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Extensible Type Unification
by Leon Smith 08 Feb '13

08 Feb '13

I've been toying with some type-level programming ideas I just can't quite make work, and it seems what I really want is a certain kind of type unification. Basically, I'd like to introduce two new kind operators: kind Set as -- a finite set of ground type terms of kind as kind Range as = Range (Set as) (Set as) -- a greatest lower bound and a least upper bound A type expression of kind (Set as) would either be a type variable of kind (Set as), or set of *ground* type terms of kind (as). A type expression of kind (Range as) would be a type variable of kind (Range as), or two type expressions of kind (Set as). To unify ground terms of these types, one would compute as follows: unifySets xs ys | xs == ys = Just xs | otherwise = Nothing unifyRanges (Range glb0 lub0) (Range glb1 lub1) | glb' `subset` lub' = Just (Range glb' lub') | otherwise = Nothing where glb' = glb0 `union` glb1 lub' = lub0 `isect` lub1 I say sets of ground types, because I have no idea what unification between sets of non-ground types would mean, and I really don't need it. Among applications that came to mind, one could use this to create a restricted IO abstraction that could tell you which kinds of actions might be taken (read from mouse, talk to network, etc), and be able to run only those scripts that are restricted to certain resources. Or, one could define a singular GADT that represents messages/events decorated by various categories, and then define functions that only operate on selected categories of messages. E.G. for something vaguely IRC-ish, one could write something like: data EventCategory = Channel | Presence | Information | Conversation data Event :: Range EventCategory -> * where ChanCreate :: ... -> Event (Range (Set '[Channel]) a) ChanJoin :: ... -> Event (Range (Set '[Channel]) a) ChanLeave :: ... -> Event (Range (Set '[Channel]) a) PresAvailable :: ... -> Event (Range (Set '[Presence]) a) PresAway :: ... -> Event (Range (Set '[Presence]) a) WhoisQuery :: ... -> Event (Range (Set '[Information]) a) WhoisResponse :: ... -> Event (Range (Set '[Information]) a) Message :: ... -> Event (Range (Set '[Conversation]) a) And then be able to write functions such as dispatch :: Event (Range a (Set '[Channel, Conversation])) -> IO () dispatch e = case e of ChanCreate{..} -> ... ChanJoin{..} -> ... ChanLeave{..} -> ... Message{..} -> ... In this case, the case analysis tool would be able to warn me if I'm missing any possible events in this dispatcher, or if I have extraneous events that I can't be passed (according to my type.) Anyway, I've been trying to see if I can't come up with something similar using existing type-level functionality in 7.6, with little success. (Though I'm not very experienced with type-level programming.) If not, might it be possible to add some kind of extensible unification mechanism, in furtherance of type-level programming? Best, Leon

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Announce: LA Haskell User Group February Meeting
by Phil Freeman 07 Feb '13

07 Feb '13

Dear Haskellers, There will be a meeting of the LA Haskell User Group on Wednesday February 13th at 7pm. The details, including a list of discussion topics, as they become available, can be found here: http://www.meetup.com/Los-Angeles-Haskell-User-Group/events/102199892/ Thanks, Phil Freeman.

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Either example
by Jacob Thomas 07 Feb '13

07 Feb '13

Hello I'm new to Haskell, and need help with figuring out the Either type... Any example to show how this works? Jacob

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Sol vingt simple problem in various languages and systems
by Doaitse Swierstra 07 Feb '13

07 Feb '13

At https://gist.github.com/ckirkendall/2934374 you find solutions in many languages to a simple programming problem: the evaluation of an expression in an environment. I want to point your attention to the currently last three entries: - one using the UUAGC compiler to solve the problem in an aspect oriented way (entered by Jeroen Bransen (jbransen)) - one using some Haskell features, libraries and extensions (entered by me (doaitse)) - one using our attribute grammar combinators from the AspectAG package, which expresses the various aspects in an attribute grammar style (entered by Marcos Viera (marcosomarviera)) In case you did not manage to avoid getting wound up in a discussion about programming languages you can find ammunition here. Doaitse

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GHCi (7.4.2) is working on ARM
by kenny lu 07 Feb '13

07 Feb '13

I've got a Cubieboard a few weeks back. I started it off with Linaro (Ubuntu) 12.06. Today I started to upgrade the OS to 12.11, which surprisingly came with ghc 7.4.2. And ghci magically works too. Here are the links http://www.cubieboard.org sudo apt-get install update-manager-core sudo apt-get update sudo apt-get upgrade sudo do-release-upgrade Regards, Kenny

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Haskell Weekly News: Issue 257
by Daniel Santa Cruz 07 Feb '13

07 Feb '13

Welcome to issue 257 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of January 27 to February 02, 2013. Quotes of the Week * shachaf: Everyone forgets about Agda Lovelace, the first constructivist. * applicative: I have my new ghc and am reinstalling my feeble world * wuttf: My world is starting to feel like isToyLanguage x = x /= Haskell * PenguinOfDoom: Being enlightened gentlemen, we split all programming languages into two groups, sucks and doesn't-suck and put all of them into the first group. * Taneb: lens has got to be the only library with more contributors than people who know how it works * shachaf: I like to put all my application's modules under GHC.* That's because I use the GHC API, and I want them not to conflict. Top Reddit Stories * The FP Complete School of Haskell goes beta Domain: fpcomplete.com, Score: 84, Comments: 45 On Reddit: [1] http://goo.gl/dEEyU Original: [2] http://goo.gl/0eKLt * My reaction to: How To Exclude Women From Your Technical Community: A Tutorial Domain: self.haskell, Score: 75, Comments: 145 On Reddit: [3] http://goo.gl/dVnVv Original: [4] http://goo.gl/dVnVv * Edward Yang: The GHC scheduler Domain: blog.ezyang.com, Score: 72, Comments: 1 On Reddit: [5] http://goo.gl/pkCmD Original: [6] http://goo.gl/SP1s3 * GHC HEAD now has support for named holes; an extension of -XTypeHoles. Feedback welcome! Domain: github.com, Score: 44, Comments: 32 On Reddit: [7] http://goo.gl/87Pbw Original: [8] http://goo.gl/cqX9N * LiquidHaskell - Safely Catching A List By Its Tail Domain: goto.ucsd.edu, Score: 44, Comments: 2 On Reddit: [9] http://goo.gl/uxU64 Original: [10] http://goo.gl/9aS35 * ProseDoc: A literate programming tool for Haskell inspired by docco Domain: shangaslammi.github.com, Score: 43, Comments: 12 On Reddit: [11] http://goo.gl/x4YGE Original: [12] http://goo.gl/sM4bN * ANNOUNCE: GHC version 7.6.2 Domain: haskell.org, Score: 39, Comments: 16 On Reddit: [13] http://goo.gl/BS6Sm Original: [14] http://goo.gl/RPF00 * What you consider "advanced" Haskell topics? Domain: self.haskell, Score: 34, Comments: 45 On Reddit: [15] http://goo.gl/JRgzG Original: [16] http://goo.gl/JRgzG * How to write unmaintainable Haskell code Domain: github.com, Score: 34, Comments: 49 On Reddit: [17] http://goo.gl/V0nxC Original: [18] http://goo.gl/G8wNv * Introduction to Haskell, Lecture 3 is Live (Types and Typeclasses) Domain: shuklan.com, Score: 32, Comments: 5 On Reddit: [19] http://goo.gl/JyH81 Original: [20] http://goo.gl/bve7N * Proposal: A trivial Cabal package signing utility for use with Hackage Domain: github.com, Score: 32, Comments: 61 On Reddit: [21] http://goo.gl/R2l9g Original: [22] http://goo.gl/IpYSo Top StackOverflow Questions * Haskell/GHC UndecidableInstances - example for non-terminating type check? votes: 19, answers: 2 Read on SO: [23] http://goo.gl/KX8oj * Efficient parallel strategies votes: 15, answers: 2 Read on SO: [24] http://goo.gl/aDnW2 * Is it possible to implement MonadFix for `Free`? votes: 15, answers: 2 Read on SO: [25] http://goo.gl/uKOrO * Do Haskell imports have side effects? votes: 15, answers: 1 Read on SO: [26] http://goo.gl/sjltX * What monads can be expressed as Free over some functor? votes: 10, answers: 1 Read on SO: [27] http://goo.gl/Q8MF9 * What is the point of having a lazy/strict version of Writer? votes: 10, answers: 1 Read on SO: [28] http://goo.gl/InUsm * Why can applicative functors have side effects, but functors can't? votes: 9, answers: 4 Read on SO: [29] http://goo.gl/LxUi5 * How to write fixed point build rules in Shake, e.g. Latex votes: 9, answers: 1 Read on SO: [30] http://goo.gl/Ef82G * Event handling in Netwire compared to conventional FRP frameworks votes: 9, answers: 2 Read on SO: [31] http://goo.gl/mA4Kc Until next time, Daniel Santa Cruz References 1. http://fpcomplete.com/school-of-haskell-goes-beta/ 2. http://www.reddit.com/r/haskell/comments/17ngtq/the_fp_complete_school_of_h… 3. http://www.reddit.com/r/haskell/comments/17jy0e/my_reaction_to_how_to_exclu… 4. http://www.reddit.com/r/haskell/comments/17jy0e/my_reaction_to_how_to_exclu… 5. http://blog.ezyang.com/2013/01/the-ghc-scheduler/ 6. http://www.reddit.com/r/haskell/comments/17fd8c/edward_yang_the_ghc_schedul… 7. https://github.com/ghc/ghc/commit/677144b858f4a425e77399bdfbfcd43dbabd1488 8. http://www.reddit.com/r/haskell/comments/17l9xm/ghc_head_now_has_support_fo… 9. http://goto.ucsd.edu/~rjhala/liquid/haskell/blog/blog/2013/01/31/safely-cat… 10. http://www.reddit.com/r/haskell/comments/17ohbm/liquidhaskell_safely_catchi… 11. http://shangaslammi.github.com/prose-doc/ 12. http://www.reddit.com/r/haskell/comments/17keu0/prosedoc_a_literate_program… 13. http://www.haskell.org/pipermail/glasgow-haskell-users/2013-January/023661.… 14. http://www.reddit.com/r/haskell/comments/17jyw9/announce_ghc_version_762/ 15. http://www.reddit.com/r/haskell/comments/17gavl/what_you_consider_advanced_… 16. http://www.reddit.com/r/haskell/comments/17gavl/what_you_consider_advanced_… 17. https://github.com/quchen/unmaintainable_haskell/blob/master/README.md 18. http://www.reddit.com/r/haskell/comments/17idma/how_to_write_unmaintainable… 19. http://shuklan.com/haskell/lec03.html 20. http://www.reddit.com/r/haskell/comments/17ic23/introduction_to_haskell_lec… 21. https://github.com/chrisdone/cabal-sign 22. http://www.reddit.com/r/haskell/comments/17njda/proposal_a_trivial_cabal_pa… 23. http://stackoverflow.com/questions/14661073/haskell-ghc-undecidableinstance… 24. http://stackoverflow.com/questions/14624376/efficient-parallel-strategies 25. http://stackoverflow.com/questions/14636048/is-it-possible-to-implement-mon… 26. http://stackoverflow.com/questions/14666666/do-haskell-imports-have-side-ef… 27. http://stackoverflow.com/questions/14641864/what-monads-can-be-expressed-as… 28. http://stackoverflow.com/questions/14644830/what-is-the-point-of-having-a-l… 29. http://stackoverflow.com/questions/14576543/why-can-applicative-functors-ha… 30. http://stackoverflow.com/questions/14622169/how-to-write-fixed-point-build-… 31. http://stackoverflow.com/questions/14654385/event-handling-in-netwire-compa… 32. https://plus.google.com/105107667630152149014/about

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A list of Haskell-related quotes?
by Petr Pudlák 06 Feb '13

06 Feb '13

Dear haskellers, over the time I've read many funny or inspiring quotes related to Haskell, but I forgot them later. For example I vaguely remember: - "What I really like about Haskell: It's completely unlike PHP." - "To learn Haskell your brain will have to get seriously rewired." Does anybody collect them or know about such a collection? Thanks, Petr Pudlak

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Re: [Haskell-cafe] How far compilers are allowed to go with optimizations?
by Artyom Kazak 06 Feb '13

06 Feb '13

Ouch, forgot the Cafe. Would you object to this particular optimisation (replacing an algorithm with an entirely different one) if you were guaranteed that the space behaviour would not change?

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Next Meetup
by Christopher Olah 05 Feb '13

05 Feb '13

Hey All, I think we were planning on meeting on the second Wednesday of this month (Feb 13) but Hacklab has a members meeting that day. How do people feel about the 14th or 15th? Chris

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CfP: OdHac (Haskell Hackathon)
by Roman Cheplyaka 05 Feb '13

05 Feb '13

------------------------------------------------------------------------ OdHac International Haskell Hackathon May 3-5, 2013 Odessa, Ukraine http://haskell.org/haskellwiki/OdHac Co-organized with Provectus IT ------------------------------------------------------------------------ We are pleased to announce the upcoming Haskell Hackathon! The event will be held over 3 days, May 3-5, in Odessa, Ukraine. This is a great opportunity to meet your fellow haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! To attend, please register at http://bit.ly/OdHacReg. Please propose projects at http://www.haskell.org/haskellwiki/OdHac/Projects Consider participating in our accommodation program: http://www.haskell.org/haskellwiki/OdHac/Accommodation

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