- Haskell-Cafe - Haskell.org

Optional EOF in Parsec.
by Kannan Goundan 04 Apr '09

04 Apr '09

I'm writing a parser with Parsec. In the input language, elements of a sequence are separated by commas: [1, 2, 3] However, instead of a comma, you can also use an EOL: [1, 2 3] Anywhere else, EOL is considered ignorable whitespace. So it's not as simple as just making EOL a token and looking for (comma | eol). I've implemented this functionality in a hand-written parser (basically a hack that keeps track of whether the last read token was preceded by an EOL, without making EOL itself a token). Does anybody have ideas about how to do this with Parsec?

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Monad transformer, liftIO
by Michael Roth 04 Apr '09

04 Apr '09

Hello list, maybe I'm just stupid, I'm trying to do something like this: import Control.Monad import Control.Monad.Trans import Control.Monad.List foobar = do a <- [1,2,3] b <- [4,5,6] liftIO $ putStrLn $ (show a) ++ " " ++ (show b) return (a+b) main = do sums <- foobar print sums But this apparently doesn't work... I'm total clueless how to achieve the correct solution. Maybe my mental image on the monad transformer thing is totally wrong? Michael

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Generating functions for games
by gwern0＠gmail.com 04 Apr '09

04 Apr '09

So some time ago I saw mentioned the game of Zendo https://secure.wikimedia.org/wikipedia/en/wiki/Zendo_(game) as a good game for programmers to play (and not just by Okasaki). The basic idea of Zendo is that another player is creating arrangements of little colored plastic shapes and you have to guess what rule they satisfy. I thought it'd be fun to play, but not knowing anyone who has it, I figured a Haskell version would be best. 3D graphics and that sort of geometry is a bit complex, though. Better to start with a simplified version to get the fundamentals right. Why not sequences of numbers? For example: [2, 4, 6] could satisfy quite a few rules - the rule could be all evens, or it could be ascending evens, or it could be incrementing by 2, or it could just be ascending period. Now, being in the position of the player who created the rule is no good. A good guesser is basically AI, which is a bit far afield. But it seems reasonable to have the program create a rule and provide examples. Have a few basic rules, some ways to combine them (perhaps a QuickCheck generator), and bob's your uncle. So I set off creating a dataype. The user could type in their guessed rules and then read could be used to compare. I got up to something like 'data Function = Not | Add' and began writing a 'translate' function, along the lines of 'translate Not = not\ntranslate Add = (+)', at which point I realized that translate was returning different types and this is a Bad Thing in Haskell. After trying out a few approaches, I decided the basic idea was flawed and sought help. Someone in #haskell suggested GADTs, which I've never used. Before I plunge into the abyss, I was wondering: does anyone know of any existing examples of this sort of thing or alternative approachs? I'd much rather crib than create. :) -- gwern

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Possible floating point bug in GHC?
by Peter Verswyvelen 03 Apr '09

03 Apr '09

For days I'm fighting against a weird bug. My Haskell code calls into a C function residing in a DLL (I'm on Windows, the DLL is generated using Visual Studio). This C function computes a floating point expression. However, the floating point result is incorrect. I think I found the source of the problem: the C code expects that all the Intel's x86's floating point register tag bits are set to 1, but it seems the Haskell code does not preserve that. Since the x86 has all kinds of floating point weirdness<http://www.informit.com/articles/article.aspx?p=770362> - it is both a stack based and register based system - so it is crucially important that generated code plays nice. For example, when using MMX one must always emit an EMMS instruction<http://msdn.microsoft.com/en-us/library/590b9ks9(VS.80).aspx>to clear these tag bits. If I manually clear these tags bits, my code works fine. Is this something other people encountered as well? I'm trying to make a very simple test case to reproduce the behavior... I'm not sure if this is a visual C compiler bug, GHC bug, or something I'm doing wrong... Is it possible to annotate a foreign imported C function to tell the Haskell code generator the functioin is using floating point registers somehow?

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Problem with prepose.lhs and ghc6.10.1
by Henry Laxen 03 Apr '09

03 Apr '09

Dear Group, I'm trying to read the paper: "Functional Pearl: Implicit Configurations" at http://www.cs.rutgers.edu/~ccshan/prepose/ and when running the code in prepose.lhs I get: ../haskell/prepose.lhs:707:0: Parse error in pattern which is pointing at: normalize a :: M s a = M (mod a (modulus (undefined :: s))) The paper says it uses lexically scoped type variables. I tried reading about them at: http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensio… so I added -XScopedTypeVariables to my OPTIONS but I still get the same error message. I would really like to play with the code in the paper, but I'm stuck at this point. Any pointers would be appreciated. Best wishes, Henry Laxen

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A fair number of updates to Buster since the other day - 0.99.5
by Jeff Heard 03 Apr '09

03 Apr '09

http://vis.renci.org/jeff/2009/04/03/major-updates-to-buster/ Added several new widgets and several new behaviours related to file reading and writing, exceptions, and the system/program environment. -- Jeff

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ANNOUNCE hgettext-0.1.5 - GetText based internationalization of Haskell programs
by Vasyl Pasternak 03 Apr '09

03 Apr '09

Hello, I have extended my previous version of the library to support distribution and installation of PO files. Source tarball - http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hgettext Also I described how to use this feature to distribute haskell packages in my blog entry http://progandprog.blogspot.com/2009/04/configure-and-install-international… Same description was added to the Haskell Wiki - http://www.haskell.org/haskellwiki/Internationalization_of_Haskell_programs Complete example, which uses internationalization capabilities is here: http://hgettext.googlecode.com/files/hello-0.1.3.tar.gz --- Best regards, Vasyl Pasternak

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Re: [Haskell-cafe] ANNOUNCE: fad 1.0 -- Forward Automatic Differentiation library
by Edward Kmett 03 Apr '09

03 Apr '09

A somewhat tricky concern is that that the extra functionality in question depends on a bunch of primitive definitions that lie below this in the package and the AD engine is used by a layer on top. So moving it out would introduce a circular dependency back into the package or require me to stratify into two packages. When I looked into partitioning the package for another reason I found that I couldn't do so without introducing some orphan instances, so it'll probably be a tricky bit of surgery to split out. That said, it's probably still worth doing. I also agree that I should be somewhat more pedantic about the name. =) -Edward Kmett On Fri, Apr 3, 2009 at 10:49 AM, Barak A. Pearlmutter <barak(a)cs.nuim.ie>wrote: > I feel silly, did not even notice that! Thanks for the pointer. > > Would be sensible to merge the functionalities; will try to import > functionality in Data.Ring.Module.AutomaticDifferentiation currently > missing from Numeric.FAD. > > (One pedantic note: should really be named > Data.Ring.Module.AutomaticDifferentiation.Forward, since it is doing > forward-mode accumulation automatic differentiation; reverse is > an adjoint kettle of fish.) > -- > Barak A. Pearlmutter > Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland > http://www.bcl.hamilton.ie/~barak/ >

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List and description of language extensions
by Michael Snoyman 03 Apr '09

03 Apr '09

It's been multiple times now that I've been confounded by something in Haskell which was then solved by a language extension (first FunctionalDependencies, most recently ScopedTypeVariables). I'm wondering if there is a list anywhere of all the language extensions supported by GHC and a brief description of them. I looked around, but couldn't find one. If there isn't one, would others be willing to fill one in on the Haskell wiki? I'll do what I can, but I obviously have a very limited knowledge of the extensions available. Thanks, Michael

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Program using 500MB RAM to process 5MB file
by lucas＠die.net.au 03 Apr '09

03 Apr '09

I'm relatively new to haskell so as one does, I am rewriting an existing program in haskell to help learn the language. However, it eats up all my RAM whenever I run the program. http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3175#a3175 Obviously I'm doing something wrong, but without my magical FP pants I don't know what that might be. -- Lucas Hazel <lucas(a)die.net.au>

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