November 2010 - Haskell-Cafe

Unexpected results from ExitCode
by Peter Schmitz 04 Nov '10

04 Nov '10

I have a program (test.hs): > module Main (main) where > import System.Exit > main :: IO ExitCode > main = do > return (ExitFailure 1) In another program, I invoke it via 'system': > exitCode <- system ".\\test.exe" > case (exitCode) of > ExitFailure failCnt -> do > putStrLn $ "-- Fail count is: " ++ show failCnt > exitFailure > ExitSuccess -> do > putStrLn $ "-- OK." > exitSuccess but it always gets ExitSuccess (not ExitFailure failCnt as I expected). (I am running under Windows.) My other use of system (such as with ghc commands) works okay, sometimes getting success, sometimes failure, as expected. Any suggestions much appreciated. Thanks, -- Peter

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Haskell Weekly News: Issue 157 - November 03, 2010
by Daniel Santa Cruz 03 Nov '10

03 Nov '10

Welcome to issue 157 of the HWN, a newsletter covering developments in the [1]Haskell community in the week of October 24 - 30. Seems that things quieted down a bit this week. We saw 85 new or updated packages in Hackage, 29 (-5) stories posted to Reddit, 25 (-22) questions posted to StackOverflow, and 302 (-20) messages posted on the -cafe mailing list. Maybe some of the effects of ICFP have died down a bit? Don Stewart posted some interesting numbers comparing different programming communities in Reddit. As we can see, we seem to be very well represented: /r/python 16.3k users /r/javascript 8k users /r/ruby 6k users /r/php 5.7k users /r/haskell 5.5k users /r/cpp 5.3k users /r/lisp 3.9k users /r/perl 3.5k users /r/java/ 3k users /r/erlang 1.8k users /r/c_programming 1.7k users /r/scheme 1.2k users /r/clojure 1.2k users /r/asm 1.1k users /r/scala 1k users We hope to see pictures, videos, and blog posts from the people that are attending BelHac this weekend! So, what was hot last week? Announcements Anton Kholomiov [2]announced his new csound-expression combinator libary. It features liberation from id-style csound code, haskore-like composition structures, type-safe composable opcodes and simple instrument interface. Maciej Piechotka [3]announced a new version of iteratee-compress, which provides compressing and decompressing enumerators including flushing. Currently only gzip is available, but bzip is planned. Tim Docker [4]announced an update to his Chart (v0.14) library. This is a library for drawing 2D data plots. Eugene Kirpichov [5]announced an update to his timeplot library. This library is a tool for visualizing time series from log files. He also provided us with some [6]pretty pictures. Niklas Broberg couldn't find a library to manipulate Java source code, so he [7]announced the creation of language-java. One of the [8]bigger updates last week was brought to us by the darcs team. It contains many improvements over the 2.4 series. Most notable are the performance improvements in record and pull, and the --bisect option for trackdown. Way to go guys! John Lato [9]announced the initial release of mutable-iter, an extension to the iteratee package which uses mutable buffers. Top Reddit Stories * Neat Haskell commenting trick Domain: self.haskell Score: 77, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/dyvsi/neat_haskell_commenting_tric… Original: /r/haskell/comments/dyvsi/neat_haskell_commenting_trick/ * Haskell Halloween! Domain: i.imgur.com Score: 53, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/dyznc/haskell_halloween/ Original: http://i.imgur.com/t7qWf.jpg * OCaml for Haskellers (Inside 245s) Domain: blog.ezyang.com Score: 41, Comments: 27 On Reddit: http://www.reddit.com/r/haskell/comments/dw2cy/ocaml_for_haskellers_inside_… Original: http://blog.ezyang.com/2010/10/ocaml-for-haskellers/ * MTL 2.0 has been uploaded good bye old mtl, hello monads-fd! Domain: hackage.haskell.org Score: 40, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/dx8ml/mtl_20_has_been_uploaded_goo… Original: http://hackage.haskell.org/package/mtl-2.0.0.0 * Linear vs. compositional type checking Domain: gergo.erdi.hu Score: 34, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/dwip1/linear_vs_compositional_type… Original: http://gergo.erdi.hu/blog/2010-10-23-the_case_for_compositional_type_checki… * Tour of a real toy Haskell program Domain: mainisusuallyafunction.blogspot.com Score: 29, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/dwjvo/tour_of_a_real_toy_haskell_p… Original: http://mainisusuallyafunction.blogspot.com/2010/10/tour-of-real-toy-haskell… * Ride the snake: Calling Python libraries from Haskell Domain: john-millikin.com Score: 26, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/dxj0e/ride_the_snake_calling_pytho… Original: http://john-millikin.com/articles/ride-the-snake/ * Announcing: Snap Framework v0.2.15 Domain: snapframework.com Score: 25, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dy94z/announcing_snap_framework_v0… Original: http://snapframework.com/blog/2010/10/27/snap-0.2.15-released * Yesod 0.6.0 Released Domain: docs.yesodweb.com Score: 23, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/dwn5v/yesod_060_released/ Original: http://docs.yesodweb.com/blog/yesod-0-6/ * darcs 2.5 released Domain: wiki.darcs.net Score: 21, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/dyo7d/darcs_25_released/ Original: http://wiki.darcs.net/Releases/2.5 Top StackOverflow Questions * type constructor or class error Question (votes: 0, answers: 4): Read on SO: http://stackoverflow.com/questions/4060486/type-constructor-or-class-error * parse error in else Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4053638/parse-error-in-else * xmonad could not find module `XMonad': broken package Question (votes: 0, answers: 1): Read on SO: http://stackoverflow.com/questions/4037101/xmonad-could-not-find-module-xmo… * Haskell: I/O and Returning From a Function Question (votes: 1, answers: 3): Read on SO: http://stackoverflow.com/questions/4056867/haskell-i-o-and-returning-from-a… * Collapse arrow to list Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4030439/collapse-arrow-to-list * how to force the evaluation of this expression? Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4027097/how-to-force-the-evaluation-of-t… * How to get type parameters from SYBs dataTypeOf Question (votes: 1, answers: 1): Read on SO: http://stackoverflow.com/questions/4007693/how-to-get-type-parameters-from-… * How to sort the list by its accountID using quick sort in Haskell. Question (votes: 1, answers: 2): Read on SO: http://stackoverflow.com/questions/4006811/how-to-sort-the-list-by-its-acco… * Parsec parse many question Question (votes: 2, answers: 2): Read on SO: http://stackoverflow.com/questions/4058765/parsec-parse-many-question * Type error using Criterion Question (votes: 2, answers: 2): Read on SO: http://stackoverflow.com/questions/4033103/type-error-using-criterion HackageDB Updates Who contributed, and what got uploaded? * AdamVogt: xmonad-extras * AlexSuraci: atomo * AndreasVoellmy: nettle-openflow * AndrewCoppin: AC-EasyRaster-GTK, AC-Terminal * BenLippmeier: buildbox, buildbox-tools * BryanOSullivan: text-icu * ChrisDone: throttle * ChrisKuklewicz: hprotoc, protocol-buffers-descriptor * ChristiaanBaaij: clash * ClaudeHeilandAllen: gmndl * DanDoel: bytestring-show * DanielDiaz: DTC * DougBeardsley: heist * ErikHesselink: flock * EugeneKirpichov: timeplot * GaneshSittampalam: HTTP * GregoryCollins: snap-core, snap-server * GregoryWeber: sifflet, sifflet-lib * GrzegorzChrupala: morfette * HeinrichApfelmus: operational * IlyaPortnov: todos * JacobStanley: network-info * JanRochel: SoccerFun * JasperVanDerJeugt: hakyll * JeremyShaw: web-routes-happstack * JinjingWang: practice-room * JohanTibell: network * JohnGoerzen: twidge * JohnLato: ListLike, hCsound, iteratee, iteratee-mtl, mutable-iter, sndfile-enumerators * JohnMacFarlane: texmath * JohnMillikin: cpython, expat-enumerator, libxml-enumerator, libxml-sax, ncurses, yajl-enumerator * JudeNagurney: augeas * KetilMalde: bio, dephd, flower * KiYungAhn: yices * MaciejPiechotka: iteratee-compress * MarcosViera: AspectAG * MattBrown: MissingPy * MaxBolingbroke: ansi-terminal * MichaelIlseman: yaml-light * MichaelSnoyman: authenticate, hamlet, neither, persistent, persistent-postgresql, persistent-sqlite, wai, yesod, yesod-auth, yesod-examples * MichelBoucey: ip6addr * NiklasBroberg: language-java * PetrRockai: shellish * ReinierLamers: darcs * RichardSmith: linux-ptrace, posix-waitpid * RossPaterson: mtl * SebastianFischer: weighted-regexp * SinisaBidin: websockets * ThomasDuBuisson: crypto-api, hsXenCtrl, pureMD5 * ThomasSchilling: murmur-hash * TimDocker: Chart * TomHawkins: improve * TonyMorris: JSON-Combinator * VincentHanquez: cryptohash * YoshikuniJujo: mtlparse, regexpr About the Haskell Weekly News New editions are posted to [10]the Haskell mailing list as well as to [11]the Haskell Sequence and [12]Planet Haskell. [13]RSS is also available, and headlines appear on [14]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://hackage.haskell.org/package/csound-expression 3. http://groups.google.com/group/haskell-cafe/browse_thread/thread/faf1da04ba… 4. http://groups.google.com/group/haskell-cafe/browse_thread/thread/0759530428… 5. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4215122a77… 6. http://www.haskell.org/haskellwiki/Timeplot 7. http://groups.google.com/group/haskell-cafe/browse_thread/thread/4c9b3e1499… 8. http://groups.google.com/group/haskell-cafe/browse_thread/thread/229c751ebc… 9. http://article.gmane.org/gmane.comp.lang.haskell.general/18274 10. http://www.haskell.org/mailman/listinfo/haskell 11. http://sequence.complete.org/ 12. http://planet.haskell.org/ 13. http://sequence.complete.org/node/feed 14. http://haskell.org/

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Generalized monadic exception handling with monad-peel
by Anders Kaseorg 03 Nov '10

03 Nov '10

I just released the monad-peel library to Hackage. http://hackage.haskell.org/package/monad-peel MonadPeelIO is a simple class that allows lifting monadic control operations, such as those in Control.Exception and Foreign.Marshal.Alloc, through layers of monad transformers. It comes with instances for every transformer in the transformers package except ContT, and uses no Haskell extensions. class MonadTrans t => MonadTransPeel t where peel :: (Monad m, Monad n, Monad o) => t n (t m a -> m (t o a)) class MonadIO m => MonadPeelIO m where peelIO :: m (m a -> IO (m a)) I’ve included a wrapped version of Control.Exception with types generalized to all monads in MonadPeelIO, for example: catch :: (MonadPeelIO m, Exception e) => m a -> (e -> m a) -> m a catch a handler = do k <- peelIO join $ liftIO $ Control.Exception.catch (k a) (\e -> k $ handler e) I’m hoping this package can become a suitable replacement for MonadCatchIO, whose implementation of ‘finally’ was recently shown to be broken with certain monads (see “MonadCatchIO, finally and the error monad” on this list). The home page has links to a Git repository and has references to older mailing list threads providing some background: http://andersk.mit.edu/haskell/monad-peel/ Thanks to everyone who contributed to those threads and helped shape the ideas that led to this package (and if it turns out more ideas are needed, thanks for those too!). Anders

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Bug in HTTP (bad internal error handling)
by Bit Connor 03 Nov '10

03 Nov '10

Hello, I have reported this problem to the maintainer of the HTTP package about 2 weeks ago, but have not yet received a response, so I am reporting it here. I am using a recent git check out of HTTP 4000.0.10 The bufferReadLine function from Network.TCP has a bug in how it handles an IOException after it catches one. It checks for isEOFError e, and that branch of code seems to be ok. The problem is how it handles other types of IO errors. One type of such an error, for example, is: <socket: 117>: Data.ByteString.hGetLine: resource vanished (Connection reset by peer) After it catches this error, the function returns (line 376): return (fail (show e)) The "fail" is running in the Either monad (The Result type = Either). This calls the default Monad implementation of fail, which is just a call to plain old error. This basically causes the entire program to crash. A simple fix, is to instead just reraise the original error: ioError e This will propagate the error through as a normal IOException, which can then be caught by the caller (such as the caller of simpleHTTP). By looking at the library code, the same bug seems to be in other places (bufferGetBlock, bufferPutBlock), but I have only observed the bug actually occurring in bufferReadLine. Actually, it appears that simpleHTTP isn't actually supposed to throw an IOException, and it is instead supposed to return a ConnError result. So the real fix is to fix the code to make this happen. But I've found simpleHTTP to throw an IOException in a lot of circumstances, so for now I think the fix above is a good immediate solution. Thanks, Bit Connor

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Benchmark
by André Batista Martins 03 Nov '10

03 Nov '10

Hi, exist some benchmark "active" to use in my libraries, i want test memory usage and performance? Cheers, André

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Re: [Haskell-cafe] ffmpeg-tutorials fails to build
by John Lato 03 Nov '10

03 Nov '10

> > > Message: 10 > Date: Wed, 3 Nov 2010 00:38:53 -0200 > From: Victor Oliveira <rhapsodyv(a)gmail.com> > > Hi, > > Info: > ghc 6.12.3 (installed from haskell plataform) > Mac os x snow leopard 10.6.4 > ffmpeg instaled from ports - ffmpeg @0.6.1 > hs-ffmpeg 0.3.4 - installed from cabal-install > > When I try compile this package, I get this erros: > >

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Re: [Haskell-cafe] [ANNAUNCE] ghcjs-0.1.0 Haskell to Javascript compiler
by Martijn Schrage 03 Nov '10

03 Nov '10

On 02-11-10 18:57, Aaron Gray wrote: > On 2 November 2010 14:00, Martijn Schrage <martijn(a)oblomov.com > <mailto:martijn@oblomov.com>> wrote: > > On 01-11-10 22:35, Aaron Gray wrote: >> >> Right, FF seems okay now on Vista too. >> >> Chrome :- >> ... > It works on Firefox, Chrome and Safari now, both on Windows and > Mac. The IE problems seem a bit more subtle, as the script goes > into an infinite loop. > > > Great, thanks, I will check it out later when I get some time. > > Aaron Ok, after a bit of nasty IE Javascript debugging, I got it to work on IE8 as well. This only holds for the example at http://tryout.oblomov.com/ghcjs/ghcjs.html though. The ghcjs sources still need to be updated. Until then, you might run into some other problems using IE when building more fancy examples. -- Martijn

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darcs hacking sprint 5 report
by Eric Kow 03 Nov '10

03 Nov '10

Hi everybody, Here's a little report from the most recent Darcs hacking sprint (held 15-17 October in Orleans) http://blog.darcs.net/2010/10/darcs-hacking-sprint-5-report.html -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

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-fwhole-program (for gcc) in cabal files?
by Johannes Waldmann 03 Nov '10

03 Nov '10

Hi. In a project with Haskell + FFI, I recently split my C program into modules (files). I don't want to lose optimization though, so I want to use something like gcc -c -o complete.o -fwhole-program -combine *.c (doing whole program optimization/inlining etc., I hope). How do I write this in the cabal file? It seems ghc compiles each C source individually. Or should I use "-flto" on individual compilations? (That's what Gimple was invented for, actually?) But does this play nicely with ghc's linker? (Which is gcc, so it should be fine?) J.W.

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Scrap your rolls/unrolls
by Max Bolingbroke 03 Nov '10

03 Nov '10

Hi Cafe, In generic programming, e.g. as in "Data Types a la Carte" and Compos, you often wish to work with data types in their fix-point of a functor form. For example: """ data ListF a rec = Nil | Cons a rec newtype List a = Roll { unroll :: ListF a (List a) } """ In some presentations, this is done by factoring the fix point like so: """ newtype FixF f = Roll { unroll :: f (FixF f) } type List a = FixF (ListF a) """ This is all well and good, but it means when working with data types defined in this manner you have to write Roll and unroll everywhere. This is tedious :-( Something I have long wished for is equirecursion, which would let you write this instead: """ type List a = ListF a (List a) """ Note that we now have a type instead of a newtype, so we won't need to write any injection/extraction operators. However, currently GHC rightly rejects this with a complaint about a cycle in type synonym declarations. However, you can use type families to hide the cycle from GHC: """ type List a = ListF a (Force (ListThunk a)) data ListThunk a type family Force a type instance Force (ListThunk a) = List a """ Unfortunately, this requires UndecidableInstances (the type instance RHS is not smaller than the instance head). And indeed when we turn that extension on, GHC goes into a loop, so this may not seem very useful. However, we can make this slight modification to the data type to make things work again: """ data ListF a rec = Nil | Cons a (Force rec) type List a = ListF a (ListThunk a) """ Note that the application of Force has moved into the *use site* of rec rather than the *application site*. This now requires no extensions other than TypeFamilies, and the client code of the library is beautiful (i.e. has no rolls/unrolls): """ example, ones :: List Int example = Cons 1 (Cons 2 Nil) ones = Cons 1 ones """ We can factor this trick into a fix point combinator that does not require roll/unroll: """ type Fix f = f (FixThunk f) type List a = Fix (ListF a) data FixThunk f type family Force a type instance Force (FixThunk f) = Fix f """ The annoying part of this exercise is the the presence of a "Force" in the functor definition (e.g ListF) means that you can't make them into actual Functor instances! The fmap definition gives you a function of type (a -> b) and you need one of type (Force a -> Force b). However, you can make them into a category-extras:Control.Functor.QFunctor instance (http://hackage.haskell.org/packages/archive/category-extras/0.53.5/doc/html…) by choosing the "s" Category to be: """ newtype ForceCat a b = ForceCat { unForceCat :: Force a -> Force b } instance Category ForceCat where id = ForceCat id ForceCat f . ForceCat g = ForceCat (f . g) """ Rather than the usual "Hask" Category which is used in the standard Functor. This is somewhat unsatisfying, but I'm not sure there is a better way. I haven't seen this trick to "emulate equirecursion" before. Has it showed up anywhere else? Cheers, Max

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