Haskell Communities and Activities Report

UHC is the Utrecht Haskell Compiler, supporting almost all Haskell98 features and most of Haskell2010, plus experimental extensions.

StatusCurrent work is on a strictness analyser (Augusto Pasalaqua) and incrementality of analysis via the Attribute Grammar system used to construct UHC (Jeroen Bransen).

Background.UHC actually is a series of compilers of which the last is UHC, plus infrastructure for facilitating experimentation and extension. The distinguishing features for dealing with the complexity of the compiler and for experimentation are (1) its stepwise organisation as a series of increasingly more complex standalone compilers, the use of DSL and tools for its (2) aspectwise organisation (called Shuffle) and (3) tree-oriented programming (Attribute Grammars, by way of the Utrecht University Attribute Grammar (UUAG) system (→5.3.2).

Further reading

• UHC Homepage: http://www.cs.uu.nl/wiki/UHC/WebHome

• UHC Github repository: https://github.com/UU-ComputerScience/uhc

• UHC Javascript backend: http://uu-computerscience.github.com/uhc-js/

• Attribute grammar system: http://www.cs.uu.nl/wiki/HUT/AttributeGrammarSystem

The FreeBSD Haskell Team is a small group of contributors who maintain Haskell software on all actively supported versions of FreeBSD. The primarily supported implementation is the Glasgow Haskell Compiler together with Haskell Cabal, although one may also find Hugs and NHC98 in the ports tree. FreeBSD is a Tier-1 platform for GHC (on both i386 and amd64) starting from GHC 6.12.1, hence one can always download vanilla binary distributions for each recent release.

We have a developer repository for Haskell ports that features around 486 ports of many popular Cabal packages. The updates committed to this repository are continuously integrated to the official ports tree on a regular basis. However, the FreeBSD Ports Collection already includes many popular and important Haskell software: GHC 7.6.3, Haskell Platform 2013.2.0.0, Gtk2Hs, wxHaskell, XMonad, Pandoc, Gitit, Yesod, Happstack, Snap, Agda, git-annex, and so on – all of them have been incorporated into the recent 9.2-RELEASE.

In cooperation with fellow developers, Konstantin Belousov and Dimitry Andric, we have managed to restore the ability to build GHC on 32-bit 10.x FreeBSD systems, so now it is ready to be included in the upcoming 10.0-RELEASE. In addition, it turned out that this bug (in thread signal delivery) can also affect the building process for other platforms as well, which explains some of the strange build breakages our users might have experienced in the past.

If you find yourself interested in helping us or simply want to use the latest versions of Haskell programs on FreeBSD, check out our page at the FreeBSD wiki (see below) where you can find all important pointers and information required for use, contact, or contribution.

Further reading

http://wiki.FreeBSD.org/Haskell

The Debian Haskell Group aims to provide an optimal Haskell experience to users of the Debian GNU/Linux distribution and derived distributions such as Ubuntu. We try to follow the Haskell Platform versions for the core package and package a wide range of other useful libraries and programs. At the time of writing, we maintain 628 source packages.

A system of virtual package names and dependencies, based on the ABI hashes, guarantees that a system upgrade will leave all installed libraries usable. Most libraries are also optionally available with profiling enabled and the documentation packages register with the system-wide index.

The recently released stable Debian release (“wheezy”) provides the Haskell Platform 2012.3.0.0 and GHC 7.4.1, while in Debian unstable, we provide the platform 2013.2.0.0 and GHC 7.6.3.

Debian users benefit from the Haskell ecosystem on 13 architecture/kernel combinations, including the non-Linux-ports KFreeBSD and Hurd.

Further reading

http://wiki.debian.org/Haskell

The Fedora Haskell SIG works to provide good Haskell support in the Fedora Project Linux distribution.

Fedora 19 shipped in July with ghc-7.4.2, haskell-platform-2012.4.0.0, and version updates to many other packages. New packages added since the previous release included cabal-rpm, and a bunch of libraries. Recently the Fedora Haskell Packaging Guidelines have been updated and revised which will make it easier for general package reviewers and give better control to packagers.

Fedora 20 development is now at the Beta milestone with ghc-7.6.3, haskell-platform-2013.2.0.0, and numerous version updates: it is due to ship in December. New packages added during the Fedora 20 cycle include idris and various libraries. In Fedora 20 armv7 is now a Fedora primary architecture, and the inclusion of llvm-3.3 has caused problems since it is not properly supported by ghc-7.6.3. Work on packaging Yesod has slowly proceeded.

At the time of writing we have around 250 Haskell source packages in Fedora. The cabal-rpm packaging tool has improved: it can now resolve C library and pkgconfig build dependencies, word-wrap descriptions, and yum install build dependencies.

Work on Fedora 21 is also under way. I expect we will ship ghc-7.8 rather than waiting for Haskell Platform this time: it will be great to have shared library support beyond intel archs, and the many other enhancements. Static libraries will probably be moved into static subpackages

If you want to help with Fedora Haskell packaging, please join our low-traffic mailing-list and the Freenode #fedora-haskell channel. You can also follow @fedorahaskell for irregular updates.

Further reading

• Homepage: http://fedoraproject.org/wiki/SIGs/Haskell
• Mailing-list: https://admin.fedoraproject.org/mailman/listinfo/haskell
• Package list: https://admin.fedoraproject.org/pkgdb/users/packages/haskell-sig
• Package changes: http://git.fedorahosted.org/cgit/haskell-sig.git/tree/packages/diffs/f19-f20.compare

Agda is a dependently typed functional programming language (developed using Haskell). A central feature of Agda is inductive families, i.e., GADTs which can be indexed by values and not just types. The language also supports coinductive types, parameterized modules, and mixfix operators, and comes with an interactive interface—the type checker can assist you in the development of your code.

A lot of work remains in order for Agda to become a full-fledged programming language (good libraries, mature compilers, documentation, etc.), but already in its current state it can provide lots of fun as a platform for experiments in dependently typed programming.

Since the release of Agda 2.3.2 in November 2012 the following has happened in the Agda project and community:

• Ulf Norell gave a keynote speech at ICFP 2013 on dependently typed programming in Agda.
• Agda has attracted new users, the traffic on the mailing list (and bug tracker) is increasing.
• About 100 bugs of Agda 2.3.2 have been fixed; and small enhancements improve the usability.
• Copatterns are being added to Agda as a new way to define record and coinductive values.

Further reading

The Agda Wiki: http://wiki.portal.chalmers.se/agda/

MiniAgda is a tiny dependently-typed programming language in the style of Agda (→4.1). It serves as a laboratory to test potential additions to the language and type system of Agda. MiniAgda’s termination checker is a fusion of sized types and size-change termination and supports coinduction. Bounded size quantification and destructor patterns for a more general handling of coinduction. Equality incorporates eta-expansion at record and singleton types. Function arguments can be declared as static; such arguments are discarded during equality checking and compilation.

Recently, I have added pattern inductive families that need not store their indices even at type-checking time. MiniAgda is available as Haskell source code and compiles with GHC 6.12.x – 7.6.3.

Further reading

http://www2.tcs.ifi.lmu.de/~abel/miniagda/

Disciple Core is an explicitly typed language based on System-F2, intended as an intermediate representation for a compiler. In addition to the polymorphism of System-F2 it supports region, effect and closure typing. Evaluation order is left-to-right call-by-value by default, but explicit lazy evaluation is also supported. The language includes a capability system to track whether objects are mutable or constant, and to ensure that computations that perform visible side effects are not suspended with lazy evaluation.

The Disciplined Disciple Compiler (DDC) is being rewritten to use the redesigned Disciple Core language. This new DDC is at a stage where it will parse and type-check core programs, and compile first-order functions over lists to executables via C or LLVM backends. There is also an interpreter that supports the full language.

What is new?

• Over the last month we’ve been working on a new core language fragment, Disciple Core Flow, to support work on array fusion for Data Parallel Haskell (DPH). We’re writing a GHC plugin that translates GHC core programs to Disciple Core Flow, performs array fusion, and translates back. We’re using Disciple Core Flow instead of GHC Core directly because it has a simple (and working) external core format, which we use to test the fusion transform.

Further reading

http://disciple.ouroborus.net

Eden extends Haskell with a small set of syntactic constructs for explicit process specification and creation. While providing enough control to implement parallel algorithms efficiently, it frees the programmer from the tedious task of managing low-level details by introducing automatic communication (via head-strict lazy lists), synchronization, and process handling.

Eden’s primitive constructs are process abstractions and process instantiations. The Eden logo consists of four λ turned in such a way that they form the Eden instantiation operator (#). Higher-level coordination is achieved by defining skeletons, ranging from a simple parallel map to sophisticated master-worker schemes. They have been used to parallelize a set of non-trivial programs.

Eden’s interface supports a simple definition of arbitrary communication topologies using Remote Data. A PA-monad enables the eager execution of user defined sequences of Parallel Actions in Eden.

Survey and standard reference

Rita Loogen, Yolanda Ortega-Mallén, and Ricardo Peña: Parallel Functional Programming in Eden, Journal of Functional Programming 15(3), 2005, pages 431–475.

Tutorial

Rita Loogen: Eden - Parallel Functional Programming in Haskell, in: V. Zsok, Z. Horvath, and R. Plasmeijer (Eds.): CEFP 2011, Springer LNCS 7241, 2012, pp. 142-206.
(see also: http://www.mathematik.uni-marburg.de/~eden/?content=cefp)

Implementation

Eden is implemented by modifications to the Glasgow-Haskell Compiler (extending its runtime system to use multiple communicating instances). Apart from MPI or PVM in cluster environments, Eden supports a shared memory mode on multicore platforms, which uses multiple independent heaps but does not depend on any middleware. Building on this runtime support, the Haskell package edenmodules defines the language, and edenskels provides a library of parallel skeletons.

A new version based on GHC-7.8.1 (including binary packages and prepared source bundles) will be released as soon as its baseline GHC is released (expected release date: November 25). The new version fixes a number of issues related to error shut-down and recovery, and features extended support for serialising Haskell data structures. The previous stable release with binary packages and bundles was based on GHC 7.4.2, and is still available on the Eden web pages.

The source code repository for Eden releases is http://james.mathematik.uni-marburg.de:8080/gitweb, the Eden libraries (Haskell-level) are also available via Hackage.

Tools and libraries

The Eden trace viewer tool EdenTV provides a visualisation of Eden program runs on various levels. Activity profiles are produced for processing elements (machines), Eden processes and threads. In addition message transfer can be shown between processes and machines. EdenTV is written in Haskell and is freely available on the Eden web pages and on hackage.

The Eden skeleton library is under constant development. Currently it contains various skeletons for parallel maps, workpools, divide-and-conquer, topologies and many more. Take a look on the Eden pages.

Recent and Forthcoming Publications

• Mischa Dieterle, Thomas Horstmeyer, Jost Berthold, Rita Loogen: Iterating Skeletons — Structured Parallelism by Composition, Selected Papers of the Symposium on the Implementation and Application of Functional Languages (IFL 2012), LNCS 8241, Springer 2013, pp. 18-36.

• M. KH. Aswad, P. W. Trinder, A. D. Al-Zain, G. J. Michaelson, J. Berthold: Comparing Low-Pain and No-Pain Multicore Haskells, revised and extended version of TFP 2009 paper, in Special Issue of Higher-Order Symbol Computation (HOSC), to appear.

• Thomas Horstmeyer and Rita Loogen: Graph-Based Communication in Eden, revised and extended version of TFP 2009 paper, in Special Issue of Higher-Order Symbol Computation (HOSC), to appear.

• Oleg Lobachev, Michael Guthe, Rita Loogen: Estimating parallel performance, Journal of Parallel and Distributed Computing, Vol 73, No. 6, June 2013, pp. 876 - 887.

• Jost Berthold: Run-time supported Haskell Serialisation - an API. Talk at the Haskell Implementors’ Workshop, Boston, September 2013.

Further reading

http://www.mathematik.uni-marburg.de/~eden

Microsoft Research funded a 2-year project, which is now coming to an end, to promote the real-world use of parallel Haskell. The project involved industrial partners working on their own tasks using parallel Haskell, and consulting and engineering support from Well-Typed (→8.1). The overall goal has been to demonstrate successful serious use of parallel Haskell, and along the way to apply engineering effort to any problems with the tools that the organisations might run into. In addition we have put significant engineering work into a new implementation of Cloud Haskell.

The participating organisations are working on a diverse set of complex real world problems:

• Dragonfly (New Zealand): Hierarchical Bayesian Modeling
• Los Alamos National Laboratory (USA): high performance Monte Carlo algorithms to model the flow of radiation and other physical phenomena
• IIJ Innovation Institute Inc. (Japan): network servers handling a massive number of concurrent connections
• Telefonica I+D: processing large graphs representing social networks

As the project winds down, we will be publishing more details about the outcomes of these projects.

On the engineering side, the two main areas of focus in the project recently have been ThreadScope and Cloud Haskell.

ThreadScope.The latest release of ThreadScope (version 0.2.2) provides detailed statistics about heap and GC behaviour. It is much like the output that can be obtained by running your program with +RTS -s but presented in a more friendly way and with the ability to see the same statistics for any period within the program, not just the entire program run. This work could be extended to show graphs of the heap size over time. Compared to GHC’s traditional heap profiling this does not require recompiling in profiling mode and is very low overhead, but what is lost is the detailed breakdown of the heap by type, cost centre or retainer.

In addition there is a new feature to emit phase markers from user code and have these visualised in the ThreadScope timeline window.

These new features rely on the development version of GHC, and so will become generally available with GHC-7.8.

Finally, there is an alpha release of an ambitious new feature to integrate data from Linux’s “perf” system into ThreadScope. The Linux “perf” system lets us see events in the OS such as system calls and other internal kernel trace points, and also to collect detailed CPU performance counters. Our work has focused on capturing and transforming this data source, and integrating it with the existing RTS event tracing system which we believe will enable many useful new visualisations. Our initial new visualisation in ThreadScope lets us see when system calls are occurring. We hope that this and other future work in this area will help developers who are trying to optimise the performance of applications like network servers.

Cloud Haskell.For about the last year we have been working on a new implementation of Cloud Haskell. This is the same idea for concurrent distributed programming in Haskell that Simon Peyton Jones has been telling everyone about, but it’s a new implementation designed to be robust and flexible.

The summary about the new implementation is that it exists, it works, it’s on hackage, and we think it is now ready for serious experiments.

Compared to the previous prototype:

• it is much faster;
• it can run on multiple kinds of network;
• has backends to support different environments (like cluster or cloud);
• has a new system for dealing with node disconnect and reconnect;
• has a more precisely defined semantics;
• supports composable, polymorphic serialisable closures;
• and internally the code is better structured and easier to work with.

By the time you read this, we will have also released a backend for the Windows Azure cloud platform. Backends for other environments should be relatively straightforward to develop.

Further details including papers, videos and blog posts are on the Cloud Haskell homepage.

Further reading

• Parallel GHC project homepage: http://www.haskell.org/haskellwiki/Parallel_GHC_Project
• Cloud Haskell homepage: http://www.haskell.org/haskellwiki/Cloud_Haskell
• ThreadScope homepage: http://www.haskell.org/haskellwiki/ThreadScope

The Web Application Interface (WAI) is an interface between Haskell web applications and Haskell web servers. By targeting the WAI, a web framework or web application gets access to multiple deployment platforms. Platforms in use include CGI, the Warp web server, and desktop webkit.

WAI has mostly been stable since the last HCAR, with the exception of a newly added field to represent the request body length. This avoids repeatedly doing a costly integer parse, and correctly handling the case of chunked bodies at the type level. WAI has also been updated to allow the newest version of the conduit (→7.1.1) package.

WAI is also a platform for re-using code between web applications and web frameworks through WAI middleware and WAI applications. WAI middleware can inspect and transform a request, for example by automatically gzipping a response or logging a request. The Yesod (→5.2.6) web framework provides the ability to embed arbitrary WAI applications as subsites, making them a part of a larger web application.

By targeting WAI, every web framework can share WAI code instead of wasting effort re-implementing the same functionality. There are also some new web frameworks that take a completely different approach to web development that use WAI, such as webwire (FRP) and dingo (GUI). The Scotty web framework also continues to be developed, and provides a lighter-weight alternative to Yesod. Other frameworks- whether existing or newcomers- are welcome to take advantage of the existing WAI architecture to focus on the more innovative features of web development.

WAI applications can send a response themselves. For example, wai-app-static is used by Yesod to serve static files. However, one does not need to use a web framework, but can simply build a web application using the WAI interface alone. The Hoogle web service targets WAI directly.

The WAI standard has proven itself capable for different users and there are no outstanding plans for changes or improvements.

Further reading

http://www.yesodweb.com/book/wai

Warp is a high performance, easy to deploy HTTP server backend for WAI (→5.2.1). Since the last HCAR, Warp has switched from enumerators to conduits (→7.1.1), added SSL support, and websockets integration.

Due to the combined use of ByteStrings, blaze-builder, conduit, and GHC’s improved I/O manager, WAI+Warp has consistently proven to be Haskell’s most performant web deployment option.

Warp is actively used to serve up most of the users of WAI (and Yesod).

“Warp: A Haskell Web Server” by Michael Snoyman was published in the May/June 2011 issue of IEEE Internet Computing:

• Issue page: http://www.computer.org/portal/web/csdl/abs/mags/ic/2011/03/mic201103toc.htm
• PDF: http://steve.vinoski.net/pdf/IC-Warp_a_Haskell_Web_Server.pdf

Description

The Holumbus framework consists of a set of modules and tools for creating fast, flexible, and highly customizable search engines with Haskell. The framework consists of two main parts. The first part is the indexer for extracting the data of a given type of documents, e.g., documents of a web site, and store it in an appropriate index. The second part is the search engine for querying the index.

An instance of the Holumbus framework is the Haskell API search engine Hayoo! (http://holumbus.fh-wedel.de/hayoo/).

The framework supports distributed computations for building indexes and searching indexes. This is done with a MapReduce like framework. The MapReduce framework is independent of the index- and search-components, so it can be used to develop distributed systems with Haskell.

The framework is now separated into four packages, all available on Hackage.

• The Holumbus Search Engine
• The Holumbus Distribution Library
• The Holumbus Storage System
• The Holumbus MapReduce Framework

The search engine package includes the indexer and search modules, the MapReduce package bundles the distributed MapReduce system. This is based on two other packages, which may be useful for their on: The Distributed Library with a message passing communication layer and a distributed storage system.

Features

• Highly configurable crawler module for flexible indexing of structured data
• Customizable index structure for an effective search
• find as you type search
• Suggestions
• Fuzzy queries
• Customizable result ranking
• Index structure designed for distributed search
• Git repository containing the current development version of all packages under https://github.com/fortytools/holumbus
• Distributed building of search indexes

Current Work

Currently there are activities to optimize the index structures of the framework. In the past there have been problems with the space requirements during indexing. The data structures and evaluation strategies have been optimized to prevent space leaks. A second index structure working with cryptographic keys for document identifiers is under construction. This will further simplify partial indexing and merging of indexes.

There is a small project extracting the sources of the data structure used for the index to build a separate package. The search tree used in Holumbus is a space optimised version of a radix tree, which enables fast prefix and fuzzy search.

The second project, a specialized search engine for the FH-Wedel web site, has been finished http://w3w.fh-wedel.de/. The new aspect in this application is a specialized free text search for appointments, deadlines, announcements, meetings and other dates.

The Hayoo! and the FH-Wedel search engine have been adopted to run on top of the Snap framework (→5.2.7).

Further reading

The Holumbus web page (http://holumbus.fh-wedel.de/) includes downloads, Git web interface, current status, requirements, and documentation. Timo Kranz’s master thesis describing the Holumbus index structure and the search engine is available at http://holumbus.fh-wedel.de/branches/develop/doc/thesis-searching.pdf. Sebastian Gauck’s thesis dealing with the crawler component is available at http://holumbus.fh-wedel.de/src/doc/thesis-indexing.pdf The thesis of Stefan Schmidt describing the Holumbus MapReduce is available via http://holumbus.fh-wedel.de/src/doc/thesis-mapreduce.pdf.

Happstack is a fast, modern framework for creating web applications. Happstack is well suited for MVC and RESTful development practices. We aim to leverage the unique characteristics of Haskell to create a highly-scalable, robust, and expressive web framework.

Happstack pioneered type-safe Haskell web programming, with the creation of technologies including web-routes (type-safe URLS) and acid-state (native Haskell database system). We also extended the concepts behind formlets, a type-safe form generation and processing library, to allow the separation of the presentation and validation layers.

Some of Happstack’s unique advantages include:

• a large collection of flexible, modular, and well documented libraries which allow the developer to choose the solution that best fits their needs for databases, templating, routing, etc.
• the most flexible and powerful system for defining type-safe URLs.
• a type-safe form generation and validation library which allows the separation of validation and presentation without sacrificing type-safety
• a powerful, compile-time HTML templating system, which allows the use of XML syntax

A recent addition to the Happstack family is the happstack-foundation library. It combines what we believe to be the best choices into a nicely integrated solution. happstack-foundation uses:

• happstack-server for low-level HTTP functionality
• acid-state for type-safe database functionality
• web-routes for type-safe URL routing
• reform for type-safe form generation and processing
• HSP for compile-time, XML-based HTML templates
• JMacro for compile-time Javascript generation and syntax checking

Future plans

Happstack is the oldest, actively developed Haskell web framework. We are continually studying and applying new ideas to keep Happstack fresh. By the time the next release is complete, we expect very little of the original code will remain. If you have not looked at Happstack in a while, we encourage you to come take a fresh look at what we have done.

Some of the projects we are currently working on include:

• a fast pipes-based HTTP and websockets backend with a high level of evidence for correctness
• a dynamic plugin loading system
• a more expressive system for weakly typed URL routing combinators
• a new system for processing form data which allows fine grained enforcement of RAM and disk quotas and avoids the use of temporary files
• a major refactoring of HSP (fewer packages, migration to Text/Builder, a QuasiQuoter, and more).

One focus of Happstack development is to create independent libraries that can be easily reused. For example, the core web-routes and reform libraries are in no way Happstack specific and can be used with other Haskell web frameworks. Additionally, libraries that used to be bundled with Happstack, such as IxSet, SafeCopy, and acid-state, are now independent libraries. The new backend will also be available as an independent library.

When possible, we prefer to contribute to existing libraries rather than reinvent the wheel. For example, our preferred templating library, HSP, was created by and is still maintained by Niklas Broberg. However, a significant portion of HSP development in the recent years has been fueled by the Happstack team.

We are also working directly with the Fay team to bring an improved type-safety to client-side web programming. In addition to the new happstack-fay integration library, we are also contributing directly to Fay itself.

For more information check out the happstack.com website — especially the “Happstack Philosophy” and “Happstack 8 Roadmap”.

Further reading

• http://www.happstack.com/
• http://www.happstack.com/docs/crashcourse/index.html

Mighttpd (called mighty) version 2 is a simple but practical Web server in Haskell. It is now working on Mew.org serving static files, CGI (mailman and contents search) and reverse proxy for back-end Yesod applications.

Mighttpd is based on Warp providing performance on par with nginx. You can use the mightyctl command to reload configuration files dynamically and shutdown Mighttpd gracefully.

You can install Mighttpd 2 (mighttpd2) from HackageDB.

Michael Snoyman and I are now working on WAI 2. So far, we improved the performance of Warp much. Mighty version 3 will be based on this new Warp and also provides much faster logger than the fast-logger package.

Further reading

• http://www.mew.org/~kazu/proj/mighttpd/en/
• http://www.iij.ad.jp/en/company/development/tech/mighttpd/
• http://aosabook.org/en/posa/warp.html

Yesod is a traditional MVC RESTful framework. By applying Haskell’s strengths to this paradigm, Yesod helps users create highly scalable web applications.

Performance scalablity comes from the amazing GHC compiler and runtime. GHC provides fast code and built-in evented asynchronous IO.

But Yesod is even more focused on scalable development. The key to achieving this is applying Haskell’s type-safety to an otherwise traditional MVC REST web framework.

Of course type-safety guarantees against typos or the wrong type in a function. But Yesod cranks this up a notch to guarantee common web application errors won’t occur.

• declarative routing with type-safe urls — say goodbye to broken links
• no XSS attacks — form submissions are automatically sanitized
• database safety through the Persistent library (→7.6.2) — no SQL injection and queries are always valid
• valid template variables with proper template insertion — variables are known at compile time and treated differently according to their type using the shakesperean templating system.

When type safety conflicts with programmer productivity, Yesod is not afraid to use Haskell’s most advanced features of Template Haskell and quasi-quoting to provide easier development for its users. In particular, these are used for declarative routing, declarative schemas, and compile-time templates.

MVC stands for model-view-controller. The preferred library for models is Persistent (→7.6.2). Views can be handled by the Shakespeare family of compile-time template languages. This includes Hamlet, which takes the tedium out of HTML. Both of these libraries are optional, and you can use any Haskell alternative. Controllers are invoked through declarative routing and can return different representations of a resource (html, json, etc).

Yesod is broken up into many smaller projects and leverages Wai (→5.2.1) to communicate with the server. This means that many of the powerful features of Yesod can be used in different web development stacks that use WAI such as Scotty.

The new 1.2 release of Yesod, introduces a number of simplifications, especially to the subsite handling. Most applications should be able to upgrade easily. Some of the notable features are:

• Much more powerful multi-representation support via the selectRep/provideRep API.
• More efficient session handling.
• All Handler functions live in a typeclass, providing you with auto-lifting.
• Type-based caching of responses via the cached function.
• More sensible subsite handling, switch to HandlerT/WidgetT transformers.
• Simplified dispatch system, including a lighter-weight Yesod.
• Simplified streaming data mechanism, for both database and non-database responses.
• Completely overhauled yesod-test, making it easier to use and providing cleaner integration with hspec.
• yesod-auth’s email plugin now supports logging in via username in addition to email address.
• Refactored persistent module structure for clarity and ease-of-use.
• Easy asset combining for static javascript and css files
• Faster shakespeare template reloading and support for TypeScript templates.

Since the 1.0 release, Yesod has maintained a high level of API stability, and we intend to continue this tradition. The 1.2 release introduces a lot of potential code breakage, but most of the required updates should be very straightforward. Future directions for Yesod are now largely driven by community input and patches. We’ve been making progress on the goal of easier client-side interaction, and have high-level interaction with languages like Fay, TypeScript, and CoffeScript.

The Yesod site (http://www.yesodweb.com/) is a great place for information. It has code examples, screencasts, the Yesod blog and — most importantly — a book on Yesod.

To see an example site with source code available, you can view Haskellers (→1.2) source code: (https://github.com/snoyberg/haskellers).

Further reading

http://www.yesodweb.com/

The Snap Framework is a web application framework built from the ground up for speed, reliability, and ease of use. The project’s goal is to be a cohesive high-level platform for web development that leverages the power and expressiveness of Haskell to make building websites quick and easy.

In the six months since the last HCAR Snap has had two major new releases. In 0.12 we added features enabling more efficient and granular reloading of snap applications, allowing a much faster development cycle. The 0.13 release centered around a new high-level API for Heist’s compiled splices. The new API greatly simplifies compiled Heist. In addition, we introduced a new syntax that makes defining splices more convenient.

Our next major milestone on the roadmap is to put the finishing touches on our complete rewrite of the Snap web server using io-streams. If you would like to contribute, stop by the #snapframework IRC channel on Freenode to keep up with the latest activity.

Further reading

• 0.12 release announcement: http://snapframework.com/blog/2013/05/15/snap-0.12-released
• 0.13 release announcement: http://snapframework.com/blog/2013/09/09/snap-0.13-released
• A blog post demonstrating the faster reloading capability: http://devblog.soostone.com/posts/2013-06-17-snap-template-reloading.html
• Snaplet Directory: http://snapframework.com/snaplets
• http://snapframework.com

Sunroof is a Domain Specific Language (DSL) for generating JavaScript. It is built on top of the JS-monad, which, like the Haskell IO-monad, allows read and write access to external resources, but specifically JavaScript resources. As such, Sunroof is primarily a feature-rich foreign function API to the browser’s JavaScript engine, and all the browser-specific functionality, like HTML-based rendering, event handling, and drawing to the HTML5 canvas.

Furthermore, Sunroof offers two threading models for building on top of JavaScript, atomic and blocking threads. This allows full access to JavaScript APIs, but using Haskell concurrency abstractions, like MVars and Channels. In combination with the push mechanism Kansas-Comet, Sunroof offers a great platform to build interactive web applications, giving the ability to interleave Haskell and JavaScript computations with each other as needed.

It has successfully been used to write smaller applications. These applications range from 2D rendering using the HTML5 canvas element, over small GUIs, up to executing the QuickCheck tests of Sunroof and displaying the results in a neat fashion. The development has been active over the past 6 months and there is a drafted paper submitted to TFP 2013.

Further reading

• Homepage: http://www.ittc.ku.edu/csdl/fpg/software/sunroof.html
• Tutorial: https://github.com/ku-fpg/sunroof-compiler/wiki/Tutorial
• Main Repository: https://github.com/ku-fpg/sunroof-compiler

MateVM is a method-based Java Just-In-Time Compiler. That is, it compiles a method to native code on demand (i.e. on the first invocation of a method). We use existing libraries:

hs-java

for proccessing Java Classfiles according to The Java Virtual Machine Specification.

harpy

enables runtime code generation for i686 machines in Haskell, in a domain specific language style.

In the current state we are able to execute simple Java programs. The compiler eliminates the JavaVM stack via abstract interpretation, does a liveness analysis, linear scan register allocation and finally code emission. The architecture enables easy addition of further optimization passes on an intermediate representation.

Future plans are, to add an interpreter to gather profile information for the compiler and also do more aggressive optimizations (e.g. method inlining or stack allocation) , using the interpreter as fallback path via deoptimization if a assumption is violated.

Apart from that, many features are missing for a full JavaVM, most noteable are the concept of Classloaders, Floating Point or Threads. We would like to use GNU Classpath as base library some day. Other hot topics are Hoopl and Garbage Collection.

If you are interested in this project, do not hesitate to join us on IRC (#MateVM @ OFTC) or contact us on Github.

Further reading

• https://github.com/MateVM
• http://docs.oracle.com/javase/specs/jvms/se7/html/
• http://hackage.haskell.org/package/hs-java
• http://hackage.haskell.org/package/harpy
• http://www.gnu.org/software/classpath/
• http://hackage.haskell.org/package/hoopl-3.8.7.4
• http://en.wikipedia.org/wiki/Club-Mate

UUAG is the Utrecht University Attribute Grammar system. It is a preprocessor for Haskell that makes it easy to write catamorphisms, i.e., functions that do to any data type what foldr does to lists. Tree walks are defined using the intuitive concepts of inherited and synthesized attributes, while keeping the full expressive power of Haskell. The generated tree walks are efficient in both space and time.

An AG program is a collection of rules, which are pure Haskell functions between attributes. Idiomatic tree computations are neatly expressed in terms of copy, default, and collection rules. Attributes themselves can masquerade as subtrees and be analyzed accordingly (higher-order attribute). The order in which to visit the tree is derived automatically from the attribute computations. The tree walk is a single traversal from the perspective of the programmer.

Nonterminals (data types), productions (data constructors), attributes, and rules for attributes can be specified separately, and are woven and ordered automatically. These aspect-oriented programming features make AGs convenient to use in large projects.

The system is in use by a variety of large and small projects, such as the Utrecht Haskell Compiler UHC (→3.3), the editor Proxima for structured documents (http://www.haskell.org/communities/05-2010/html/report.html#sect6.4.5), the Helium compiler (http://www.haskell.org/communities/05-2009/html/report.html#sect2.3), the Generic Haskell compiler, UUAG itself, and many master student projects. The current version is 0.9.50 (August 2013), is extensively tested, and is available on Hackage. There is also a Cabal plugin for easy use of AG files in Haskell projects.

Some of the recent changes to the UUAG system are:

OCaml and Clean support.

We have added OCaml and Clean code generation such that UUAG can also be used in OCaml projects and in Clean projects.

We are currently working on the following enhancements:

Evaluation scheduling and fixed point computation.

We are running a project to improve the scheduling algorithms in combination with fixed point computations. The currently implemented algorithms for scheduling AG computations do not fully satisfy our needs; the code we write goes beyond the class of OAGs, but the algorithm by Kennedy and Warren (1976) results in an undesired increase of generated code due to non-linear evaluation orders. However, because we know that our code belongs to the class of linear orderable AGs, we would like to find and algorithm that can find this linear order, and thus lies in between the two existing approaches.

Fixed point computations and ordered AG computations do not easily mix. In some cases however we would like to include fixed point computations inside an ordered AG, so the second aim of this project is to introduce way of letting the user specify such computations, and evaluate these in the right way.

Incremental evaluation.

We are currently also running a Ph.D. project that investigates incremental evaluation of AGs. In this ongoing work we hope to improve the UUAG compiler by adding support for incremental evaluation, for example by statically generating different evaluation orders based on changes in the input.

Further reading

• http://www.cs.uu.nl/wiki/bin/view/HUT/AttributeGrammarSystem
• http://hackage.haskell.org/package/uuagc

LQPL (Linear Quantum Programming Language) is a functional quantum programming language inspired by Peter Selinger’s paper “Towards a Quantum Programming Language”.

The LQPL system consists of a compiler, a GUI based front end and an emulator. LQPL incorporates a simple module / include system (more like C’s include than Haskell’s import), predefined unitary transforms, quantum control and classical control, algebraic data types, and operations on purely classical data.

Starting with the 0.9 series, LQPL is now split into separate components:

• The compiler (Haskell) — available at the command line and via a TCP/IP interface;
• The emulator (which emulates a virtual quantum machine) (Haskell) — available as a server via a TCP/IP interface;
• The front end (JRuby/Swing) — which connects to both the compiler and the emulator via TCP/IP.

Version 0.9.1 was a bugfix release.

A screenshot of the interface (showing a probabilistic list) is included below.

Quantum programming allows us to provide a fair coin toss:

The next major items on the road map are:

• Change the TCP/IP data format to something less verbose;
• Implementing a translation of the virtual machine code into quantum circuits.

Further reading

Documentation and executable downloads may be found at http://pll.cpsc.ucalgary.ca/lqpl/index.html. The source code, along with a wiki and bug tracker, is available at https://bitbucket.org/BrettGilesUofC/lqpl.

EclipseFP is a set of Eclipse plugins to allow working on Haskell code projects. Its goal is to offer a fully featured Haskell IDE in a platform developers coming from other languages may already be familiar with. It provides the following features, among others:

Cabal Integration

Provides a .cabal file editor, uses Cabal settings for compilation, allows the user to install Cabal packages from within the IDE. Can also use cabal-dev to provide sandboxing and project dependencies inside an Eclipse workspace.

GHC Integration

Compilation is done via the GHC API, syntax coloring uses the GHC Lexer.

Productive Coding

Quick fixes for common errors, warnings, and HLint suggestions. Automatic organization of imports. Autocompletion. Find and rename across modules and projects. Stylish-haskell integration for consistent code formatting.

Debugging

Easy to launch GHCi sessions on any module with proper parameters. Manages breakpoints, the evaluation of variables and expressions uses the Eclipse debugging framework, and requires no knowledge of GHCi syntax. Also integrates with Yesod (launch the web application from EclipseFP). Running a program with profiling options results in profiling graphs being displayed in the UI for easy analysis.

Browsing

The Haskell Browser perspective allows the user to navigate the list of packages and their documentation. It integrates seamlessly with Hackage. The Haskell module editor provides code folding, outline view of the module, popup of types and documentation mouse hovers, etc.

Testing

EclipseFP integrates with Haskell test frameworks, most notably HTF, to provide UI feedback on test failures.

The source code is fully open source (Eclipse License) on github and anyone can contribute. Current version is 2.5.5, released in October 2013, and more versions with additional features are planned and actively worked on. Most notably, version 2.6 should include support for Cabal 1.8 sandboxes. Feedback on what is needed is welcome! The website has information on downloading binary releases and getting a copy of the source code. Support and bug tracking is handled through Sourceforge forums and github issues.

Further reading

• http://eclipsefp.github.com/
• http://jpmoresmau.blogspot.com/

ghc-mod is a backend command to enrich Haskell programming on editors including Emacs, Vim and Sublime. The ghc-mod package on Hackage includes the ghc-mod command and Emacs front-end.

Emacs front-end provides the following features:

Completion

You can complete a name of keyword, module, class, function, types, language extensions, etc.

Code template

You can insert a code template according to the position of the cursor. For instance, “module Foo where” is inserted in the beginning of a buffer.

Syntax check

Code lines with error messages are automatically highlighted thanks to flymake. You can display the error message of the current line in another window. hlint can be used instead of GHC to check Haskell syntax.

Document browsing

You can browse the module document of the current line either locally or on Hackage.

Expression type

You can display the type/information of the expression on the cursor.

There are two Vim plugins:

• ghcmod-vim
• syntastic

Here are new features:

• ghc-mod now supports the sandbox of cabal 1.18 and stopped supporting the sandbox of cabal-dev.
• The ghc-mod package also provides a library now.

Further reading

http://www.mew.org/~kazu/proj/ghc-mod/en/

Heat is an interactive development environment (IDE) for learning and teaching Haskell. Heat was designed for novice students learning the functional programming language Haskell. Heat provides a small number of supporting features and is easy to use. Heat is distributed as a single, portable Java jar-file and works on top of GHCi.

Version 5.05, with small improvements and bug-fixes, was released end of April 2013.

Heat provides the following features:

• Editor for a single module with syntax-highlighting and matching brackets.
• Shows the status of compilation: non-compiled; compiled with or without error.
• Interpreter console that highlights the prompt and error messages.
• If compilation yields an error, then the relevant source line is highlighted and no further expression can be evaluated in the console until the source has been changed and successfully recompiled.
• A tree structure provides a program summary, giving definitions of types and types of functions.
• Automatic checking of either Boolean or QuickCheck properties of a program; results shown in summary.

Further reading

http://www.cs.kent.ac.uk/projects/heat/

Refactorings are source-to-source program transformations which change program structure and organization, but not program functionality. Documented in catalogs and supported by tools, refactoring provides the means to adapt and improve the design of existing code, and has thus enabled the trend towards modern agile software development processes.

Our project, Refactoring Functional Programs, has as its major goal to build a tool to support refactorings in Haskell. The HaRe tool is now in its seventh major release. HaRe supports full Haskell 2010, and is integrated with (X)Emacs. All the refactorings that HaRe supports, including renaming, scope change, generalization and a number of others, are module-aware, so that a change will be reflected in all the modules in a project, rather than just in the module where the change is initiated.

Snapshots of HaRe are available from our GitHub repository (see below) and Hackage. There are related presentations and publications from the group (including LDTA’05, TFP’05, SCAM’06, PEPM’08, PEPM’10, TFP’10, Huiqing’s PhD thesis and Chris’s PhD thesis). The final report for the project appears on the University of Kent Refactoring Functional Programs page (see below).

There is also a Google+ community called HaRe, and an IRC channel on freenode called #haskell-refactorer.

Recent developments

• HaRe 0.7, which is a major change from 0.6 as it makes use of the GHC library for analysis, has been released; HaRe 0.7 is available on Hackage, and also downloadable from our GitHub page
• HaRe 0.7 is alpha software, and comes with a limited number of refactorings, as the work so far has concentrated on getting the new architecture in place to make use of the GHC AST.
• There is plenty to do, so anyone who has an interest is welcome to fork the repo and get stuck in.
• Stephen Adams is starting his PhD at the University of Kent and will be working on data refactoring in Haskell.

Further reading

• http://www.cs.kent.ac.uk/projects/refactor-fp/
• https://github.com/alanz/HaRe

Darcs is a distributed revision control system written in Haskell. In Darcs, every copy of your source code is a full repository, which allows for full operation in a disconnected environment, and also allows anyone with read access to a Darcs repository to easily create their own branch and modify it with the full power of Darcs’ revision control. Darcs is based on an underlying theory of patches, which allows for safe reordering and merging of patches even in complex scenarios. For all its power, Darcs remains a very easy to use tool for every day use because it follows the principle of keeping simple things simple.

Our most recent release, Darcs 2.8.4 (with GHC 7.6 support), was in Februrary 2013. Some key changes in Darcs 2.8 include a faster and more readable darcs annotate, a darcs obliterate -O which can be used to conveniently “stash” patches, and hunk editing for the darcs revert command.

Our work on the next Darcs release continues. In our sights are the new ‘darcs rebase‘ command (for merging and amending patches that would be hard to do with patch theory alone), the patch index optimisation (for faster local lookups on repositories with long histories), and the packs optimisation (for faster darcs get).

This summer we have been lucky to get a chance to work on not just one, but two Google Summer of Code projects.(We participate in the Summer of Code under the Haskell.org umbrella, with an special agreement with Google to have an extra slot allocated for the purposes of Darcs work. This year, one of the Darcs related submissions was also selected in addition to the Darcs slot.) Our projects this year were on improving the local patch recording experience (Jose Neder), and the Darcs hub (BSRK Aditya).

For the work on patch recording, Jose Neder brought three major improvements to the Darcs user interface: the patience diff algorithm, making patches much more reflective of human intention; automatic detection of file-move operations, and automatic detection of token replaces, resulting in patches that take better advantage of Darcs’ high-level representation of changes.

As for the Darcs hub project (the Darcsden instance running on http://hub.darcs.net), BSRK Aditya has brought us a series of now live changes that make the hub easier to get up and running with. OAuth support means that users can now log in with their GitHub or Google accounts avoiding the need to create yet-another-password. A handful of nice new features make the site more useful than just a repository store: file edit for convenient quick edits, repository compare and patch bundle support for enhanced collaboration with forks, changes/annotate on files for better insight on the evolution of you code. Finally, the new build packs feature (see repository settings) enables an optimisation (GSoC 2010) which we hope will pave the way to much faster darcs get over networks.

These changes have been merged into the mainline Darcs and Darcsden repositories, and are expected to available in the next stable release of Darcs.

Darcs is free software licensed under the GNU GPL (version 2 or greater). Darcs is a proud member of the Software Freedom Conservancy, a US tax-exempt 501(c)(3) organization. We accept donations at http://darcs.net/donations.html.

Further reading

• http://darcs.net
• http://darcs.net/Development/Priorities
• Post GSoC 2013 hub.darcs.net update http://joyful.com/blog/2013-09-26-darcsden-darcs-hub-gsoc-complete.html

DarcsWatch is a tool to track the state of Darcs (→6.2.1) patches that have been submitted to some project, usually by using the darcs send command. It allows both submitters and project maintainers to get an overview of patches that have been submitted but not yet applied.

DarcsWatch continues to be used by the xmonad project, the Darcs project itself, and a few developers. At the time of writing (November 2013), it was tracking 39 repositories and 4602 patches submitted by 248 users.

Further reading

• http://darcswatch.nomeata.de/
• http://darcs.nomeata.de/darcswatch/documentation.html

cab is a MacPorts-like maintenance command of Haskell cabal packages. Some parts of this program are a wrapper to ghc-pkg and cabal.

If you are always confused due to inconsistency of ghc-pkg and cabal, or if you want a way to check all outdated packages, or if you want a way to remove outdated packages recursively, this command helps you.

cab now supports the sandbox of cabal 1.18 and stopped supporting cabal-dev. Some new command-line options are added, including -j for parallel jobs, -p for library profiling and -e for executable profiling.

Further reading

http://www.mew.org/~kazu/proj/cab/en/

Background

Cabal is the standard packaging system for Haskell software. It specifies a standard way in which Haskell libraries and applications can be packaged so that it is easy for consumers to use them, or re-package them, regardless of the Haskell implementation or installation platform.

Hackage is a distribution point for Cabal packages. It is an online archive of Cabal packages which can be used via the website and client-side software such as cabal-install. Hackage enables users to find, browse and download Cabal packages, plus view their API documentation.

cabal-install is the command line interface for the Cabal and Hackage system. It provides a command line program cabal which has sub-commands for installing and managing Haskell packages.

Recent progress

The Cabal packaging system has always faced growing pains. We have been through several cycles where we’ve faced chronic problems, made major improvements which bought us a year or two’s breathing space while package authors and users become ever more ambitious and start to bump up against the limits again. In the last few years we have gone from a situation where 10 dependencies might be considered a lot, to a situation now where the major web frameworks have a 100+ dependencies and we are again facing chronic problems.

The Cabal/Hackage maintainers and contributors have been pursuing a number of projects to address these problems:

The IHG sponsored Well-Typed (→8.1) to work on cabal-install resulting in a new package dependency constraint solver. This was incorporated into the cabal-install-0.14 release in the spring, and which is now in the latest Haskell Platform release. The new dependency solver does a much better job of finding install plans. In addition the cabal-install tool now warns when installing new packages would break existing packages, which is a useful partial solution to the problem of breaking packages.

We had two Google Summer of Code projects on Cabal this year, focusing on solutions to other aspects of our current problems. The first is a project by Mikhail Glushenkov (and supervised by Johan Tibell) to incorporate sandboxing into cabal-install. In this context sandboxing means that we can have independent sets of installed packages for different projects. This goes a long way towards alleviating the problem of different projects needing incompatible versions of common dependencies. There are several existing tools, most notably cabal-dev, that provide some sandboxing facility. Mikhail’s project was to take some of the experience from these existing tools (most of which are implemented as wrappers around the cabal-install program) and to implement the same general idea, but properly integrated into cabal-install itself. We expect the results of this project will be incorporated into a cabal-install release within the next few months.

The other Google Summer of Code project this year, by Philipp Schuster (and supervised by Andres Löh), is also aimed at the same problem: that of different packages needing inconsistent versions of the same common dependencies, or equivalently the current problem that installing new packages can break existing installed packages. The solution is to take ideas from the Nix package manager for a persistent non-destructive package store. In particular it lifts an obscure-sounding but critical limitation: that of being able to install multiple instances of the same version of a package, built against different versions of their dependencies. This is a big long-term project. We have been making steps towards it for several years now. Philipp’s project has made another big step, but there’s still more work before it is ready to incorporate into ghc, ghc-pkg and cabal.

Looking forward

Johan Tibell and Bryan O’Sullivan have volunteered as new release managers for Cabal. Bryan moved all the tickets from our previous trac instance into github, allowing us to move all the code to github. Johan managed the latest release and has been helping with managing the inflow of patches. Our hope is that these changes will increase the amount of contributions and give us more maintainer time for reviewing and integrating those contributions. Initial indications are positive. Now is a good time to get involved.

The IHG is currently sponsoring Well-Typed to work on getting the new Hackage server ready for switchover, and helping to make the switchover actually happen. We have recruited a few volunteer administrators for the new site. The remaining work is mundane but important tasks like making sure all the old data can be imported, and making sure the data backup system is comprehensive. Initially the new site will have just a few extra features compared to the old one. Once we get past the deployment hurdle we hope to start getting more contributions for new features. The code is structured so that features can be developed relatively independently, and we intend to follow Cabal and move the code to github.

We would like to encourage people considering contributing to take a look at the bug tracker on github, take part in discussions on tickets and pull requests, or submit their own. The bug tracker is reasonably well maintained and it should be relatively clear to new contributors what is in need of attention and which tasks are considered relatively easy. For more in-depth discussion there is also the cabal-devel mailing list.

Further reading

• Cabal homepage: http://www.haskell.org/cabal
• Hackage package collection: http://hackage.haskell.org/
• Bug tracker: https://github.com/haskell/cabal/

This tool by Ralf Hinze and Andres Löh is a preprocessor that transforms literate Haskell or Agda code into LaTeX documents. The output is highly customizable by means of formatting directives that are interpreted by lhs2TeX. Other directives allow the selective inclusion of program fragments, so that multiple versions of a program and/or document can be produced from a common source. The input is parsed using a liberal parser that can interpret many languages with a Haskell-like syntax.

The program is stable and can take on large documents.

The current version is 1.18 and has been released in September 2012. The main change is compatibility with GHC 7.6. Development repository and bug tracker are on GitHub. There are still plans for a rewrite of lhs2TeX with the goal of cleaning up the internals and making the functionality of lhs2TeX available as a library.

Further reading

• http://www.andres-loeh.de/lhs2tex
• https://github.com/kosmikus/lhs2tex

The library ghc-heap-view provides means to inspect the GHC’s heap and analyze the actual layout of Haskell objects in memory. This allows you to investigate memory consumption, sharing and lazy evaluation.

This means that the actual layout of Haskell objects in memory can be analyzed. You can investigate sharing as well as lazy evaluation using ghc-heap-view.

The package also provides the GHCi command :printHeap, which is similar to the debuggers’ :print command but is able to show more closures and their sharing behaviour:

The graphical tool ghc-vis (→6.4.3) builds on ghc-heap-view.

Further reading

• http://www.joachim-breitner.de/blog/archives/548-ghc-heap-view-Complete-referential-opacity.html
• http://www.joachim-breitner.de/blog/archives/580-GHCi-integration-for-GHC.HeapView.html
• http://www.joachim-breitner.de/blog/archives/590-Evaluation-State-Assertions-in-Haskell.html

The tool ghc-vis visualizes live Haskell data structures in GHCi. Since it does not force the evaluation of the values under inspection it is possible to see Haskell’s lazy evaluation and sharing in action while you interact with the data.

Ghc-vis supports two styles: A linear rendering similar to GHCi’s :print, and a graph-based view where closures in memory are nodes and pointers between them are edges. In the following GHCi session a partially evaluated list of fibonacci numbers is visualized:

At this point the visualization can be used interactively: To evaluate a thunk, simply click on it and immediately see the effects. You can even evaluate thunks which are normally not reachable by regular Haskell code.

Ghc-vis can also be used as a library and in combination with GHCi’s debugger.

Further reading

http://felsin9.de/nnis/ghc-vis

Hat is a source-level tracer for Haskell. Hat gives access to detailed, otherwise invisible information about a computation.

Hat helps locating errors in programs. Furthermore, it is useful for understanding how a (correct) program works, especially for teaching and program maintenance. Hat is not a time or space profiler. Hat can be used for programs that terminate normally, that terminate with an error message or that terminate when interrupted by the programmer.

Tracing a program with Hat consists of two phases: First the program needs to be run such that it additionally writes a trace to file. To add trace-writing, hat-trans translates all the source modules Module of a Haskell program into tracing versions Hat.Module. These are compiled as normal and when run the program does exactly the same as the original program except for additionally writing a trace to file. Second, after the program has terminated, you view the trace with a tool. Hat comes with several tools for selectively viewing fragments of the trace in different ways: hat-observe for Hood-like observations, hat-trail for exploring a computation backwards, hat-explore for freely stepping through a computation, hat-detect for algorithmic debugging, …

Hat is distributed as a package on Hackage that contains all Hat tools and tracing versions of standard libraries. Currently Hat supports Haskell 98 plus some language extensions such as multi-parameter type classes and functional dependencies. For portability all viewing tools have a textual interface; however, many tools use some Unix-specific features and thus run on Unix / Linux / OS X, but not on Windows.

Hat was mostly built around 2000–2004 and then disappeared because of lack of maintenance. Now it is back and new developments have started.

The source-to-source transformation of hat-trans has been completely rewritten to use the haskell-src-exts parser. Thus small bugs of the old parser disappeared and in the future it will be easier to cover more Haskell language extensions. This work was released on Hackage as Hat 2.8.

When a traced program uses any libraries besides the standard Haskell 98 / 2010 ones, these libraries currently have to be transformed (in trusted mode). So the plan for the next release of Hat is to enable Hat to use trusted libraries without having to transform them.

Feedback on Hat is welcome.

Further reading

• Initial website: http://projects.haskell.org/hat

• Hackage package: http://hackage.haskell.org/package/hat

While lazy I/O has served the Haskell community well for many purposes in the past, it is not a panacea. The inherent non-determinism with regard to resource management can cause problems in such situations as file serving from a high traffic web server, where the bottleneck is the number of file descriptors available to a process.

Left fold enumerators have been the most common approach to dealing with streaming data without using lazy I/O. While it is certainly a workable solution, it requires a certain inversion of control to be applied to code. Additionally, many people have found the concept daunting. Most importantly for our purposes, certain kinds of operations, such as interleaving data sources and sinks, are prohibitively difficult under that model.

The conduit package was designed as an alternate approach to the same problem. The root of our simplification is removing one of the constraints in the enumerator approach. In order to guarantee proper resource finalization, the data source must always maintain the flow of execution in a program. This can lead to confusing code in many cases. In conduit, we separate out guaranteed resource finalization as its own component, namely the ResourceT transformer.

Once this transformation is in place, data producers, consumers, and transformers (known as Sources, Sinks, and Conduits, respectively) can each maintain control of their own execution, and pass off control via coroutines. The user need not deal directly with any of this low-level plumbing; a simple monadic interface (inspired greatly by the pipes package) is sufficient for almost all use cases.

Since its initial release, conduit has been through many design iterations, all the while keeping to its initial core principles. Since the last HCAR, we’ve released version 1.0. This release introduces a simplification of the public facing API, optimizing for the common use cases. This was a minor change, and the conduit ecosystem has already caught up. The package has been in a mature state for quite some time now, and can be relied upon for most streaming data needs.

There is a rich ecosystem of libraries available to be used with conduit, including cryptography, network communications, serialization, XML processing, and more. The Web Application Interface was the original motivator for creating the library, and continues to use it for expressing request and response bodies between servers and applications. As such, conduit is also a major player in the Yesod ecosystem.

The library is available on Hackage. There is an interactive tutorial available on the FP Complete School of Haskell. You can find many conduit-based packages in the Conduit category on Hackage as well.

Further reading

• http://hackage.haskell.org/package/conduit
• https://www.fpcomplete.com/user/snoyberg/library-documentation/conduit-overview
• http://hackage.haskell.org/packages/archive/pkg-list.html#cat:conduit

Holmes is a tool for detecting plagiarism in Haskell programs. A prototype implementation was made by Brian Vermeer under supervision of Jurriaan Hage, in order to determine which heuristics work well. This implementation could deal only with Helium programs. We found that a token stream based comparison and Moss style fingerprinting work well enough, if you remove template code and dead code before the comparison. Since we compute the control flow graphs anyway, we decided to also keep some form of similarity checking of control-flow graphs (particularly, to be able to deal with certain refactorings).

In November 2010, Gerben Verburg started to reimplement Holmes keeping only the heuristics we figured were useful, basing that implementation on haskell-src-exts. A large scale empirical validation has been made, and the results are good. We have found quite a bit of plagiarism in a collection of about 2200 submissions, including a substantial number in which refactoring was used to mask the plagiarism. A paper has been written, which has been presented at CSERC’13, and should become available in the ACM Digital Library.

The tool will be made available through Hackage at some point, but before that happens it can already be obtained on request from Jurriaan Hage.

Contact

<J.Hage at uu.nl>

The Ideas project at Open Universiteit Nederland and Utrecht University aims at developing domain reasoners for stepwise exercises on various topics. These reasoners assist students in solving exercises incrementally by checking intermediate steps, providing feedback on how to continue, and detecting common mistakes. The reasoners are based on a strategy language, from which feedback is derived automatically. The calculation of feedback is offered as a set of web services, enabling external (mathematical) learning environments to use our work. We currently have a binding with the Digital Mathematics Environment of the Freudenthal Institute (first/left screenshot), the ActiveMath learning system of the DFKI and Saarland University (second/right screenshot), and our own online exercise assistant that supports rewriting logical expressions into disjunctive normal form.

We have continued working on the Ask-Elle functional programming tutor. This tool lets you practice introductory functional programming exercises in Haskell. The tutor can both guide a student towards developing a correct program, as well as analyze intermediate, incomplete programs and check whether or not certain properties are satisfied. We have recently extended the tutor with QuickCheck properties for testing the correctness of student programs, and for the generation of counterexamples. Normalization of functional programs is used for dealing with all kinds of variations in programs; this is ongoing research. We also want to make it as easy as possible for teachers to add programming exercises to the tutor, and to adapt the behavior of the tutor by disallowing or enforcing particular solutions. Teachers can adapt feedback by annotating the model solutions of an exercise. The tutor has an improved web-interface and is used in an introductory FP course at Utrecht University.

We have modeled the artificial intelligence of a real-time video game on top of the strategy combinator language used in the domain reasoners. In the future we expect to develop a serious game for communication skills using a similar approach.

The library for developing domain reasoners with feedback services is available as a Cabal source package. We have written a tutorial on how to make your own domain reasoner with this library. We have also released our domain reasoner for mathematics and logic as a separate package.

Further reading

• Online exercise assistant (for logic), accessible from our project page.

• Bastiaan Heeren, Johan Jeuring, and Alex Gerdes. Specifying Rewrite Strategies for Interactive Exercises. Mathematics in Computer Science, 3(3):349–370, 2010.
• Johan Jeuring, Alex Gerdes, and Bastiaan Heeren. A Programming Tutor for Haskell. Lecture Notes Central European School on Functional Programming, (CEFP 2011). Try our tutor at http://ideas.cs.uu.nl/FPTutor/.

• Tom Hastjarjanto, Johan Jeuring, and Sean Leather. A DSL for describing the artificial intelligence in real-time video games. Third International Workshop on Games and Software Engineering (GAS 2013).

FliPpr (“flip” + “ppr” (pretty-printer)) is a program transformation tool that takes a pretty-printing program and returns a parser consistent with the pretty-printer. It is common that, when we implement a programming language, we have to write a pair of programs: a pretty-printer and a parser, and it is expected that, especially early in the development, the syntax of the language changes frequently. In such a case, we have to update both the pretty-printer and the parser so that they continue to work with each other. FliPpr removes this maintenance burden from the programmers through program inversion techniques such that a consistent parser (in the sense that pretty-printed code is always correctly parsed) is automatically generated.

Pretty-printers in FliPpr are written with Wadler’s pretty-printing combinators as recursive functions on an AST datatype, which is very similar to what we normally do in Haskell. The differences are that there is a new combinator for embedding additional information for effective parsing, and there are syntactic restrictions in place. For details, please see our ESOP paper (doi: 10.1007/978-3-642-37036-6_6).

Currently, the implementation is an experimental prototype. So bugs and unhelpful error messages are expected. However, you shall be able to play with the system by using the examples from the implementation page below.

Further reading

http://www-kb.is.s.u-tokyo.ac.jp/~kztk/FliPpr/

Library for parsing and manipulating epub OPF package data. Now with epub3 support.

• Added support for epub3 documents. This was done using a single set of datatypes, not specific to either epub2 or epub3.
• Redesigned the book file querying API to be an edsl. Actions are to be combined together based on what the developer needs from the document.
• Data structures to contain epub metadata “sections” were redesigned to no longer be nested. Part of this change includes a typeclass-based pretty-print API for displaying this data.
• Documentation rewrites and additions, including a working code example in the API docs.

epub-metadata is available from Hackage and the Darcs repository below.

See also epub-tools (→7.9.2).

Further reading

• Project page: http://ui3.info/d/proj/epub-metadata.html

• Source repository: darcs get http://ui3.info/darcs/epub-metadata

With respect to the previous version the code for building interleaved parsers was split off into a separate package uu-interleaved, such that it can be used by other parsing libraries too. Based on this another small package uu-options was constructed which can be used to parse command line options and files with preferences. The internals of these are described in a technical report: http://www.cs.uu.nl/research/techreps/UU-CS-2013-005.html.

As an example of its use we show how to fill a record from the command line. We start out by defining the record which is to hold the options to be possibly set: data Prefers  =  Agda | Haskell deriving Show
data Address  =  Address  {  city_ :: String
                          ,  street_ :: String} 
                 deriving Show
data Name     =  Name  {  name_:: String 
                       ,  prefers_:: Prefers
                       ,  ints_ :: [Int]
                       ,  address_ :: Address} 
                 deriving Show
$(deriveLenses ''Name)
$(deriveLenses ''Address)

The Haskell Equational Reasoning Model-to-Implementation Tunnel (HERMIT) is an NSF-funded project being run at KU (→9.9), which aims to improve the applicability of Haskell-hosted Semi-Formal Models to High Assurance Development. Specifically, HERMIT uses a Haskell-hosted DSL and a new refinement user interface to perform rewrites directly on Haskell Core, the GHC internal representation.

This project is a substantial case study of the application of Worker/Wrapper on larger examples. In particular, we want to demonstrate the equivalences between efficient Haskell programs, and their clear specification-style Haskell counterparts. In doing so there are several open problems, including refinement scripting and managing scaling issues, data representation and presentation challenges, and understanding the theoretical boundaries of the worker/wrapper transformation.

We have reworked KURE (http://www.haskell.org/communities/11-2008/html/report.html#sect5.5.7), a Haskell-hosted DSL for strategic programming, as the basis of our rewrite capabilities, and constructed the rewrite kernel making use of the GHC Plugins architecture. A journal writeup of the KURE internals has been submitted to JFP, and is available on the group webpage. As for interfaces to the kernel, we currently have a command-line REPL, and an Android version is under development. Thus far, we have used HERMIT to successfully mechanize many smaller examples of program transformations, drawn from the literature on techniques such as concatenate vanishes, tupling transformation, and worker/wrapper. We are scaling up our capabilities, and working on larger examples.

HERMIT has also been used in two larger case studies. The first, led by Michael Adams from Portland State University in Oregon, uses HERMIT to mechanize the optimization of scrap your boilerplate generics, leading to execution speeds that were as fast as hand-optimized code (→7.4.1). The second uses HERMIT to implement a custom GHC optimization pass which enables fusion of nested streams within the Stream Fusion framework.

Further reading

http://www.ittc.ku.edu/csdl/fpg/Tools/HERMIT

Datatype-generic programming increases program reliability by reducing code duplication and enhancing reusability and modularity. However, it is known that datatype-generic programs often run slower than type-specific variants, and this factor can prevent adoption of generic programming altogether. There can be multiple reasons for the performance penalty, but often it is caused by conversions to and from representation types that do not get eliminated during compilation.

Fortunately, it is also known that generic functions can be specialised to concrete datatypes, removing any overhead from the use of generic programming. We have investigated compilation techniques to specialise generic functions and remove the performance overhead of generic programs in Haskell. We used a representative generic programming library and inspected the generated code for a number of example generic functions. After understanding the necessary compiler optimisations for producing efficient generic code, we benchmarked the runtime of our generic functions against handwritten variants, and concluded that all the overhead can indeed be removed automatically by the compiler. More details can be found in the IFL’12 paper linked below.

We have also investigated how to optimise the popular Scrap Your Boilerplate (SYB) generic programming library. Using a HERMIT (→7.3.5) script for implementing an optimisation pass in the compiler, we have removed all runtime overhead from SYB functions. More details can be found in the draft paper linked below.

Further reading

• Optimisation of Generic Programs through Inlining
• Optimizing SYB Is Easy!

Bed and breakfast is a pure Haskell linear algebra library that I built between getting out of the bed and having breakfast — thus the name. It features basic operations on matrices, for which it makes use of boxed and unboxed mutable arrays as necessary to improve performance — everything is pure though, as stateful computations happen in the ST monad. It currently excels in inverting matrices and finding determinants.

Bed and breakfast is published under the MIT license and available via hackage as bed-and-breakfast. You are more than welcome to suggest improvements. Development happens at http://hub.darcs.net/scravy/bed-and-breakfast.

HList is a comprehensive, general purpose Haskell library for typed heterogeneous collections including extensible polymorphic records and variants. HList is analogous to the standard list library, providing a host of various construction, look-up, filtering, and iteration primitives. In contrast to the regular lists, elements of heterogeneous lists do not have to have the same type. HList lets the user formulate statically checkable constraints: for example, no two elements of a collection may have the same type (so the elements can be unambiguously indexed by their type).

An immediate application of HLists is the implementation of open, extensible records with first-class, reusable, and compile-time only labels. The dual application is extensible polymorphic variants (open unions). HList contains several implementations of open records, including records as sequences of field values, where the type of each field is annotated with its phantom label. We and others have also used HList for type-safe database access in Haskell. HList-based Records form the basis of OOHaskell. The HList library relies on common extensions of Haskell 2010. HList is being used in AspectAG (http://www.haskell.org/communities/11-2011/html/report.html#sect5.4.2), typed EDSL of attribute grammars, and in HaskellDB.

The October 2012 version of HList library marks the significant re-write to take advantage of the fancier types offered by GHC 7.4 and 7.6. HList now relies on type-level booleans, natural numbers and lists, symbols, and on kind polymorphism. A number of operations are implemented as type functions. Another notable addition is unfold for heterogeneous lists. Many operations (projection, splitting) are now implemented in terms of unfold. Such a refactoring moved more computations to type-level, with no run-time overhead.

Since the last update, almost the entire HList has been rewritten in this new style. There are new operations: |HReplicate|, |HScanr|, |HFoldl|. Keyword function arguments become a part of the HList distribution. A new version of the |Apply| constraint, |ApplyAB|, improves the type inference obviating many type annotations. The Haddoc documentation has been greatly extended, with many doctest.

Further reading

• HList repository: http://code.haskell.org/HList/
• HList: http://okmij.org/ftp/Haskell/types.html#HList
• OOHaskell: http://homepages.cwi.nl/~ralf/OOHaskell/

Persistent is a type-safe data store interface for Haskell. Haskell has many different database bindings available, but they provide few usefeul static guarantees. Persistent uses knowledge of the data schema to provide a type-safe interface that re-uses existing database binding libraries. Persistent is designed to work across different databases, and works on Sqlite, PostgreSQL, MongoDB, and MySQL, with an experimental backend for CouchDB.

The 1.2 release features a refactoring of the module hierarchy. We’re taking this opporunity to clean up a few idiosyncracies in the API and make the documentation a bit more helpful, but otherwise the library is remaining unchanged.

The MongoDB backend features new helpers, query operators, and bug fixes for working with embedded/nested models. One can store a list of Maps or records inside a column/field. This is required for proper usage of MongoDB. In SQL an embedded object is stored as JSON, which is convenient as long as the column is not queried.

In order to accomodate various different backend types, Persistent is broken up into multiple components (separated by type classes). There is one for storage/serialization, one for uniqueness, and one for querying. This means that anyone wanting to create database abstractions can re-use the battle-tested persistent storage/serialization layer without having to implement the full query interface.

Persistent’s query layer is the same for any backend that implement the query interface, although backends can define their own additional operators. The interface is a straightforward usage of combinators:

selectList [  PersonFirstName ==. "Simon", 
              PersonLastName ==. "Jones"] []

Most Haskellers would like to use a mature FRP-based API for creating graphical user interfaces. But FRP may not be the best tool for special user interfaces, like interfaces which consist mainly of buttons, checkboxes, combo boxes, text entries, tabs and menus. The goal of the LGtk project is to give a lens-based API which better fits these user interfaces. LGtk is built on Gtk2Hs.

The first release of LGtk was announced on 15 April 2013. Currently the first version of the API is available with a demo application.

LGtk is being actively developed. I currently work on the following items:

• Give an approximation of the semantics of LGtk and adjust the API to the given semantics.
• Support for asynchronous effects.
• Make a tutorial for developers with lots of small examples.

Further reading

http://people.inf.elte.hu/divip/LGtk/index.html

Gtk2Hs is a set of Haskell bindings to many of the libraries included in the Gtk+/Gnome platform. Gtk+ is an extensive and mature multi-platform toolkit for creating graphical user interfaces.

GUIs written using Gtk2Hs use themes to resemble the native look on Windows. Gtk is the toolkit used by Gnome, one of the two major GUI toolkits on Linux. On Mac OS programs written using Gtk2Hs are run by Apple’s X11 server but may also be linked against a native Aqua implementation of Gtk.

Gtk2Hs features:

• Automatic memory management (unlike some other C/C++ GUI libraries, Gtk+ provides proper support for garbage-collected languages)
• Unicode support
• High quality vector graphics using Cairo
• Extensive reference documentation
• An implementation of the “Haskell School of Expression” graphics API
• Bindings to many other libraries that build on Gtk: gio, GConf, GtkSourceView 2.0, glade, gstreamer, vte, webkit

Since the last release, there have been many bugfixes, and Peter Davies and Hamish Mackenzie have begun adding experimental Gtk3 support, enabled by the “gtk3” cabal flag.

Further reading

• News and downloads: http://haskell.org/gtk2hs/

• Development version: darcs get http://code.haskell.org/gtk2hs/

Reactive-banana is a practical library for functional reactive programming (FRP).

FRP offers an elegant and concise way to express interactive programs such as graphical user interfaces, animations, computer music or robot controllers. It promises to avoid the spaghetti code that is all too common in traditional approaches to GUI programming.

The goal of the library is to provide a solid foundation.

• Writing graphical user interfaces with FRP is made easy. The library can be hooked into any existing event-based framework like wxHaskell or Gtk2Hs. A plethora of example code helps with getting started. You can mix FRP and imperative style. If you don’t know how to express functionality in terms of FRP, just temporarily switch back to the imperative style.
• Programmers interested in implementing FRP will have a reference for a simple semantics with a working implementation. The library stays close to the semantics pioneered by Conal Elliott.
• It features an efficient implementation. No more spooky time leaks, predicting space &time usage should be straightforward.

Status. The latest version of the reactive-banana library is 0.7.1.3. Compared to the previous report, there has been no new public release as the API and its semantics have reached a stable plateau.

It turned out that the library suffered from a large class of space leaks concerning accumulated behaviors. This has been fixed in the development version, but not yet incorporated into a new release.

Current development. As foreshadowed in the last report, not much development has occurred on the reactive-banana library itself. Most of my efforts have been spent on the threepenny-gui project, which is a library for writing graphical user interfaces in Haskell (→7.7.5).

Fortunately, these development efforts are directly relevant to reactive-banana; graphical user interfaces have always been my main motivation for FRP in the first place. In particular, I have implemented a new FRP module specifically for the threepenny-gui project, and this reimplementation has taught me many valuable lessons, which I hope to reintegrate into reactive-banana soon. The most important lesson is that the current API for reactive-banana is too complex — the type parameter t that indicates starting times just isn’t worth it. I have also learned that recursion is best implemented differently from how I did it before, which leads to a fix for certain situations where reactive-banana couldn’t handle recursion.

Further reading

• Project homepage: http://haskell.org/haskellwiki/Reactive-banana
• Example code: http://haskell.org/haskellwiki/Reactive-banana/Examples
• threepenny-gui: http://haskell.org/haskellwiki/Threepenny-gui

The diagrams framework provides an embedded domain-specific language for declarative drawing. The overall vision is for diagrams to become a viable alternative to DSLs like MetaPost or Asymptote, but with the advantages of being declarative—describing what to draw, not how to draw it—and embedded—putting the entire power of Haskell (and Hackage) at the service of diagram creation. There is still much more to be done, but diagrams is already quite fully-featured, with a comprehensive user manual, a large collection of primitive shapes and attributes, many different modes of composition, paths, cubic splines, images, text, arbitrary monoidal annotations, named subdiagrams, and more.

What’s new

Since the last HCAR edition, version 0.7 was released in August 2013. New features in 0.7 include:

• A big refactoring of the way segments, trails, and paths are represented; the new API is more powerful and semantically consistent.
• Functions to compute the curvature of path segments at a given point.
• Segment offsets (paths lying a constant distance away from a given segment). A fuller implementation with offsets for entire trails will be in the upcoming 1.0 release.
• A generalized color API, allowing backends to use whatever color space they want.
• Additions to the diagrams-contrib library, including a symmetric layout algorithm for binary trees, circle packing layout, a generalized turtle drawing interface, factorization diagrams, and iterated subset fractals.
• Many documentation improvements, using diagrams-haddock to generate example images.
• Big improvements to diagrams-builder, including much smarter rebuilding and hsenv support.
• Official support for the SVGFonts package, providing Haskell-native code for handling font data and converting strings into diagrams paths.

There have been many improvements and changes to the core diagrams libraries as well. A 1.0 release is planned for on or around November 20, to coincide with Brent’s presentation at the New York Haskell Users’ Group. Features slated for the 1.0 release include:

• A nice API for drawing arrows between arbitrary points or diagrams.
• Convenient integration with the lens (→7.1.2) package.
• Path offsets and expansions: for example, the operation of “stroking” a path can now be internalized within diagrams, returning a closed path representing the outline of the stroke.
• Performance improvements: across-the-board improvements of around 30%, and more optimized SVG output.

Contributing

There is plenty of exciting work to be done; new contributors are welcome! Diagrams has developed an encouraging, responsive, and fun developer community, and makes for a great opportunity to learn and hack on some “real-world” Haskell code. Because of its size, generality, and enthusiastic embrace of advanced type system features, diagrams can be intimidating to would-be users and contributors; however, we are actively working on new documentation and resources to help combat this. For more information on ways to contribute and how to get started, see the Contributing page on the diagrams wiki: http://haskell.org/haskellwiki/Diagrams/Contributing, or come hang out in the #diagrams IRC channel on freenode.

Further reading

• http://projects.haskell.org/diagrams
• http://projects.haskell.org/diagrams/gallery.html
• http://haskell.org/haskellwiki/Diagrams
• http://github.com/diagrams
• https://byorgey.wordpress.com/2012/08/28/creating-documents-with-embedded-diagrams/
• http://www.cis.upenn.edu/~byorgey/pub/monoid-pearl.pdf
• http://www.youtube.com/watch?v=X-8NCkD2vOw

Chordify is a music player that extracts chords from musical sources like Soundcloud, Youtube, or your own files, and shows you which chord to play when. The aim of Chordify is to make state-of-the-art music technology accessible to a broader audience. Our interface is designed to be simple: everyone who can hold a musical instrument should be able to use it.

Behind the scenes, we use the sonic annotator for extraction of audio features. These features consist of the downbeat positions and the tonal content of a piece of music. Next, the Haskell program HarmTrace takes these features and computes the chords. HarmTrace uses a model of Western tonal harmony to aid in the chord selection. At beat positions where the audio matches a particular chord well, this chord is used in final transcription. However, in case there is uncertainty about the sounding chords at a specific position in the song, the HarmTrace harmony model will select the correct chords based on the rules of tonal harmony.

Chordify is free for everyone to use. We have recently added the ability to download PDF and MIDI transcriptions of the chords for a small fee, and plan to release multiple new features soon. The code for HarmTrace is available on Hackage, and we have ICFP’11 and ISMIR’12 publications describing some of the technology behind Chordify.

Further reading

http://chordify.net

Overview

Euterpea is a Haskell library for computer music applications. It is a descendent of Haskore and HasSound, and is intended for both educational purposes as well as serious computer music development. Euterpea can be thought of as a “wide-spectrum” DSL, suitable for high-level music representation, algorithmic composition, and analysis; mid-level concepts such as MIDI; and low-level audio processing, sound synthesis, and instrument design. It also includes a musical user interface (MUI), a set of GUI widgets such as sliders, buttons, and so on.

The audio and MIDI-stream processing aspects of Euterpea are based on arrows, which makes programs analogous to signal processing diagrams. Using arrows prevents certain kinds of space leaks, and facilitates significant optimization strategies (in particular, the use of causal commutative arrows.

Euterpea is being developed at Yale in Paul Hudak’s research group, where it has become a key component of Yale’s new Computing and the Arts major. Hudak is teaching a two-term sequence in computer music using Euterpea, and is developing considerable pedagogical material, including a new textbook tentatively titled The Haskell School of Music — From Signals to Symphonies (HSoM). The name “Euterpea” is derived from “Euterpe”, who was one of the nine Greek Muses (goddesses of the arts), specifically the Muse of Music.

Status

The system is stable enough for experimental computer music applications, and for use in coursework either to teach Haskell programming or to teach computer music concepts.

All source code, papers, and a draft of the HSoM textbook can be found on the Yale Haskell Group website at: http://haskell.cs.yale.edu/.

History

Haskore is a Haskell library developed over 15 years ago by Paul Hudak and his students at Yale for high-level computer music applications. HasSound was a later development that served as a functional front-end to csound’s sound synthesis capabilities. Euterpea combines Haskore with a native Haskell realization of HasSound (i.e. no csound dependencies).

Future Plans

Euterpea is a work in progress, as is the HSoM textbook. Computer-music specific MUI widgets (such as keyboards and guitar frets), further optimization strategies, better support for real-time MIDI and audio processing, and a parallel (multicore) implementation are amongst the planned future goals.

Anyone who would like to contribute to the project, please contact Paul Hudak at <paul.hudak at yale.edu>.

FurtherReading

Please visit http://haskell.cs.yale.edu/. Click on “Euterpea” to learn more about the library, “Publications” to find our papers on computer music (including HSoM), and “CS431” or “CS432” to see the course material used in two computer music classes at Yale that use Euterpea.

Description

The Haskell XML Toolbox (HXT) is a collection of tools for processing XML with Haskell. It is itself purely written in Haskell 98. The core component of the Haskell XML Toolbox is a validating XML-Parser that supports almost fully the Extensible Markup Language (XML) 1.0 (Second Edition). There is a validator based on DTDs and a new more powerful one for Relax NG schemas.

The Haskell XML Toolbox is based on the ideas of HaXml and HXML, but introduces a more general approach for processing XML with Haskell. The processing model is based on arrows. The arrow interface is more flexible than the filter approach taken in the earlier HXT versions and in HaXml. It is also safer; type checking of combinators becomes possible with the arrow approach.

HXT is partitioned into a collection of smaller packages: The core package is hxt. It contains a validating XML parser, an HTML parser, filters for manipulating XML/HTML and so called XML pickler for converting XML to and from native Haskell data.

Basic functionality for character handling and decoding is separated into the packages hxt-charproperties and hxt-unicode. These packages may be generally useful even for non XML projects.

HTTP access can be done with the help of the packages hxt-http for native Haskell HTTP access and hxt-curl via a libcurl binding. An alternative lazy non validating parser for XML and HTML can be found in hxt-tagsoup.

The XPath interpreter is in package hxt-xpath, the XSLT part in hxt-xslt and the Relax NG validator in hxt-relaxng. For checking the XML Schema Datatype definitions, also used with Relax NG, there is a separate and generally useful regex package hxt-regex-xmlschema.

The old HXT approach working with filter hxt-filter is still available, but currently only with hxt-8. It has not (yet) been updated to the hxt-9 mayor version.

Features

• Validating XML parser
• Very liberal HTML parser
• Lightweight lazy parser for XML/HTML based on Tagsoup (http://www.haskell.org/communities/05-2010/html/report.html#sect5.11.3)
• Binding to the expat parser via hexpat package
• Easy de-/serialization between native Haskell data and XML by pickler and pickler combinators
• XPath support
• Full Unicode support
• Support for XML namespaces
• Cabal package support for GHC
• HTTP access via Haskell bindings to libcurl and via Haskell HTTP package
• Tested with W3C XML validation suite
• Example programs
• Relax NG schema validator
• XML Schema validator (next release)
• Lightweight regex library with full support of Unicode and XML Schema Datatype regular expression syntax
• An HXT Cookbook for using the toolbox and the arrow interface
• Basic XSLT support
• GitHub repository with current development versions of all packages http://github.com/UweSchmidt/hxt

Current Work

The master thesis and project implementing an XML Schema validator started in October 2011 has been finished. The validator will be released in a separate module hxt-xmlschema. Integration with hxt has been prepared in hxt-9.3. The XML Schema datatype library has also been completed, all datatypes including date and time types are implemented. But there is still a need for testing the validator, especially with the W3C test suite. Hopefully testing will be done in the next few months. With the release of the schema validator the the master thesis will also be published on the HXT homepage. The current state of the validator can be found in the HXT repository on github.

Further reading

The Haskell XML Toolbox Web page (http://www.fh-wedel.de/~si/HXmlToolbox/index.html) includes links to downloads, documentation, and further information.

The latest development version of HXT can be found on github under (https://github.com/UweSchmidt/hxt).

A getting started tutorial about HXT is available in the Haskell Wiki (http://www.haskell.org/haskellwiki/HXT ). The conversion between XML and native Haskell data types is described in another Wiki page (http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML).

A suite of command-line utilities for creating and manipulating epub book files. Included are: epubmeta, epubname, epubzip.

epubmeta is a command-line utility for examining and editing epub book metadata. With it you can export, import and edit the raw OPF Package XML document for a given book. Or simply dump the metadata to stdout for viewing in a friendly format.

epubname is a command-line utility for renaming epub ebook files based on the metadata. It tries to use author names and title info to construct a sensible name.

epubzip is a handy utility for zipping up the files that comprise an epub into an .epub zip file. Using the same technology as epubname, it can try to make a meaningful filename for the book.

This project is built on the latest epub-metadata library and so supports epub3 for the first time.

See also epub-metadata (→7.3.2).

epub-tools is available from Hackage and the Darcs repository below.

Further reading

• Project page: http://ui3.info/d/proj/epub-tools.html

• Source repository: darcs get http://ui3.info/darcs/epub-tools

The Haskell Natural Language Processing community aims to make Haskell a more useful and more popular language for NLP. The community provides a mailing list, Wiki and hosting for source code repositories via the Haskell community server.

The Haskell NLP community was founded in March 2009. The list is still growing slowly as people grow increasingly interested in both natural language processing, and in Haskell.

New packages

• multext-east-msd 0.1.0.4 (Jan Snajder)

  This package is an implementation of the MULTEXT-East morphosyntactic descriptors. It could be useful for work with Eastern European languages.

• concraft 0.8 (Jakub Waszczuk)

  Concraft is a morphological disambiguation library designed for highly-inflectional languages. It is based on conditional random fields extended with additional, position-wise restrictions on the output domain, which are used to impose consistency between the modeled label sequences and morphosyntactic analysis results (Waszczuk 2012; see further reading).

  See also the package concraft-pl, a morphosyntactic tagging tool for the Polish language which relies on the Concraft library.

• nerf 0.5.0 (Jakub Waszczuk)

  The package provides a named entity (NE) recognition tool which can be used to model tree-like structures of NEs. It combines the IOB encoding method (used to translate between the original, forest representation of NEs and the sequence of atomic labels) with the sequence labeler based on linear-chain conditional random fields.

• dawg 0.11 (Jakub Waszczuk)

  The library implements directed acyclic word graphs internally represented as minimal acyclic deterministic finite-state automata. It provides fast insert and delete operations which can be used to build the automaton on-the-fly and a static hashing functionality. The library can be particularly useful to store language dictionaries (e.g. morphological dictionaries or resources of named entities). The implementation is not very efficient at the moment, but it provides a convenient map-like interface and should be easy to use.

Updated packages

• GenI 0.24.1 (Eric Kow)

  GenI is a surface realiser (part of a natural language generation system) using Feature Based Tree Adjoining Grammar. This latest version can be customised to work with alternative semantic inputs, making it easier to integrate with wider applications.

• sequor 0.4.2 (Grzegorz Chrupala)

  Sequor is a sequence labeler based on Collins’s (2002) perceptron. Sequor has a flexible feature template language and is meant mainly for NLP applications such as Named Entity labeling, Part of Speech tagging or syntactic chunking. This release includes the SemiNER named entity recognizer, with pre-trained models for German and English.

  https://bitbucket.org/gchrupala/sequor

• hiera 0.1.0.0 (Grzegorz Chrupala)

  Hiera implements the algorithm for hierarchical clustering of word-class probability distributions described in Chrupala 2012 (see Further Reading): it is an agglomerative clustering algorithm where the distance between clusters is defined as the Jensen-Shannon divergence between the probability distributions over classes associated with each word-type.

  https://bitbucket.org/gchrupala/hiera

At the present, the mailing list is mainly used to make announcements to the Haskell NLP community. We hope that we will continue to expand the list and expand our ways of making it useful to people potentially using Haskell in the NLP world.

Further reading

• The Haskell NLP page http://projects.haskell.org/nlp
• Grzegorz Chrupala. 2012. Hierarchical clustering of word class distributions. NAACL-HLT 2012 Workshop on the Induction of Linguistic Structure.
• Jakub Waszczuk. 2012. Harnessing the CRF complexity with domain-specific constraints. The case of morphosyntactic tagging of a highly inflected language. In Proceedings of the 24th International Conference on Computational Linguistics (COLING 2012).

GenI is a surface realizer for Tree Adjoining Grammars. Surface realization can be seen a subtask of natural language generation (producing natural language utterances, e.g., English texts, out of abstract inputs). GenI in particular takes a Feature Based Lexicalized Tree Adjoining Grammar and an input semantics (a conjunction of first order terms), and produces the set of sentences associated with the input semantics by the grammar. It features a surface realization library, several optimizations, batch generation mode, and a graphical debugger written in wxHaskell. It was developed within the TALARIS project and is free software licensed under the GNU GPL, with dual-licensing available for commercial purposes.

GenI is now mirrored on GitHub, with its issue tracker and wiki and homepage also hosted there. The most recent release, GenI 0.24 (2013-09-18), allows for custom semantic inputs, making it simpler to use GenI in a wider variety for applications. This has recently been joined by a companion geni-util package which offers a rudimentary geniserver client and a reporting tool for grammar debugging.

GenI is available on Hackage, and can be installed via cabal-install, along with its GUI and HTTP server user interfaces. For more information, please contact us on the geni-users mailing list.

Further reading

• http://github.com/kowey/GenI
• http://projects.haskell.org/GenI
• Paper from Haskell Workshop 2006:

  http://hal.inria.fr/inria-00088787/en

• http://websympa.loria.fr/wwsympa/info/geni-users

ADPfusion provides a domain-specific language (DSL) for the formulation of dynamic programs with a special emphasis on computational biology. Following ideas established in Algebraic dynamic programming (ADP) a problem is separated into a grammar defining the search space and one or more algebras that score and select elements of the search space. The DSL has been designed with performance and a high level of abstraction in mind.

As an example, consider a grammar that recognizes palindromes. Given the non-terminal p, as well as parsers for single characters c and the empty input e, the production rule for palindromes can be formulated as p ->c p c || e.

The corresponding ADPfusion code is similar:

We need a number of combinators as “glue” and additional evaluation functions f, g, and h. With f c₁ p c₂ = p && (c₁=c₂) scoring a candidate, g e = True, and h xs = or xs determining if the current substring is palindromic.

As of now, code written in ADPfusion achieves performance close to hand-optimized C, and outperforms similar approaches (Haskell-based ADP, GAPC producing C++) thanks to stream fusion. The figure shows running times for the Nussinov algorithm.

Starting with ADPfusion 0.2, dynamic programs on more than one input sequence can be written. This allows efficient dynamic programs that compute, say, the alignment of two or more inputs. More complicated algorithms of coupled context-free grammars also become possible with this new, multi-dimensional expansion. Together with generalised index spaces, more algorithms can be implemented efficiently, while at the same time reducing the effort required to implement these more complicated algorithms correctly.

Further reading

• http://www.tbi.univie.ac.at/~choener/adpfusion
• http://hackage.haskell.org/package/ADPfusion
• http://dx.doi.org/10.1145/2364527.2364559

Bioinformatics in Haskell is a steadily growing field, and the Bio section on Hackage now contains 53 libraries and applications. The biohaskell web site coordinates this effort, and provides documentation and related information. Anybody interested in the combination of Haskell and bioinformatics is encouraged to sign up to the mailing list (currently by emailing Ketil), and to register and document their contributions on the http://biohaskell.org wiki.

BioHaskell now contains a fledgling collection of libraries for structural analysis of biomolecules. hPDB is a parallel parser of Protein DataBank file format, which parses the largest structures faster than single-threaded Python code, and faster than parallel BioJava parser. It uses an octree data structure for fast querying of atom positions and contacts, and |Iterable| class to allow for iteration over deeply contained objects within hierarchical collections. The author of the library, Michal J. Gajda plans to soon release other libraries for processing biomolecular data, in particular parseSTAR for parsing STAR* format used for BMRB database of nuclear magnetic resonance data, and hDat library for processing small-angle scattering data.

Further reading

• http://biohaskell.org
• http://blog.malde.org
• http://hackage.haskell.org/package/hPDB
• http://hackage.haskell.org/package/Octree
• http://hackage.haskell.org/package/Iterable
• http://www.tbi.univie.ac.at/~choener/haskell/
• http://adp-multi.ruhoh.com

Feldspar is a domain-specific language for digital signal processing (DSP). The language is embedded in Haskell and is currently developed at Chalmers University of Technology (→9.8).

The motivating application of Feldspar is telecoms processing, but the language is intended to be useful for DSP in general. The aim is to allow DSP functions to be written in pure functional style in order to raise the abstraction level of the code and to enable more high-level optimizations. The current version consists of an extensive library of numeric and array processing operations as well as a code generator producing C code for running on embedded targets.

At present, Feldspar can express the data-intensive numeric algorithms which are at the core of any DSP application. There is also support for the expression and compilation of parallel algorithms. As future work remains to extend the language to deal with interaction with the environment (e.g., processing of streaming data) and to support compilation to heterogeneous multi-core targets.

Further reading

• https://github.com/Feldspar/feldspar-language
• http://hackage.haskell.org/package/feldspar-language
• http://hackage.haskell.org/package/feldspar-compiler

Kansas Lava is a Domain Specific Language (DSL) for expressing hardware descriptions of computations, and is hosted inside the language Haskell. Kansas Lava programs are descriptions of specific hardware entities, the connections between them, and other computational abstractions that can compile down to these entities. Large circuits have been successfully expressed using Kansas Lava, and Haskell’s powerful abstraction mechanisms, as well as generic generative techniques, can be applied to good effect to provide descriptions of highly efficient circuits.

• The Fabric monad is now a Monad transformer. The Fabric monad historically provided access to named input/output ports, and now also provides named variables, implemented by ports that loop back on themselves. This additional primitive capability allows for a typed state machine monad. This design gives an elegant stratospheric pattern: purely functional circuits using streams; a monad for layout over space; and a monad for state generation, that acts over time.

• On top of the Fabric monad, we are implementing an atomic transaction layer, which provides a BSV-like interface, but in Haskell. An initial implementation has been completed, and this is being reworked to include BSV’s Ephemeral History Registers.

Further reading

http://www.ittc.ku.edu/csdl/fpg/Tools/KansasLava

clckwrks (pronounced “clockworks”) is a blogging and content management system (CMS). It is intended to compete directly with popular PHP-based systems. Pages and posts are written in markdown and can be edited directly in the browser. The system can be extended via plugins and themes packages.

At present, clckwrks is still alpha, and requires Haskell knowledge to install and configure. However, the goal is to create an end user system that requires zero Haskell knowledge. It will be possible to one-click install plugins and themes and perform all other administrative functions via the browser.

Future plans

We are currently focused on four tasks:

1. Overhaul of the plugin system to support one-click installation of plugins and themes
2. Improvements to the user experience in the core blogging and page editing functionality
3. Simplifying installation
4. Improved documentation

Once the core is solid, we will focus development efforts on creating plugins to extend the core functionality.

Further reading

http://www.clckwrks.com/

The program arbtt, the automatic rule-based time tracker, allows you to investigate how you spend your time, without having to manually specify what you are doing. arbtt records what windows are open and active, and provides you with a powerful rule-based language to afterwards categorize your work. And it comes with documentation!

Since the last report, version 0.7 was released, sporting a progress bar while processing the data. Furthermore, Waldir Pimenta contributed a slick web site for the project.

Further reading

• http://arbtt.nomeata.de/
• http://www.joachim-breitner.de/blog/archives/336-The-Automatic-Rule-Based-Time-Tracker.html
• http://arbtt.nomeata.de/doc/users_guide/

Well-Typed is a Haskell services company. We provide commercial support for Haskell as a development platform, including consulting services, training, and bespoke software development. For more information, please take a look at our website or drop us an e-mail at <info at well-typed.com>.

We are working for a variety of commercial clients, but naturally, only some of our projects are publically visible.

Well-Typed is now five years old! We’ve been sad to see Ian Lynagh, one of Well-Typed’s founders, leaving the company this summer. We are grateful for all the hard work that Ian has done over the years.

We have hired two new consultants: Austin Seipp and Adam Gundry. Austin in particular is going to be taking over a lot of Ian’s responsibilities in the development and maintenance of GHC (→3.1). We’re currently busy preparing the GHC 7.8.1 release.

On behalf of the Industrial Haskell Group (IHG) (→8.3), we have been working on getting Hackage 2 (→6.3.1) ready for release for a while, and we’re happy to say that the release has finally happened. Hackage 2 is live and works well since a couple of weeks now. Now is a good time to become involved with Hackage development, because it’s now much easier to get changes merged and see them in production soon.

We continue to be involved in the community, maintaining several packages on Hackage and giving talks at a number of conferences. Some of our recent appearances are available online, such as Edsko’s talk on Lazy I/O at Skills Matter, Andres’s talk on Cloud Haskell at the Big Tech Day in Munich, and Andres’s talk on Free Monads at the Haskell eXchange (links below).

We are continuing to offer training services. We offer regular courses in London and (new) in New York City, and on-demand on-site training courses elsewhere as well. For our London courses, the 2014 dates are now public, and the NYC dates will be known within the next few weeks.

We are of course always looking for new clients and projects, so if you have something we could help you with, just drop us an e-mail.

Further reading

• Company page: http://www.well-typed.com
• Blog: http://blog.well-typed.com/
• Training page: http://www.well-typed.com/services_training
• Edsko’s Lazy I/O talk: http://skillsmatter.com/podcast/haskell/lazy-io-and-alternatives-in-haskell
• Andres’s Cloud Haskell talk: http://www.techcast.com/events/bigtechday6/odeon-1345
• Andres’s Free Monads talk: http://skillsmatter.com/podcast/haskell/monads-for-free

Bluespec, Inc. provides an industrial-strength language (BSV) and tools for high-level hardware design. Components designed with these are shipping in some commercial smartphones and tablets today.

BSV is used for all aspects of ASIC and FPGA design — specification, synthesis, modeling, and verification. All hardware behavior is expressed using rewrite rules (Guarded Atomic Actions). BSV borrows many ideas from Haskell — algebraic types, polymorphism, type classes (overloading), and higher-order functions. Strong static checking extends into correct expression of multiple clock domains, and to gated clocks for power management. BSV is universally applicable, from algorithmic “datapath” blocks to complex control blocks such as processors, DMAs, interconnects, and caches.

Bluespec’s core tool synthesizes (compiles) BSV into high-quality Verilog, which can be further synthesized into netlists for ASICs and FPGAs using third-party tools. Atomic transactions enable design-by-refinement, where an initial executable approximate design is systematically transformed into a quality implementation by successively adding functionality and architectural detail. The synthesis tool is implemented in Haskell (well over 100K lines).

Bluesim is a fast simulation tool for BSV. There are extensive libraries and infrastructure to make it easy to build FPGA-based accelerators for compute-intensive software, including for the Xilinx XUPv6 board popular in universities, and the Convey HC-1 high performance computer.

BSV is also enabling the next generation of computer architecture education and research. Students implement and explore architectural models on FPGAs, whose speed permits evaluation using whole-system software.

Status and availability

BSV tools, available since 2004, are in use by several major semiconductor and electronic equipment companies, and universities. The tools are free for academic teaching and research.

Further reading

• Abstraction in Hardware System Design, R.S. Nikhil, in Communications of the ACM, 54:10, October 2011, pp. 36-44.

• Bluespec, a General-Purpose Approach to High-Level Synthesis Based on Parallel Atomic Transactions, R.S. Nikhil, in High Level Synthesis: from Algorithm to Digital Circuit, Philippe Coussy and Adam Morawiec (editors), Springer, 2008, pp. 129-146.

• BSV by Example, R.S. Nikhil and K. Czeck, 2010, book available on Amazon.com.

• http://bluespec.com/SmallExamples/index.html: from BSV by Example.

• http://www.cl.cam.ac.uk/~swm11/examples/bluespec/: Simon Moore’s BSV examples (U. Cambridge).

• http://csg.csail.mit.edu/6.375: Complex Digital Systems, MIT courseware.

• http://www.bluespec.com/products/BluDACu.htm: A fun example with many functional programming features — BluDACu, a parameterized Bluespec hardware implementation of Sudoku.

The Industrial Haskell Group (IHG) is an organization to support the needs of commercial users of Haskell.

The main activity of the IHG is to fund work on the Haskell development platform. It currently operates two schemes:

• The collaborative development scheme pools resources from full members in order to fund specific development projects to their mutual benefit.

• Associate and academic members contribute to a separate fund which is used for maintenance and development work that benefits the members and community in general.

In the past six months, the collaborative development scheme funded work on the new Hackage server (→6.3.1). Hackage 2 has now replaced the old Hackage and is in active use. The code of Hackage 2 is on Github. This is a good time to become involved with Hackage development, because it’s now much easier to get changes merged and see them in production soon.

In general, details of the tasks undertaken are appearing on the Well-Typed (→8.1) blog, on the IHG status page and on standard communication channels such as the Haskell mailing list.

The collaborative development scheme is running continuously, so if you are interested in joining as a member, please get in touch. Details of the different membership options (full, associate, or academic) can be found on the website.

Two new associate members have just joined the IHG: Systor Vest and Erudify. There are now three full members, five associate members and one academic member in the IHG.

We are very interested in new members. If you are interested in joining the IHG, or if you just have any questions or comments, please drop us an e-mail at <info at industry.haskell.org>.

Further reading

• http://industry.haskell.org/
• http://industry.haskell.org/status/

Barclays Capital has been using Haskell as the basis for our FPF (Functional Payout Framework) project for about seven years now. The project develops a DSL and associated tools for describing and processing exotic equity options. FPF is much more than just a payoff language — a major objective of the project is not just pricing but “zero-touch” management of the entire trade lifecycle through automated processing and analytic tools.

For the first half of its life the project focused only on the most exotic options — those which were too complicated for the legacy systems to handle. Over the past few years however, FPF has expanded to provide the trade representation and tooling for the vast majority of our equity exotics trades and with that the team has grown significantly in both size and geographical distribution. We now have eight permanent full-time Haskell developers spread between Hong Kong, Kiev and London (with the latter being the biggest development hub).

Our main front-end language is currently a deeply embedded DSL which has proved very successful, but we have recently been working on a new non-embedded implementation. This will allow us to bypass some of the traditional DSEL limitations (e.g., error messages and syntactical restrictions) whilst addressing some business areas which have historically been problematic. The new language is based heavily on arrows, but has a custom (restrictive but hopefully easier-to-use than raw arrow-notation) syntax. We are using a compiler from our custom DSL syntax into Haskell source (with standard transformers from Ross Paterson’s “arrows” package) to provide the semantics for the language but plan to develop a number of independent backends. Our hope is that, over time, this will gradually replace our embedded DSL as the front end for all our tools. For the parsing part of this work we have been very impressed by Doaitse Swierstra’s uu-parsinglib (→7.3.3).

We have been and remain very satisfied GHC users and feel that it would have been significantly harder to develop our systems in any other current language.

Oblomov Systems is a one-person software company based in Utrecht, The Netherlands. Founded in 2009 for the Proxima 2.0 project (http://www.haskell.org/communities/05-2010/html/report.html#sect6.4.5), Oblomov has since then been working on a number of Haskell-related projects. The main focus lies on web-applications and (web-based) editors. Haskell has turned out to be extremely useful for implementing web servers that communicate with JavaScript clients or iPhone apps.

Awaiting the acceptance of Haskell by the world at large, Oblomov Systems also offers software solutions in Java, Objective C, and C#, as well as on the iPhone/iPad. Last year, Oblomov Systems has worked together with Ordina NV on a substantial Haskell project for the Council for the Judiciary in The Netherlands.

Further reading

http://www.oblomov.com

OpenBrain Ltd. is developing a new platform for statistical computing that enables optimal decisions taking into account all the available information. We have developed a new statistical programming language (BAYSIG) that augments a Haskell-like functional programming language with Bayesian inference and first-class ordinary and stochastic differential equations. BAYSIG is designed to support a declarative style of programming where almost all the work consists in building probabilistic models of observed data. Data analysis, risk assessment, decision, hypothesis testing and optimal control procedures are all derived mechanically from the definition of these models. We are targeting a range of application areas, including financial, clinical and life sciences data.

We are building a web application (http://BayesHive.com) to make this platform accessible to a wide range of users. Users can upload and analyse varied types of data using a point-and-click interface. Models and analyses are collected in literate programming-like documents that can be published by users as blogs.

We use Haskell for almost all aspects of implementing this platform. The BAYSIG compiler is written in Haskell, which is particularly well suited for implementing the recursive syntactical transformations underlying statistical inference. BayesHive.com is being developed in Yesod.

Contact

<tomn at openbrain.org>

Further reading

http://BayesHive.com

Education

There are many different courses on Haskell and Agda that run at Eötvös Lorand University, Faculty of Informatics.

• Programming for first-year BSc students using Haskell, it is officially in the curriculum. It is also taught for foreign language students as part of their program.

• Advanced functional programming using Haskell, it is an optional course for BSc and MSc students.

• Programming in Agda as an optional course for BSc and MSc students.

• Other Haskell-related courses on Lambda Calculus, Type Theory and Implementation of Functional Languages.

There is an interactive online evaluation and testing system, called ActiveHs. It contains several hundred systematized exercises and it may be also used as a teaching aid. There is also some experimenting going on about supporting SVG graphics, and extending the embedded interpreter and testing environment with safe emulation of IO values, providing support for Agda.

We have been translating our course materials to English, some of the materials is already available.

Further reading

• Haskell course materials (in English): http://pnyf.inf.elte.hu/fp/Overview_en.xml

• Agda course materials (in English): http://people.inf.elte.hu/divip/AgdaTutorial/Index.html (→2.3)

• ActiveHs: http://hackage.haskell.org/package/activehs

Semantics of Functional Programming Languages. Extended call-by-need lambda calculi model the semantics of Haskell. Our investigations of those calculi include correctness of strictness analysis using abstract reduction, equivalence of the call-by-name and call-by-need semantics, completeness of applicative bisimilarity w.r.t. contextual equivalence, and unsoundness of applicative bisimilarity in nondeterminstic languages with letrec. We also have shown that any semantic investigation of Haskell should include the seq-operator, since extending the lazy lambda calculus by seq is not conservative, i.e. the semantics changes.

A recent result is that deciding (extended) alpha-equivalence in languages with bindings (like letrec) is graph isomorphism complete. However, if the expressions are free of garbage (i.e. have no unused bindings) the problem can be solved efficiently.

Concurrency. We analyzed a higher-order functional language with concurrent threads, monadic IO, synchronizing variables and concurrent futures which models Concurrent Haskell. We proved correctness of program transformations, correctness of an abstract machine, and we have shown that this language conservatively extends the pure core language of Haskell, i.e. all program equivalences for the pure part also hold in the concurrent language. Most recently, we proved correctness of a highly concurrent implementation of Software Transactional Memory (STM) in a similar program calculus. We also proposed an alternative approach for implementing STM Haskell and prototypically implemented a library.

Correctness of Program Transformations. An ongoing project aims at automating correctness proofs of program transformations. To compute so-called forking and commuting diagrams we implemented a sound and complete algorithm as a combination of several unification algorithms in Haskell. To conclude the correctness proofs we automated the corresponding induction proofs (which use the diagrams) using automated termination provers for term rewriting systems.

Grammar based compression. This research topic focuses on algorithms on grammar compressed data like strings, matrices, trees, &hellp;. Our goal is to reconstruct known algorithms on uncompressed data (e.g. unification, matching, matrix operations, etc.) for their use on grammars without prior decompression. We implemented several of those algorithms in Haskell.

Further reading

http://www.ki.informatik.uni-frankfurt.de/research/HCAR.html

The Functional Programming group at Kent is a subgroup of the Programming Languages and Systems Group of the School of Computing. We are a group of staff and students with shared interests in functional programming. While our work is not limited to Haskell, we use for example also Erlang and ML, Haskell provides a major focus and common language for teaching and research.

Our members pursue a variety of Haskell-related projects, several of which are reported in other sections of this report. Thomas Schilling passed his PhD viva on trace-based dynamic optimisations for Haskell programs. Three new PhD students joined the group this September. Stephen Adams is working on advanced refactoring of Haskell programs. Andreas Reuleaux is working on refactoring dependently typed functional programs. Maarten Faddegon is working on making tracing for Haskell practical and easy to use. Currently he is looking into combining ideas from the Haskell tracer Hat (→6.4.4) and the Haskell object observation debugger Hood. Olaf Chitil refactored/reimplemented Hat to use standard Hackage libraries. Thus the code is simpler and more maintainable, but the functionality is unchanged. Scott Owens is working on verified compilers for the (strict) functional language CakeML.

We are always looking for more PhD students. We are particularly keen to recruit students interested in programming tools for verification, tracing, refactoring, type checking and any useful feedback for a programmer. The school and university have support for strong candidates: more details at http://www.cs.kent.ac.uk/pg or contact any of us individually by email.

We are also keen to attract researchers to Kent to work with us. There are many opportunities for research funding that could be taken up at Kent, as shown in the website http://www.kent.ac.uk/researchservices/sciences/fellowships/index.html. Please let us know if you’re interested in applying for one of these, and we’ll be happy to work with you on this.

Finally, if you would like to visit Kent, either to give a seminar if you’re passing through London or the UK, or to stay for a longer period, please let us know.

Further reading

• PLAS group: http://www.cs.kent.ac.uk/research/groups/plas/

• Haskell: the craft of functional programming: http://www.haskellcraft.com

• Refactoring Functional Programs: http://www.cs.kent.ac.uk/research/groups/plas/hare.html

• A trace-based just-in-time compiler for Haskell: http://www.youtube.com/watch?v=PtEcLs2t9Ws

• Scion, a library for building IDEs for Haskell: http://code.google.com/p/scion-lib/

• Hat, the Haskell Tracer: http://projects.haskell.org/hat/

• CakeML, a verification friendly dialect of SML: https://cakeml.org

• Practical Lazy Typed Contracts for Haskell: http://www.cs.kent.ac.uk/~oc/contracts.html

• Heat, an IDE for learning Haskell: http://www.cs.kent.ac.uk/projects/heat/

The activities of our groups center on formal methods, covering a variety of formal languages and also translations and heterogeneous combinations of these.

We are using the Glasgow Haskell Compiler and many of its extensions to develop the Heterogeneous tool set (Hets). Hets is a parsing, static analysis and proof management tool incorporating various provers and different specification languages, thus providing a tool for heterogeneous specifications. Logic translations are first-class citizens.

The languages supported by Hets include the CASL family, such as the Common Algebraic Specification Language (CASL) itself (which provides many-sorted first-order logic with partiality, subsorting and induction), HasCASL, CoCASL, CspCASL, and an extended modal logic based on CASL. Other languages supported include propositional logic, QBF, Isabelle, Maude, VSE, TPTP, THF, FPL (logic of functional programs), LF type theory and still Haskell (via Programatica). More recently, ontology languages like OWL, RDF, Common Logic, and DOL (the Distributed Ontology Language) have been integrated.

Hets can speak to the following provers:

• minisat, zChaff (SAT solvers),
• SPASS, Vampire, Darwin, KRHyper and MathServe (automated first-order theorem provers),
• Pellet and Fact++ (description logic tableau provers),
• Leo-II and Satallax (automated higher-order theorem provers),
• Isabelle (an interactive higher-order theorem prover),
• CSPCASL-prover (an Isabelle-based prover for CspCASL),
• VSE (an interactive prover for dynamic logic).

The user interface of the Hets implementation (about 200K lines of Haskell code) is based on some Haskell sources such as bindings to uDrawGraph (formerly Davinci) and Tcl/TK that we maintain and also gtk2hs (→7.7.3). Additionally we have a command line interface and a prototypcial web interface based on warp (→5.2.2) with a RESTful API.

HasCASL is a general-purpose higher-order language which is in particular suited for the specification and development of functional programs; Hets also contains a translation from an executable HasCASL subset to Haskell. There is a prototypical translation of a subset of Haskell to Isabelle/HOL.

Further reading

• Group activities overview:

  http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/

• CASL specification language:

  http://www.cofi.info

• distributed ontology language DOL:

  http://www.ontoiop.org

• Heterogeneous tool set:

  http://hets.dfki.de
  http://www.informatik.uni-bremen.de/htk/
  http://www.informatik.uni-bremen.de/uDrawGraph/

Haskell is one of the main research topics of the new Programming Languages Group at the Department of Applied Mathematics and Computer Science at the University of Ghent, Belgium.

Teaching. UGent is a great place for Haskell-aficionados:

• Make Haskell part of your studies with the elective course Functional and Logic Programming Languages.
• Explore the theory behind Haskell in the new master course on Programming Language Fundamentals.
• Explore Haskell in depth with one of our Haskell master thesis topics.
• Attend the thriving Ghent Functional Programming Group (→9.10).

Research. Haskell-related projects of the group members and collaborators are:

• Meta-Theory a la Carte:

  Formalizing meta-theory, or proofs about programming languages, in a proof assistant has many well-known benefits. However, the considerable effort involved in mechanizing proofs has prevented it from becoming standard practice. This cost can be amortized by reusing as much of an existing formalization as possible when building a new language or extending an existing one. Unfortunately reuse of components is typically ad-hoc, with the language designer cutting and pasting existing definitions and proofs, and expending considerable effort to patch up the results. This work presents a more structured approach to the reuse of formalizations of programming language semantics through the composition of modular definitions and proofs. The key contribution is the development of an approach to induction for extensible Church encodings which uses a novel reinterpretation of the universal property of folds. These encodings provide the foundation for a framework, formalized in Coq, which uses type classes to automate the composition of proofs from modular components.

  Several interesting language features, including binders and general recursion, illustrate the capabilities of our framework. We reuse these features to build fully mechanized definitions and proofs for a number of languages, including a version of mini-ML. Bounded induction enables proofs of properties for non-inductive semantic functions, and mediating type classes enable proof adaptation for more feature-rich languages.

  This is joint work with Ben Delaware and Bruno Oliveira.

• Generic Conversions of Abstract Syntax Representations:

  This work presents a datatype-generic approach to syntax with variable binding. A universe specifies the binding and scoping structure of object languages, including binders that bind multiple variables as well as sequential and recursive scoping. Two interpretations of the universe are given: one based on parametric higher-order abstract syntax and one on well-typed de Bruijn indices. The former provides convenient interfaces to embedded domain-specific languages, but is awkward to analyse and manipulate directly, while the latter is a convenient representation in implementations, but is unusable as a surface language. We show how to generically convert from the parametric HOAS interpretation to the de Bruijn interpretation thereby taking the pain from DSL developer to write the conversion themselves.

  This is joint work with Johan Jeuring.

• Modular Reasoning about Incremental Programming:

  Incremental Programming (IP) is a programming style in which new program components are defined as increments of other components. Examples of IP mechanisms include: Object-oriented programming (OOP) inheritance, aspect-oriented programming (AOP) advice and feature-oriented programming (FOP). A characteristic of IP mechanisms is that, while individual components can be independently defined, the composition of components makes those components become tightly coupled, sharing both control and data flows. This makes reasoning about IP mechanisms a notoriously hard problem: modular reasoning about a component becomes very difficult; and it is very hard to tell if two tightly coupled components interfere with each other’s control and data flows.

  This work presents modular reasoning about interference (MRI), a purely functional model of IP embedded in Haskell. MRI models inheritance with mixins and side-effects with monads. It comes with a range of powerful reasoning techniques: equational reasoning, parametricity and reasoning with algebraic laws about effectful operations. These techniques enable modular reasoning about interference in the presence of side-effects.

  MRI formally captures harmlessness, a hard-to-formalize notion in the interference literature, in two theorems. We prove these theorems with a non-trivial combination of all three reasoning techniques.

  This is joint work with Bruno Oliveira and William Cook.

• Search Combinators:

  Search heuristics often make all the difference between effectively solving a combinatorial problem and utter failure. Hence, the ability to swiftly design search heuristics that are tailored towards a problem domain is essential to performance improvement. In other words, this calls for a high-level domain-specific language (DSL).

  The tough technical challenge we face when designing a DSL for search heuristics, is to bridge the gap between a conceptually simple specification language (high-level, purely functional and naturally compositional) and an efficient implementation (typically low-level, imperative and highly non-modular). We overcome this challenge with a systematic approach in Haskell that disentangles different primitive concepts into separate monadic modular mixin components, each of which corresponds to a feature in the high-level DSL. The great advantage of mixin components to provide a semantics for our DSL is its modular extensibility.

  This is joint work with Guido Tack, Pieter Wuille, Horst Samulowitz and Peter Stuckey, following up on Monadic Constraint Programming, a monadic DSL for Constraint Programming in Haskell.

Further reading

• http://users.ugent.be/~tschrijv/haskell.html
• http://users.ugent.be/~tschrijv/SearchCombinators/
• http://hackage.haskell.org/package/Monatron
• http://hackage.haskell.org/package/monadiccp

In Romania, Haskell is taught at several universities across the country: in Bucharest at both University POLITEHNICA of Bucharest and University of Bucharest, in Bacãu at “Vasile Alecsandri” University, in Braçov at “Transilvania” University, …. However, everywhere the courses are only centered on the theoretical aspects of functional programming and (sometimes) type systems. As a result, very few students will use this language after the exam is taken.

However, small communities are created to promote the language. That was the case of the Ro/Haskell group from Bacãu or FPBucharest group. Right now, almost all of these groups have stopped being active.

The main reason behind these failures is that the point of view in presenting the language is too deeply concerned with presenting its features and the purely functional aspect while hiding away the fact that you have to do some IO in real world applications. Basically, every activity of the previous groups and the subjects taught at universities regard Haskell only as a laboratory language.

A small group of people from Faculty of Automatic Control and Computers, University POLITEHNICA of Bucharest, decided last year to change that. The new teachers and teaching assistants from the Programming Paradigm course organised the first “Functional Programming Summer School” in June 2012 where a few real-world topics were presented among more theoretical aspects.

This year, a small subgroup of the ROSEdu (http://rosedu.org/) community developed on the feedback from the summer school and created a plan towards making Haskell a known and usable language with a community around it. There were talks on Yesod and GHC at different events (OSOM, Talks by Softbinator) or companies (IXIA), some new projects were launched – some of them being turned into bachelor or masters diploma projects – and an workshop called “Programming Haskell from N00b to Real World Programmer” was organized in June, during ROSEdu Summer Workshops (http://workshop.rosedu.org/2013/sesiuni/haskell). At the end of the workshop the students implemented IRC bots and Pacman-like games with a graphical interface and some dummy AI. Finally, some articles were published in the "Today Software Magazine" (http://www.todaysoftmag.com/tsm/en/) monthly magazine. This has prompted some Haskell-related hackathons at an byweekly Agile event called "Code and Beer".

But one of the major results of these activities is that the awareness of Haskell in Romanian communities has increased, leading to the launch of three small startup companies in Romanian towns.

We are a seminar and social group for people in Sydney, Australia, interested in Functional Programming and related fields. Members of the group include users of Haskell, Ocaml, LISP, Scala, F#, Scheme and others. We have 10 meetings per year (Feb–Nov) and meet on the third (usually, sometimes fourth) Wednesday of each month. We regularly get 20–30 attendees, with a 70/30 industry/research split. Talks this year have included material on compilers, theorem proving, type systems, Haskell web programming, Haskell database libraries, Scala and the Free Monad. We usually have about 90 mins of talks, starting at 6:30pm, then go for drinks afterwards. All welcome.

Further reading

• http://groups.google.com/group/fp-syd
• http://fp-syd.ouroborus.net/
• http://fp-syd.ouroborus.net/wiki/Past/2013

Functional Programming is an important component of the CSE department at Chalmers and University of Gothenburg. In particular, Haskell has a very important place, as it is used as the vehicle for teaching and numerous research projects. Besides functional programming, language technology, and in particular domain specific languages is a common aspect in our projects. We also hope to see all HCAR readers at ICFP 2014 in Gothenburg the first week of September! (Paper submissions due Saturday, 1 March 2014.)

Property-based testing.QuickCheck, developed at Chalmers, is one of the standard tools for testing Haskell programs. It has been ported to Erlang and used by Ericsson, Quviq, and others. QuickCheck continues to be improved and tools and related techniques are developed:

• We have shown how to successfully apply QuickCheck to test polymorphic properties.
• A new exhaustive testing tool (testing-feat on Hackage) has been developed. It is especially suited to generate test cases from large groups of mutually recursive syntax tree types. A paper describing it was presented at the Haskell Symposium 2012.
• Testing Type Class Laws: the specification of a class in Haskell often starts with stating, in comments, the laws that should be satisfied by methods defined in instances of the class, followed by the type of the methods of the class. We have developed a library (ClassLaws) that supports testing such class laws using QuickCheck.

Parsing: BNFC. The BNF Converter (BNFC) is a frontend for various parser generators in various languages. BNFC is written in Haskell and is commonly used as a frontend for the Haskell tools Alex and Happy. BNFC has recently been extended in two directions:

• A Haskell backend, which offers incremental and parallel parsing capabilities, as well as the ability to parse context-free grammars in full generality, has been added to BNFC. The underlying concepts are described in a paper published at ICFP 2013.
• BNFC has been embedded in a library (called BNFC-meta on Hackage) using Template-Haskell. An important aspect of BNFC-meta is that it automatically provides quasi-quotes for the specified language. This includes a powerful and flexible facility for anti-quotation.

Parsing: Combinators. A new package for combinator-based parsing has been released on Hackage. The combinators are based on the paper Parallel Parsing Processes. The technique is based on parsing in parallel all the possibly valid alternatives. This means that the parser never “hold onto” old input. A try combinator is also superfluous.

Parsing: Natural languages.Grammatical Framework is a declarative language for describing natural language grammars. It is useful in various applications ranging from natural language generation, parsing and translation to software localization. The framework provides a library of large coverage grammars for currently fifteen languages from which the developers could derive smaller grammars specific for the semantics of a particular application.

Generic Programming.Starting with Polytypic Programming in 1995 there is a long history of generic programming research at Chalmers. Recent developments include fundamental work on parametricity. This work has led to the development of a new kind of abstraction, to generalize notions of erasure. This means that a new kind of generic programming is available to the programmer. A paper describing the idea was presented in ICFP 2013.

Our research on generic-programming is lively, as witnessed by a constant stream of publications: Testing Type Class Laws, Functional Enumeration of Algebraic Types (FEAT), Testing versus proving in climate impact research and Dependently-typed programming in scientific computing — examples from economic modelling. The last two are part of our effort to contribute to the emerging research programme in Global Systems Science.

Program Inversion/bidirectionalization. Program transformation systems that generate pairs of programs that are some sort of inverses of each other. The pairs are guaranteed to be consistent by construction with respect to certain laws. Applications include pretty-printing/parsing (→7.3.1), XML transformation etc. The work is done in collaboration with University of Tokyo and University of Bonn.

Language-based security.SecLib is a light-weight library to provide security policies for Haskell programs. The library provides means to preserve confidentiality of data (i.e., secret information is not leaked) as well as the ability to express intended releases of information known as declassification. Besides confidentiality policies, the library also supports another important aspect of security: integrity of data. SecLib provides an attractive, intuitive, and simple setting to explore the security policies needed by real programs.

Type theory.Type theory is strongly connected to functional programming research. Many dependently-typed programming languages and type-based proof assistants have been developed at Chalmers. The Agda system (→4.1) is the latest in this line, and is of particular interest to Haskell programmers. We encourage you to experiment with programs and proofs in Agda as a “dependently typed Haskell”.

Embedded domain-specific languages.The functional programming group has developed several different domain-specific languages embedded in Haskell. The active ones are:

• Feldspar (→7.12.1) is a domain-specific language for digital signal processing (DSP), developed in co-operation by Ericsson, Chalmers FP group and Eötvös Lorand (ELTE) University in Budapest.
• Obsidian is a language for data-parallel programming targeting GPUs. Most recently we used Obsidian to implement an interesting variation of counting sort that also removes duplicate elements. This work was presented at FHPC 2013.

We are also working on general methods for EDSL development:

• Syntactic is a library that aims to support the definition of EDSLs. The core of the library was presented at ICFP 2012. The paper presents a generic model of typed abstract syntax trees in Haskell, which can serve as a basis for a library supporting the implementation of deeply embedded DSLs.
• Names For Free. A new technique for representing names and bindings of object languages represented as Haskell data types has been developed. The essence of the technique is to represent names using typed de Bruijn indices. The type captures exactly the context where the index is valid, and hence is as safe to use as a name. The technique was presented at Haskell Symposium 2013.
• Circular Higher-Order Syntax We have also developed a light-weight method for generating names while building an expression with binders. The method lends itself to be used in the front end of EDSLs based on higher-order syntax. The technique was presented at ICFP 2013.
• Simple and Compositional Monad Reification A method for reification of monads (compilation of monadic embedded languages) that is both simple and composable. The method was presented at ICFP 2013.

Automated reasoning.We are responsible for a suite of automated-reasoning tools:

• Equinox is an automated theorem prover for pure first-order logic with equality. Equinox actually implements a hierarchy of logics, realized as a stack of theorem provers that use abstraction refinement to talk with each other. In the bottom sits an efficient SAT solver. Paradox is a finite-domain model finder for pure first-order logic with equality. Paradox is a MACE-style model finder, which means that it translates a first-order problem into a sequence of SAT problems, which are solved by a SAT solver.
• Infinox is an automated tool for analysing first-order logic problems, aimed at showing finite unsatisfiability, i.e., the absence of models with finite domains. All three tools are developed in Haskell.
• QuickSpec generates algebraic specifications for an API automatically, in the form of equations verified by random testing. http://www.cse.chalmers.se/~nicsma/quickspec.pdf
• Hip (the Haskell Inductive Prover) is a new tool to automatically prove properties about Haskell programs by using induction or co-induction. The approach taken is to compile Haskell programs to first order theories. Induction is applied on the meta level, and proof search is carried out by automated theorem provers for first order logic with equality.
• On top of Hip we built HipSpec, which automatically tries to find appropriate background lemmas for properties where only doing induction is too weak. It uses the translation and structural induction from Hip. The background lemmas are from the equational theories built by QuickSpec. Both the user-stated properties and those from QuickSpec are now tried to be proven with induction. Conjectures proved to be theorems are added to the theory as lemmas, to aid proving later properties which may require them. For more information, see http://web.student.chalmers.se/~danr/hipspec-atx.pdfthe draft paper.

Teaching.Haskell is present in the curriculum as early as the first year of the BSc programme. We have four courses solely dedicated to functional programming (of which three are MSc-level courses), but we also provide courses which use Haskell for teaching other aspects of computer science, such the syntax and semantics of programming languages, compiler construction, data structures and parallel programming.

Functional Programming continues at KU and the Computer Systems Design Laboratory in ITTC! The System Level Design Group (lead by Perry Alexander) and the Functional Programming Group (lead by Andy Gill) together form the core functional programming initiative at KU. There are three major Haskell projects at KU (as well as numerous smaller ones): the GHC rewrite plugin HERMIT (→7.3.5), the VHDL generator Kansas Lava (→7.12.2) and the JavaScript generator Sunroof (→5.2.8).

Nicolas Frisby, who defended his PhD from KU last summer, spent the spring at MSR Cambridge, working on optimizations inside the Glasgow Haskell compiler. Andrew Farmer spent the spring in Portland, OR, working with functional programmers at Portland State.

Further reading

• The Functional Programming Group: http://www.ittc.ku.edu/csdl/fpg
• CSDL website: https://wiki.ittc.ku.edu/csdl/Main_Page

The Ghent Functional Programming Group is a user group aiming to bring together programmers, academics, and others interested in functional programming located in the area of Ghent, Belgium. Our goal is to have regular meetings with talks on functional programming, organize functional programming related events such as hackathons, and to promote functional programming in Ghent by giving after-hours tutorials. While we are open to all functional languages, quite frequently, the focus is on Haskell, since most attendees are familiar with this language. The group has been active for two and a half years, holding meetings on a regular basis.

We have reported in previous HCARs on the first eleven meetings. Since May 2012, we had a single meeting. The GhentFPG #12 meeting took place on May 8, 2012 and involved two talks.

• Tom Schrijvers — Discussion on the Flemish Programming Contest 2012, with a focus on using the right Haskell data types for solving several of the given problems.
• Jasper Van der Jeugt — Tutorial on parallelisation in Haskell.

The plans for the fall 2012 Hackathon have shifted due to busy schedules of the GhentFPG organisers. In this academic year, we do plan to review the approach used during the meetings, because talks seem to attract more attendees compared to problem solving or coding events.

If you want more information on GhentFPG you can follow us on twitter (@ghentfpg), via Google Groups (http://groups.google.com/group/ghent-fpg), or by visiting us at irc.freenode.net in channel #ghentfpg.

Further reading

• http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group
• http://groups.google.com/group/ghent-fpg