January 2012 - Haskell-Cafe

Haskell Weekly News: Issue 212
by Daniel Santa Cruz 27 Jan '12

27 Jan '12

Welcome to issue 212 of the HWN, a newsletter covering developments in the Haskell community. This issue covers top Haskell stories from around the web found between January 11 to 21, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/01/haskell-weekly-news-issue-212.h… Quotes of the Week * shachaf: Haskell's type system is the perfect mix of useless and stupid. * roconnor: gez, you write one paper explaining how a lens is really a higher order monoidal natural transformation, and suddenly everyone thinks you are an expert on lenses. * roconnor: a lens is a monoidal natural transformation between higher-order coalgebra functors, what's the problem? * Enigmagic: we use the 'rsync' deploy model * cmccann: someone should implement a dependently-typed unsafeCoerce in Agda, you'd have to provide a proof that the coercion is invalid * ghc: Couldn't match type `No' with `Yes' Top Reddit Stories * Idris - A language with dependent types Domain: idris-lang.org, Score: 43, Comments: 30 On Reddit: [1] http://goo.gl/pZdKX Original: [2] http://goo.gl/MxXKx * Purely Functional, Declarative Game Logic Using Reactive Programming Domain: github.com, Score: 38, Comments: 17 On Reddit: [3] http://goo.gl/ByPJj Original: [4] http://goo.gl/1PYa4 * A guided tour through the bytestring library Domain: meiersi.github.com, Score: 32, Comments: 10 On Reddit: [5] http://goo.gl/0YUQ2 Original: [6] http://goo.gl/A2u2S * Comprehensions Domain: patternsinfp.wordpress.com, Score: 26, Comments: 7 On Reddit: [7] http://goo.gl/9Zv3F Original: [8] http://goo.gl/SNA14 * Thinking of writing a tutorial series on FP and Haskell. Here's a demo. Domain: codingtales.com, Score: 24, Comments: 9 On Reddit: [9] http://goo.gl/eU99Y Original: [10] http://goo.gl/1efUI * UHC compiles Haskell to JavaScript. Web game using Haskell for Ludum Dare? Domain: chrisdone.com, Score: 23, Comments: 4 On Reddit: [11] http://goo.gl/og088 Original: [12] http://goo.gl/8Op3M * Warp Conduits Released! Domain: yesodweb.com, Score: 22, Comments: 3 On Reddit: [13] http://goo.gl/zpHnZ Original: [14] http://goo.gl/5Vl3w * Why are guards in the list comprehension faster than in the do-notation? Domain: self.haskell, Score: 19, Comments: 22 On Reddit: [15] http://goo.gl/1T8Kk Original: [16] http://goo.gl/1T8Kk * Hac Boston Domain: self.haskell, Score: 19, Comments: 7 On Reddit: [17] http://goo.gl/xWWZF Original: [18] http://goo.gl/xWWZF * Easy steps to install gtk and use all its features on OS X lion Domain: gist.github.com, Score: 16, Comments: 10 On Reddit: [19] http://goo.gl/Teh6P Original: [20] http://goo.gl/Su6Y1 Top StackOverflow Questions * What is this haskell syntax? votes: 19, answers: 1 Read on SO: [21] http://goo.gl/JCYg0 * Why is the type of this function (a -> a) -> a? votes: 18, answers: 4 Read on SO: [22] http://goo.gl/BTr8e * `Integer` vs `Int64` vs `Word64` votes: 13, answers: 1 Read on SO: [23] http://goo.gl/Pc2CX * Did I just write a continuation? votes: 12, answers: 1 Read on SO: [24] http://goo.gl/aXnTw * How lazy is Haskell's `++`? votes: 12, answers: 3 Read on SO: [25] http://goo.gl/GxPxC * Can Haskell be used to write shell scripts? votes: 12, answers: 4 Read on SO: [26] http://goo.gl/u3Qke * Why does this Haskell code compile? votes: 12, answers: 2 Read on SO: [27] http://goo.gl/yz6VB * Any nice tools for untying knots in Haskell? votes: 11, answers: 4 Read on SO: [28] http://goo.gl/YGNeq * Two-dimensional zipper votes: 10, answers: 2 Read on SO: [29] http://goo.gl/37E9J * the way merge-sort faster than insertion-sort puzzles me votes: 10, answers: 2 Read on SO: [30] http://goo.gl/mkmdc * Haskell function that takes a type and a value and checks if value has that type votes: 10, answers: 1 Read on SO: [31] http://goo.gl/tJO81 Until next time, Daniel Santa Cruz References 1. http://idris-lang.org/ 2. http://www.reddit.com/r/haskell/comments/ol90m/idris_a_language_with_depend… 3. https://github.com/leonidas/codeblog/blob/master/2012/2012-01-17-declarativ… 4. http://www.reddit.com/r/haskell/comments/ol7vm/purely_functional_declarativ… 5. http://meiersi.github.com/HaskellerZ/meetups/2012%2001%2019%20-%20The%20byt… 6. http://www.reddit.com/r/haskell/comments/oqjhy/a_guided_tour_through_the_by… 7. http://patternsinfp.wordpress.com/2012/01/19/comprehensions/ 8. http://www.reddit.com/r/haskell/comments/on8a6/comprehensions/ 9. http://codingtales.com/2012/01/16/understanding-foldl-using-foldr-from-real… 10. http://www.reddit.com/r/haskell/comments/oi4l2/thinking_of_writing_a_tutori… 11. http://chrisdone.com/posts/2012-01-06-uhc-javascript.html 12. http://www.reddit.com/r/haskell/comments/olthb/uhc_compiles_haskell_to_java… 13. http://www.yesodweb.com/blog/2012/01/warp-conduits 14. http://www.reddit.com/r/haskell/comments/on285/warp_conduits_released/ 15. http://www.reddit.com/r/haskell/comments/oolyt/why_are_guards_in_the_list_c… 16. http://www.reddit.com/r/haskell/comments/oolyt/why_are_guards_in_the_list_c… 17. http://www.reddit.com/r/haskell/comments/opgi1/hac_boston/ 18. http://www.reddit.com/r/haskell/comments/opgi1/hac_boston/ 19. https://gist.github.com/1655271 20. http://www.reddit.com/r/haskell/comments/or0ro/easy_steps_to_install_gtk_an… 21. http://stackoverflow.com/questions/8905178/what-is-this-haskell-syntax 22. http://stackoverflow.com/questions/8918990/why-is-the-type-of-this-function… 23. http://stackoverflow.com/questions/8873000/integer-vs-int64-vs-word64 24. http://stackoverflow.com/questions/8867837/did-i-just-write-a-continuation 25. http://stackoverflow.com/questions/8872714/how-lazy-is-haskells 26. http://stackoverflow.com/questions/8925780/can-haskell-be-used-to-write-she… 27. http://stackoverflow.com/questions/8935568/why-does-this-haskell-code-compi… 28. http://stackoverflow.com/questions/8945096/any-nice-tools-for-untying-knots… 29. http://stackoverflow.com/questions/8905030/two-dimensional-zipper 30. http://stackoverflow.com/questions/8919785/the-way-merge-sort-faster-than-i… 31. http://stackoverflow.com/questions/8957601/haskell-function-that-takes-a-ty…

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Wow you have to check this out Haskell
by R J 27 Jan '12

27 Jan '12

hello Haskell the holidays are coming up soon and I think this can help http://www.news13open.com

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Re: [Haskell-cafe] Need advice: Haskell in Web Client
by dokondr 26 Jan '12

26 Jan '12

On Tue, Jan 17, 2012 at 6:42 PM, John Lenz <lenz(a)math.uic.edu> wrote: > > HTML5 Canvas is great for charts. If you go this route you might as well > use a library which draws charts for you instead of writing all this code > yourself. > > Personally, I use extjs version 4 which has some amazing charts, but there > are other libraries out there. > > http://www.sencha.com/**products/extjs/examples/#**sample-3<http://www.sencha.com/products/extjs/examples/#sample-3> > > Essentially the server provides the data in JSON or XML some other format, > and the extjs code draws the charts on the client side. > > If you go with extjs, then the server side I would suggest a small, simple > yesod or snap server. You could probably get the server under a hundred > lines of code with yesod; see some of the examples in the yesod book. > yesod or snap would serve JSON of the statistics on request, and also > serve the javascript files which draw the charts. > Yes, I was thinking about using Haskell to generate everything that specific Javascript library needs to display charts in browser. Naturally charts are to be displayed by this library itself. I also would like to have Haskell tools to generate Web GUI in Javascript. As for yesod, I am not sure that I like approach which mixes HTML with code, or even worse - creates a new weird HTML-like language like 'whamlet quasi-quotation', for example:  I prefer using Turing complete PL to program web client, like the one used in GWT (Java) or Cappuccino (Objective-J). http://cappuccino.org/learn/ In this case you /almost/ don't need to know HTML, CSS, DOM, Ajax, etc. to develop WebUI and good PL lets you concentrate on problem domain instead of bothering about browser support. It is a real pity that Haskell still has no such tools to generate Web GUI in Javascript. (((

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Why were unfailable patterns removed and "fail" added to Monad?
by Gregory Crosswhite 26 Jan '12

26 Jan '12

Today I learned (tldr; TIL) that the "fail" in the Monad class was added as a hack to deal with the consequences of the decision to remove "unfailable" patterns from the language. I will attempt to describe the story as I have picked it up from reading around, but please feel free to correct me on the details. :-) An "unfailable" pattern (which is a generalization of an "irrefutable" pattern) is a pattern which can never fail (excluding the possibility of _|_), such as let (x,y) = pair Before "fail" was a method of the Monad class, using refutable patterns in a monad required the type to be an instance of MonadZero (that is, MonadPlus without the plus), so that for example do Just x <- m required that the monad be an instance of MonadZero. If you avoided such patterns, your Monad did not have to have this instance, so that for example do (x,y) <- pair would not require MonadZero because the pattern is unfailable. To me this seems like a lovely way of handling the whole matter, and much improved over the incredibly ugly wart of having a "fail" method in the Monad class. In fact, I think I remember people on this list and in other forums occasionally bringing something like this approach up as a way of getting rid of the "fail" wart. So my question is, why did we go to all of the trouble to transition away from the MonadZero approach to the current system to begin with? What was so bad about "unfailable" patterns that it was decided to remove them and in doing so replace MonadZero with a mandatory "fail" method in Monad? I mean, this *is* Haskell, so my safest assumption is that smart people were involved in making this decision and therefore the reasons much have been really good (or at least, seemed good given the information at the time). :-) Cheers, Greg

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LDTA 2012: call for participation
by Emilie Balland 26 Jan '12

26 Jan '12

*************************************************************************************** LDTA 2012 Call for Participation 12th International Workshop on Language Descriptions, Tools, and Applications http://ldta.info Tallinn, Estonia March 31 & April 1, 2012 an ETAPS workshop *************************************************************************************** You are cordially invited to LDTA'12. LDTA is an application and tool-oriented workshop focused on grammarware - software based on grammars in some form. Grammarware applications are typically language processing applications and traditional examples include parsers, program analyzers, optimizers and translators. -- REGISTRATION-- To register for LDTA'12, please follow the instructions at: http://www.etaps.org/2012/registration The early registration deadline is January 29, 2012. --HOTEL INFORMATION-- LDTA will be co-located with ETAPS. Please visit ETAPS's web site to get more information for the accomodation (http://www.etaps.org/2012/accommodation) Special rates and/or block bookings for ETAPS 2012 participants at a number of central hotels also expire Sunday, 29 January 2012. --PROGRAM-- The workshop will include an invited talk by Janis Voigtländer (University of Bonn, Germany) and the presentation of the following technical papers: - Sebastian Erdweg, Paolo G. Giarrusso and Tillmann Rendel. Language Composition Untangled - Johnni Winther. Improving Precision of Generated ASTs - Tobi Vollebregt, Lennart C. L. Kats and Eelco Visser. Declarative Specification of Template-Based Textual Editors - Marcos Viera, Doaitse Swierstra and Atze Dijkstra. Grammar Fragments Fly First-Class - Marcos Viera, Doaitse Swierstra and Arie Middelkoop. UUAG Meets AspectAG: How to make Attribute Grammars First-Class - Daniel Lincke and Sibylle Schupp. From HOT to COOL - Transforming Higher-Order Typed Languages to Concept-Constrained Object-Oriented Languages - Maartje De Jonge and Eelco Visser. A Language Generic Solution for Name Binding Preservation in Refactorings - Vadim Zaytsev. Notation-Parametric Grammar Recovery - Jean-Christophe Bach, Pierre-Etienne Moreau, Marc Pantel and Xavier Crégut. Models Transformations with Tom The workshop will also feature presentations devoted to a de-brief of the 2011 tool challenge, based on the paper currently being prepared by the challenge participants.

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Hierarchical tracing for debugging laziness
by Eugene Kirpichov 26 Jan '12

26 Jan '12

Hi cafe, Look how one can watch the evaluation tree of a computation, to debug laziness-related problems. {-# LANGUAGE BangPatterns #-} module HTrace where import Data.List (foldl') import Data.IORef import System.IO.Unsafe level = unsafePerformIO $ newIORef 0 htrace str x = unsafePerformIO $ do lvl <- readIORef level putStrLn (replicate (4*lvl) ' ' ++ str) writeIORef level (lvl+1) let !vx = x writeIORef level lvl return vx xs = map (\x -> htrace (show x) x) [1..10] s = foldl (\a b -> htrace "+" (a+b)) 0 xs s2 = foldl' (\a b -> htrace "+" (a+b)) 0 xs b = htrace "b" 2 c = htrace "c" 3 a = htrace "a" $ b + c x = htrace "x" $ b + c *HTrace> a a b c 5 *HTrace> x x 5 *HTrace> s + + + + + + + + + + 1 2 3 4 5 6 7 8 9 10 55 (reload) *HTrace> s2 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 55 -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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where to put general-purpose utility functions
by Joey Hess 26 Jan '12

26 Jan '12

I'm finding a rather unusual problem as I write haskell.. Unlike every other language I've used, large portions of my haskell code are turning out to be general-purpose, reusable code. Fully 20% of the haskell code I've written for git-annex is general purpose. Now, I came out of a decade of perl with maybe 1% reusable code. So I'm sure this is a credit to haskell, and not to me. My problem now is that as I start new projects, I want to have my haskell utility functions available, and copying them around is not ideal. So, put them on hackage. But where, exactly? It already has several grab bag utility libraries. The only one with much traction is MissingH. Using the others makes a program have an unusual dependency, which while only a cabal install away, would make work for distributions that want to package the program. I've ruled out using a couple on that basis. Doesn't encourage me to add another one. My 2000+ lines of reusable code are a grab-bag of generic utility functions. Looking them over (see Appendix), I could try to get portions into existing libraries on hackage, but it's unlikely I'd find a home for most of them, so I'm still left with this problem of what to do. I wonder if the model used for xmonad-contrib, of a big library package, that is very open to additions from contributors, would be helpful here? John, any interest in moving MissingH in this direction? I get the impression it's not otherwise changing much lately, and parts of it are becoming naturally obsolete, maybe this could inject some life into it. Any other thoughts you have on grab-bag utility libraries on hackage also appreciated. ---- Appendix: A sample of a a few of the better functions from my utility library. Some quite generic monadic control functions, few of them truely unique: whenM :: Monad m => m Bool -> m () -> m () -- also >>? unlessM :: Monad m => m Bool -> m () -> m () -- also >>! firstM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a) A module that exports functions conflicting with partial functions in the Prelude, to avoid them being accidentially used. And provides some alternatives (which overlap somewhat with Safe): headMaybe :: [a] -> Maybe a readMaybe :: Read a => String -> Maybe a beginning :: [a] -> [a] Various path manipulation functions such as: dirContains :: FilePath -> FilePath -> Bool dotfile :: FilePath -> Bool absPath :: FilePath -> IO FilePath Other stuff: separate :: (a -> Bool) -> [a] -> ([a], [a]) catchMaybeIO :: IO a -> IO (Maybe a) readSize :: [Unit] -> String -> Maybe ByteSize -- parses "100 kb" etc format :: Format -> Variables -> String findPubKeys :: String -> IO GpgKeyIds boolSystem :: FilePath -> [CommandParam] -> IO Bool withTempFile :: Template -> (FilePath -> Handle -> IO a) -> IO a -- see shy jo

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"Re: Wow you have to check this out Haskell" - appols accidental resend of this scam spam
by Steve Horne 25 Jan '12

25 Jan '12

Lesson = don't open e-mail client while borderline asleep.

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Just found GHCI can be used as a debugger
by Yves Parès 25 Jan '12

25 Jan '12

Hello, By randomly typing :<Tab> under GHCI, I discovered it could be used as a debugger (:breakpoint, :step, etc.) Three questions in that regard: 1) Is there some documentation about it? 2) I haven't investigated much, is that fairly thorough and convenient? Does some people over here use those functions on a regular basis? 3) Is it possible to save the state (notably to save the position of the breakpoints)? Thanks!

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XMonad.Shell.Prompt and unicode input
by Eugene Dzhurinsky 25 Jan '12

25 Jan '12

Hello, community! I have some strange issue with recent XMonad: with using of standard Shell module it is not possible to use unicode input. It simply freezes up and in console I can observe: Enum.toEnum{Word8} : tag (1092) is outside of bounds (0,255). What function may cause such error indirectly? I looked through the sources - but have no idea where such conversion may have place and why Word8 is used. -- Eugene N Dzhurinsky

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