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<p class="MsoNormal"><span style="color:#1F497D">We’ve made a Wiki page with some relevant information,
<a href="https://ghc.haskell.org/trac/ghc/wiki/BurningBridgesSlowly">https://ghc.haskell.org/trac/ghc/wiki/BurningBridgesSlowly</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Libraries [mailto:libraries-bounces@haskell.org]
<b>On Behalf Of </b>Augustsson, Lennart<br>
<b>Sent:</b> 27 January 2015 10:33<br>
<b>To:</b> libraries@haskell.org<br>
<b>Subject:</b> Drastic Prelude changes imminent<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The next version (7.10) of GHC is slated to have a drastically changed Prelude.<o:p></o:p></p>
<p class="MsoNormal">This message is very late in the release process, but I would urge caution before changing.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The changes are (aptly) named the Burning Bridges Proposal (BBP).<o:p></o:p></p>
<p class="MsoNormal">Even though the work has been going on for a while, it seems that this<o:p></o:p></p>
<p class="MsoNormal">change is coming as a surprise to many people (including Simon Peyton<o:p></o:p></p>
<p class="MsoNormal">Jones). In summary, it generalizes many list operation, e.g., foldr,<o:p></o:p></p>
<p class="MsoNormal">to be overloaded.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">There is much to welcome in BBP, but changing the Prelude cannot be<o:p></o:p></p>
<p class="MsoNormal">done lightly since it really is like changing the language.<o:p></o:p></p>
<p class="MsoNormal">So I think it's really important for a large number of people to be able to<o:p></o:p></p>
<p class="MsoNormal">try out such changes before they come into effect, and to have time<o:p></o:p></p>
<p class="MsoNormal">to let the changes stabilize (you rarely get it right the first time).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I've discussed this with a number of people, including Simon PJ, and<o:p></o:p></p>
<p class="MsoNormal">we have concrete proposals.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Proposal 1:<o:p></o:p></p>
<p class="MsoNormal">* Add a new pragma<o:p></o:p></p>
<p class="MsoNormal"> {-# LANGUAGE Prelude=AlternativePrelude #-}<o:p></o:p></p>
<p class="MsoNormal"> * This is a new feature, but it is easy and low-risk to implement.<o:p></o:p></p>
<p class="MsoNormal"> * Which Prelude you use really is a language choice; appropriate for a LANGUAGE pragma.<o:p></o:p></p>
<p class="MsoNormal"> * Semantics is name-space only: import Prelude (); import AlternativePrelude<o:p></o:p></p>
<p class="MsoNormal"> * No effect on desugaring or typing of built-in syntax (list comprehensions, do-notation etc).<o:p></o:p></p>
<p class="MsoNormal">* Ship with both old and new prelude.<o:p></o:p></p>
<p class="MsoNormal">* So now old and new behaviour are easy to achieve, in the module or in a .cabal file.<o:p></o:p></p>
<p class="MsoNormal">* The question becomes "what is the default".<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Proposal 2:<o:p></o:p></p>
<p class="MsoNormal">* Make the default be the old rather than the new.<o:p></o:p></p>
<p class="MsoNormal"> * Altering the default Prelude API should be done slowly, with lots of warning; because all users get it willy-nilly.<o:p></o:p></p>
<p class="MsoNormal"> * Unlike AMP, the change is controversial (clearly).<o:p></o:p></p>
<p class="MsoNormal"> * Easier to make changes to New Prelude if it isn't the default.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">That's it.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Discussing the BBP proposal we also came up with a number of technical questions:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Q1<o:p></o:p></p>
<p class="MsoNormal">An alternative to Foldable would be <o:p></o:p></p>
<p class="MsoNormal"> class Enumerable t where<o:p></o:p></p>
<p class="MsoNormal"> toList :: t a -> [a] -- Implementations should use 'build'<o:p></o:p></p>
<p class="MsoNormal">Is Foldable more general (or efficient) than a Enumerable class, plus fusion?
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Consider a new data type X a. I write<o:p></o:p></p>
<p class="MsoNormal"> foldX :: (a -> b -> b) -> b -> X a -> b<o:p></o:p></p>
<p class="MsoNormal"> foldX = ...lots of code...<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> toList :: X a -> [a] {-# INLINE toList #-}<o:p></o:p></p>
<p class="MsoNormal"> toList x = build (\c n. foldX c n x)<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">So now toList is small and easy to inline. Every good list consumer of a call to toList will turn into a call to foldX, which is what we want.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Q2<o:p></o:p></p>
<p class="MsoNormal">What are the criteria for being in Foldable?<o:p></o:p></p>
<p class="MsoNormal">For instance, why are 'sum', 'product' in Foldable, but not 'and', 'or'?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Q3<o:p></o:p></p>
<p class="MsoNormal">What's the relationship of Foldable to GHC.Exts.IsList?<o:p></o:p></p>
<p class="MsoNormal">Which also has toList, fromList, and does work with ByteString.<o:p></o:p></p>
<p class="MsoNormal">* For example, could we use IsList instead of Foldable?<o:p></o:p></p>
<p class="MsoNormal"> Specifically, Foldable does not use its potential power to apply the type constructor t to different arguments. (Unlike Traversable which does.)<o:p></o:p></p>
<p class="MsoNormal"> foldr :: IsList l => (Item l->b->b) -> b -> l -> b<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> -- Lennart<o:p></o:p></p>
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