between-two-functions-creating-a-singleton-list 42. https://plus.google.com/105107667630152149014/about -------------- next part -------------- An HTML attachment was scrubbed... URL: From caseybasichis at gmail.com Thu Jan 31 06:41:48 2013 From: caseybasichis at gmail.com (Casey Basichis) Date: Wed, 30 Jan 2013 21:41:48 -0800 Subject: [Haskell-cafe] FFI - Approaches to C/C++ Message-ID: Hi, I'm working on a project in Haskell and C++ where the former is the brains and the latter is for UI, interaction etc. I've read this http://www.altdevblogaday.com/2012/04/26/functional-programming-in-c/ and a number of other haskell posts suggesting the OOP is not the way to go. Without trying to emulate functional programming through templates or boost::phoenix, what approaches do you all favor when designing parts of an application in C++? Patterns to embrace or avoid? Should I just use functions and handle things with name spaces? I was thinking about handling the callbacks with boosts signals and slots 2 I know this is not entirely haskell centric, but it is a question for haskell users. Thanks, Casey -- Casey James Basichis Composer - Cartoon Network http://www.caseyjamesbasichis.com caseybasichis at gmail.com 310.387.7540 -------------- next part -------------- An HTML attachment was scrubbed... URL: From efiish at gmail.com Thu Jan 31 06:59:10 2013 From: efiish at gmail.com (Junior White) Date: Thu, 31 Jan 2013 13:59:10 +0800 Subject: [Haskell-cafe] list comprehansion performance has hug different In-Reply-To: References: <754643DB-A73B-47FC-9BB9-9F0BFB792482@swierstra.net> Message-ID: Thank you everyone! I like Haskell because the following two reasons: 1. It is beautifully 2. There are many great guys like you here. I will work harder on it, and forgive me for my broken English. On Thu, Jan 31, 2013 at 12:41 AM, Rustom Mody wrote: > > > On Wed, Jan 30, 2013 at 5:32 PM, Junior White wrote: > >> >> >> Thanks for your reply! I must learn more to fully understand what's >> going on inside the list comprehension. >> But when I frist learn Haskell, it says sequence doesn't matter, but now >> it is a big matter, can compiler do some thing for us? I think this >> behavior is not friendly to newbies like me, I will take a very long time >> to work through it. >> > > Good point. Being a programmer means having to juggle many hats -- two > important ones being the mathematician-hat and the machine-hat, also called > declaration and 'imperation' Get only the first and your programs will run > very inefficiently. Get only the second and your program will have bugs. > > Specifically in the case of list comprehensions the newbie needs > - to practice thinking of the comprehension like a set comprehension and > ignoring computation sequences > - to practice thinking of comprehension in terms of map/filter etc ie > operationally > > Both views are needed. > Rusi > -- > http://www.the-magus.in > http://blog.languager.org > > -------------- next part -------------- An HTML attachment was scrubbed... URL: From es at ertes.de Thu Jan 31 07:27:54 2013 From: es at ertes.de (Ertugrul =?UTF-8?B?U8O2eWxlbWV6?=) Date: Thu, 31 Jan 2013 07:27:54 +0100 Subject: [Haskell-cafe] Ticking time bomb References: <1359574031-sup-1195@javelin> <1359579588.5131.4.camel@kirk> <1359585638-sup-7671@javelin> <1359586077-sup-6738@javelin> Message-ID: <20130131072754.23345dad@tritium.streitmacht.eu> Ramana Kumar wrote: > But if you keep calling cabal a package manager, eventually you'll > have to write the patches to make it one. The combination of Cabal, cabal-install and Hackage is a package distribution system. As such, it needs the necessary cryptographic support. I don't view them as a package management system. What's important is that many programmers blindly trust the code they download, install and run from Hackage. Yes, it's a bad habit, but seriously, that's the alternative? Distributions are often some linear combination of outdated and broken with coefficients near 1. Let me restate the important fact with more emphasis: People are using Hackage! In any case there is no valid excuse for the lack of crypto. It's too easy to attack Hackage, so we need some crypto regardless of what we interpret it as. My proposal is: 1. Build the necessary machinery into Cabal to allow signing keys and packages and verifying the signatures, ideally through GnuPG. Cabal would benefit from that even without cabal-install and Hackage. 2. Update Hackage to allow uploading signatures along with packages. 3. Update cabal-install to allow signing packages and optionally checking the signatures. Do not allow signature chains. Do not support CAs for now. Have a database of trusted keys. Could be a directory below ~/.cabal with keys as files. More detailed (skip to next step, if you're not going to implement this): Before installing anything, build a map from maintainers from whom we would be installing packages to the packages they maintain. Maintainers are identified by their keys, not their usernames. If any of the keys is not trusted yet, then print this list as readable as possible. Use colors, etc. Hypothetical example: % cabal install diagrams Resolving dependencies... The following maintainers are untrusted: Diagrams Project <... at ...> [ABCD1234]: FP: 0000 1111 2222 3333 4444 5555 6666 7777 8888 9999 monoid-extras-0.2.2.2 dual-tree-0.1.0.1 diagrams-core-0.6.0.1 diagrams-lib-0.6.0.1 diagrams-svg-0.6.0.1 diagrams-contrib-0.6.0.1 diagrams-0.6 Trust them (?/y/n/t)? ? y: yes n: no (default) t: temporarily Trust them (?/y/n/t)? y Adding to trust db: Diagrams Project <... at ...> [ABCD1234] [install] Cabal should ignore the "Maintainer" field in the Cabal file. Only the signature counts here. Cabal must report a changed maintainer: % cabal install diagrams Resolving dependencies... WARNING: Package 'diagrams-core' has changed maintainer. [old key info] [new key info] Install anyway (y/n)? y The following maintainers are untrusted: [...] 4. Announce the change and urge maintainers to update their packages to include signatures. 5. Wait a few weeks. 6. Make signature verification the default in cabal-install. 7. Optionally implement CA support and establish a CA outside and offsite of Hackage. Someone with a good understanding of server security and cryptography should do that. They could be added to ~/.cabal/config to make package installations easier. Steps 1..6 should be done with high priority, otherwise they will never be finished. Step 7 is optional. If you're indeed considering this, I'm happy to turn this into a set of bug tracker issues and possibly help with the development. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 836 bytes Desc: not available URL: From efiish at gmail.com Thu Jan 31 07:31:02 2013 From: efiish at gmail.com (Junior White) Date: Thu, 31 Jan 2013 14:31:02 +0800 Subject: [Haskell-cafe] linking errors while compile hugs98 in macos Message-ID: Hi Cafe, I downloaded the latest hugs98 source package, unzip and build, I get the following link errors. It seems many symbols are not defined, am I missing same depending libraries? This is my machine info: ? hugs98-plus-Sep2006 git:(master) ? uname -a Darwin lan-seimatoMacBook-Air.local 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64 ================================ Preprocessing Network/Hackage/Version Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccuTP291.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/C/Error.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/C/Error.so" "libraries/bootlib/Foreign/C/Error.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c' Undefined symbols for architecture x86_64: "_environ", referenced from: ___hscore_environ in ccYZo1zT.o (maybe you meant: ___hscore_environ) ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status runhugs: Error occurred ERROR "libraries/bootlib/Foreign/Marshal/Alloc.hs" - Error while running compilation command 'gcc -DNDEBUG=1 -g -no-cpp-precomp -flat_namespace -shared -fPIC -D__HUGS__ "-Ihugsdir/include" -o "libraries/bootlib/Foreign/Marshal/Alloc.so" "libraries/bootlib/Foreign/Marshal/Alloc.c" -Ipackages/base/include -Ighc/includes packages/base/cbits/PrelIOUtils.c packages/base/cbits/dirUtils.c packages/base/cbits/consUtils.c' -------------- next part -------------- An HTML attachment was scrubbed... URL: From es at ertes.de Thu Jan 31 07:32:27 2013 From: es at ertes.de (Ertugrul =?UTF-8?B?U8O2eWxlbWV6?=) Date: Thu, 31 Jan 2013 07:32:27 +0100 Subject: [Haskell-cafe] FFI - Approaches to C/C++ References: Message-ID: <20130131073227.47a241a7@tritium.streitmacht.eu> Casey Basichis wrote: > I'm working on a project in Haskell and C++ where the former is the > brains and the latter is for UI, interaction etc. That's a rather odd choice. Not exactly answering your question, but questioning your project decisions, why would you do UI and interaction in C++? You have the necessary Haskell bindings and libraries to write everything cleanly in Haskell. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -------------- next part -------------- A non-text attachment was scrubbed... Name: signature.asc Type: application/pgp-signature Size: 836 bytes Desc: not available URL: From caseybasichis at gmail.com Thu Jan 31 08:08:39 2013 From: caseybasichis at gmail.com (Casey Basichis) Date: Wed, 30 Jan 2013 23:08:39 -0800 Subject: [Haskell-cafe] FFI - Approaches to C/C++ Message-ID: Hi Ertugrul, I'm not entirely sure what you mean. I'm intending on using Ogre for GUI - for which there is the Hogre bindings, but after emailing the DEV about it, I didn't get the impression from his advice that I should be using it for production code. Here is what he suggested: "It depends, really. Hogre is good for running Ogre from within Haskell, but it has its limitations. The number one thing people have been struggling with is handling input with hogre - there's Hois (Haskell wrapper for OIS) but it's not perfect (it misses input events), and the other option is having to write some C++ glue. Hogre is a solid proof of concept and you can do some demos with it, but if you're e.g. writing a game it might be a bit of a struggle. In the end it's about how much you value being able to write code in Haskell (or how allergic to C++ you are)." I'm on iOS so I imagine those difficulties are compounded. I am using several other C++ libraries for which there are no existing bindings and no Haskell alternative packages that are even remotely close. Are you suggesting it would be better to write all my own FFI bindings for all the needed libraries? Everything I read suggests that Haskells strengths are in transformation and that interaction is not its strong suit. I am interested in your thoughts and I am open to whatever, but you are the first to suggest that the mix is a bad idea. Thanks, Casey >>* I'm working on a project in Haskell and C++ where the former is the*>>* brains and the latter is for UI, interaction etc.* >That's a rather odd choice. Not exactly answering your question, but >questioning your project decisions, why would you do UI and interaction >in C++? You have the necessary Haskell bindings and libraries to write >everything cleanly in Haskell. > > >Greets, >Ertugrul -------------- next part -------------- An HTML attachment was scrubbed... URL: From alexander.kjeldaas at gmail.com Thu Jan 31 08:49:32 2013 From: alexander.kjeldaas at gmail.com (Alexander Kjeldaas) Date: Thu, 31 Jan 2013 08:49:32 +0100 Subject: [Haskell-cafe] Ticking time bomb In-Reply-To: <1359574031-sup-1195@javelin> References: <1359574031-sup-1195@javelin> Message-ID:

On Thu, Jan 31= , 2013 at 9:53 AM, Casey Basichis <caseybasichis at gmail.com> wrote:

Hi Ertugrul,

Thank you for the det= ailed reply. =C2=A0From what you wrote, partial FFI still seems like the wa= y to go.

Unfortunately Ogre isn't the only large library I&#= 39;m using, so "difficult" several times over sounds like a good = way to handicap the project early on.

I'm perfectly happy to use Haskell for the strength= s that will most benefit my project. =C2=A0I can always go back and try to = bring the C++ specific parts into the fold once a prototype is up and runni= ng.

As it seems there is a great deal of c/c++ to do either= way, I would really appreciate so thoughts towards my original question.

What practices in C++ are preferred by Haskell user= s, in the know, for the parts of the app that will not be pure Haskell?

Should I be looking to avoid OOP? =C2=A0Dependency Inje= ction? I wont reiterate all the facets of the first post, but it would help= me immensely to zero in on a few patterns and strategies that can minimize= d the damage I inflict in c++ land.

Thanks,
Casey

p.s.

With

&qu= ot;That used to be true, but the reason has nothing to do with the language= . =C2=A0The problem was that the libraries weren't there."

What do you mean? Which packages should I be look= ing at? =C2=A0I am on iOS like I said, its a stage 1 GHC compiler so I don&= #39;t have access to GHCI or template haskell.
=

>Casey Basichis <caseybasichis at=C2=A0gmail.com> wrote:
>
&g= t;> I'm not entirely sure what you mean.
>>
>> I'm intending on using Ogre for GUI - for which there is the= Hogre

>> bindings, but after emailing the DEV about it, I didn't g= et the
>> impression from his advice that I should be using= it for production
>> code. =C2=A0Here is what he suggested= :

>>
>> "It depends, really. Hogre is good fo= r running Ogre from within
>> Haskell, but it has its limit= ations. The number one thing people have
>> been struggling= with is handling input with hogre - there's Hois

>> (Haskell wrapper for OIS) but it's not perfect (it misses= input
>> events), and the other option is having to write = some C++ glue. Hogre
>> is a solid proof of concept and you= can do some demos with it, but if

>> you're e.g. writing a game it might be a bit of a struggl= e. In the end
>> it's about how much you value being ab= le to write code in Haskell (or
>> how allergic to C++ you = are)."

>>
>> I'm on iOS so I imagine those difficul= ties are compounded.
>>
&= gt;> I am using several other C++ libraries for which there are no exist= ing

>> bindings and no Haskell alternative packages that are even re= motely

>> close.
>>
>> Are you suggesti= ng it would be better to write all my own FFI bindings
>> f= or all the needed libraries?
>

>That's not what I'm suggesting. =C2=A0It was just too litt= le information to

>properly judge the difficulty of doing everything in Haskell.
>
>Binding to Ogre (or C++ in general) is indeed diff= icult. =C2=A0If Hogre
>doesn't work or is too limited, you= r best option might be to write a C

>wrapper around the Hogre functionality you need. =C2=A0Another opt= ion is to
>use SDL/OpenGL directly, which may be easier or har= der depending on your
>application.
>
&= gt;However, if you can build the bridge between your rendering library and<= /div>
>Haskell, then Haskell is certainly the better choice.
&g= t;
>
>> Everything I r= ead suggests that Haskells strengths are in
>> transformati= on and that interaction is not its strong suit.

>>
>> I am interested in= your thoughts and I am open to whatever, but you
>> are th= e first to suggest that the mix is a bad idea.
>
>That used to be true, but the reason has nothing to do with the

>language. =C2=A0The problem was that the libraries weren't the= re. =C2=A0Nowadays
>you can write all sorts of interactive app= lications in Haskell,
>including GUIs, TUIs, games, simulation= s and web applications. =C2=A0However,

>I've long been waiting for useful bindings to Ogre or Irrlicht= , but I'm
>afraid that it's not going to happen any ti= me soon.
>
>Ultimately it's your choice. =C2= =A0Let me summarize the possiblities:

>
> =C2=A0* C wrapper around Ogre. =C2=A0Easy integrat= ion, but need to write the
> =C2=A0 =C2=A0rendering code in C/= C++.
>
> =C2=A0* Full FFI bindings to Ogre. =C2= =A0Difficult integration, but you can write

> =C2=A0 =C2=A0your rendering code in Haskell.
>
=
> =C2=A0* Partial FFI bindings to Ogre. =C2=A0Integration may be so= mewhat easy, if
> =C2=A0 =C2=A0you do the initialization in Ha= skell and the actual rendering in

> =C2=A0 =C2=A0C/C++. =C2=A0However, this again requires to write t= he rendering in
> =C2=A0 =C2=A0C/C++.
>
> =C2=A0* Using SDL/OpenGL directly: =C2=A0Everything available for Has= kell. =C2=A0May
> =C2=A0 =C2=A0be difficult, because you need = to write OpenGL code.

>
>I hope, this helps.
>
>
>Greets,
>Ertugrul

--=C2=A0

Casey James Basi= chis
Composer - Cartoon Network
http://www= .caseyjamesbasichis.com
caseybasichis at gmail.com
310.387.7540

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