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happstack example code wanted
by Johannes Waldmann 04 Apr '09

04 Apr '09

Hi, I'm thinking of using happstack for a simple online registration system. [ Customer gives Name and Address and registers for certain workshops. Then the app server should just produce a bill. (Payment is handled through a different channel.) The "store owner" needs to get some overview of registrations. ] According to happs(tack) marketing department, this should be an easy exercise? Any code that I could use as a starting point? ( Or, if you do the coding, I can link to you from the workshop site. But I don't have a serious budget :-) Thanks - J.W.

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ANN: logfloat 0.12.0.1
by wren ng thornton 04 Apr '09

04 Apr '09

-------------------------------------------- -- logfloat 0.12.0.1 -------------------------------------------- This package provides a type for storing numbers in the log-domain, primarily useful for preventing underflow when multiplying many probabilities as in HMMs and other probabilistic models. The package also provides modules for dealing with floating numbers correctly. -------------------------------------------- -- Now using FFI -------------------------------------------- We now use the FFI to gain access to C's accurate log1p (and expm1) functions. This greatly increases the range of resolution, especially for very small LogFloat values. These are currently exposed from Data.Number.LogFloat, though they may move to a different module in future versions. On GHC 6.10 the use of -fvia-C had to be disabled because it conflicts with the FFI (version 0.12.0.0 still used it, which is fine on GHC 6.8). I'm still investigating the use of -fasm and getting proper benchmarking numbers. Contact me if you notice significant performance degradation. Using the FFI complicates the build process for Hugs; details are noted in the INSTALL file. It may also complicate building on Windows (due to ccall vs stdcall), though I'm not familiar with Windows FFI and don't have a machine to test on. The calling convention is "unsafe" in order to avoid overhead. However, in the past this has been noted to cause issues with multithreaded applications since it locks all RTS threads instead of just the calling thread. If you're using logfloat in a multithreaded application and notice a slowdown, or if you're more familiar with these details than I, tell me so I can fix things. If you have any difficulties with the FFI, let me know. As an interim solution the FFI can be disabled by turning off the useFFI Cabal flag during configure, which will compile the package to use the naive log1p implementation from earlier versions. -------------------------------------------- -- Other changes since 0.11.0 -------------------------------------------- * (0.11.1) Felipe Lessa added an instance for IArray UArray LogFloat. On GHC we use newtype deriving; On Hugs we fall back to unsafeCoerce to distribute the newtype over IArray UArray Double. * (0.11.2 Darcs) Moved the log/exp fusion rules from Data.Number.LogFloat into Data.Number.Transfinite where our log function is defined. * Added a Storable LogFloat instance for GHC. No implementation is available yet for Hugs. * Removed orphaned toRational/fromRational fusion rules, which were obviated by the introduction of the Data.Number.RealToFrac module in 0.11.0. * Changed the Real LogFloat instance to throw errors when trying to convert transfinite values into Rational. -------------------------------------------- -- Compatibility / Portability -------------------------------------------- The package is compatible with Hugs (September 2006) and GHC (6.8, 6.10). For anyone still using GHC 6.6, the code may still work if you replace LANGUAGE pragma by equivalent OPTIONS_GHC pragma. The package is not compatible with nhc98 and Yhc because Data.Number.RealToFrac uses MPTCs. The other modules should be compatible. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat Darcs: http://code.haskell.org/~wren/logfloat/ Haddock (Darcs version): http://code.haskell.org/~wren/logfloat/dist/doc/html/logfloat/ -- Live well, ~wren

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about import alias
by Manlio Perillo 04 Apr '09

04 Apr '09

Hi. Haskell 98 allows import alias: import qualified VeryLongModuleName as C however it does not allow aliasing for imported names import NormalModule (veryLongFunctionName as C) what is the rational? IMHO this can be very useful in some cases. Thanks Manlio

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Re: [Haskell-cafe] How to get glyph outline from ttf font.
by Tillmann Vogt 04 Apr '09

04 Apr '09

Dmitry V'yal schrieb: > Greetings. I'm trying to render some glyphs from ttf font to svg > image using gtk2hs cairo binding. Since I can find nothing appropriate > in gtk2hs API, I decided to draw outlines with bezier curves myself. > But how to get them out of font? As far as I know, freetype library is > capable of extracting outlines, but is there any haskell binding for it > which supports this functionality? > > Here [1] one such lib was mentioned, but it wasn't availible online > that time. Have situation changed today? > > Thanks in advance > > [1] > http://www.nabble.com/Re%3A-Rendering-TTF-fonts-in-Haskell-and-OpenGL-p1563… > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe Hi Dmitry, I cannot help you directly with freetype. But I have developped a library that parses the svg-font format (this is not svg). Since svg-font is XML it is much easier to parse than ttf. There is also a program called fontforge that can be used to convert nearly every format into svg-font. The only problem is that currently triangulation has some bugs and is only O(n^2), the algorithm for deleting holes is not finished, and memoization isn't implemented. Should I upload the work although it is not finished?

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g++ std:map vs GHC IntMap
by Manlio Perillo 04 Apr '09

04 Apr '09

Hi. I have tried to compare performance of the g++ std::map versus the GHC IntMap. The test consists in adding 10000000 elements to an empty map. Haskell code is here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2899 C++ code is here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2900 The execution time and CPU usage is almost the same. However the C++ version requires 305 MB, the GHC version 617 MB. gcc 4.3.2 ghc 6.8.2 on Debian Linux Lenny, i386. Isn't really possible to optimize memory usage in cases like this? I also tried with Data.HashTable: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2902 but memory usage is 703 MB, and execution time is about 4.5 times slower! Thanks Manlio Perillo

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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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