May 2008 - Haskell-Cafe

Haskell PNG Writer
by Nahuel Rullo 09 May '08

09 May '08

Hi list, i am new in Haskell. I need to make images (PNG, JPEG) with haskell, if you can give me a tutorial, thanks! -- Nahuel Rullo

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Re: [Haskell-cafe] Induction (help!)
by PR Stanley 09 May '08

09 May '08

You've got the right idea. Paul: At long last! :-) I should point out that it doesn't make sense to say p(Succ n) = Succ(p(n)), p(x) represents some statement that is either true or false, so it doesn't make sense to say Succ(p(n)). . Paul: okay, da capo: We prove/test through case analysis that the predicate p holds true for the first/starting case/element in the sequence. When dealing with natural numbers this could be 0 or 1. We try the formula with 0 and if it returns the desired result we move onto the next stage. If the formula doesn't work with 0 and so the predicate does not hold true for the base case then we've proved that it's a nonstarter. In the inductive step we'll make a couple of assumptions: we'll imagine that p(j). We'll also assume that p holds true for the successor of j - p(j+1). Then with the help of rules and the protocol available to us we'll try to establish whether the formula (f) gives us f(j) = f(j+1) - f(1) So, we know that the predicate holds for 0 or at least one element. By the way, could we have 3 or 4 or any element other than 0? Anyway, p(0). Then we set out to find out if p holds for the successor of 0 followed by the successor of the successor of 0 and so forth. However, rather than laboriously applying p to every natural number we innstead try to find out if f(j+1) - f(1) will take us back to fj). I think this was the bit I wasn't getting. The assumptions in the inductive step and the algebraic procedures are not to prove the formula or premise per se. That's sort of been done with the base case. Rather, they help us to illustrate that f remains consistent while allowing for any random element to be succeeded or broken down a successive step at a time until we reach the base/starting element/value. Okay so far? Cheers Paul

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[Haskell-cafe] Announce: SCC, Streaming Component Combinator library
by Mario Blazevic 09 May '08

09 May '08

I'm pleased to announce the 0.1 release of Streaming Component Combinators in Haskell, based on my earlier work done in OmniMark and published in paper Mario Blažević, Streaming component combinators, Extreme Markup Languages, 2006. http://www.idealliance.org/papers/extreme/proceedings/html/2006/Blazevic01/… The release consists of the following: - Yet another monad transformer with coroutine functionality. Its central feature is the function pipe :: forall context x m r1 r2. Monad m => Producer m x r1 -> Consumer m x r2 -> Pipe context m (r1, r2) - Based on the above, streaming component types Transducer and Splitter are defined, as well as a number of primitive component instances. - Next are component combinators that can be used to synthesize larger components. - Finally, there is a rudimentary executable, shsh (Streaming Haskell SHell), that exposes most of the above within a command-line shell. It's useful for experiments and testing of new components. Here are a few simple command examples: wc -c count | show | concatenate wc -l foreach line then substitute x else suppress end | count | show | concatenate grep "foo" foreach line having substring "foo" then append "\n" else suppress end sed "s:foo:bar:" foreach substring "foo" then substitute "bar" end sed "s:foo:[\\&]:" foreach substring "foo" then prepend "[" | append "]" end The SCC library and shell are available on Hackage, at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/scc-0.1

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The Monad.Reader (11) - Call for Copy
by Wouter Swierstra 09 May '08

09 May '08

Call for Copy The Monad.Reader - Issue 11 Please consider writing something for the next issue of The Monad.Reader. The deadline for Issue 11 is ** August 1, 2008 ** It doesn't matter if you're a respected researcher or if you have only just started learning Haskell, get your thoughts together and write an article for The Monad.Reader! The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but more enduring than a wiki-page or blog. There have been a wide variety of articles, including: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. * Submission Details * Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wss at cs.nott.ac.uk). Articles should be written according to the guidelines available from http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll sort something out. All the best, Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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FFI: Creating a Storable for a C-struct composed of char arrays
by Olivier Boudry 09 May '08

09 May '08

Hi all, I'm trying to make RFC calls to SAP using the nwsaprfc library. Some structs defined in the library contains arrays (byte or word arrays). For example: typedef struct _RFC_ATTRIBUTES { SAP_UC dest[64+1]; /* RFC destination */ SAP_UC host[100+1]; /* Own host name */ SAP_UC partnerHost[100+1]; /* Partner host name */ SAP_UC sysNumber[2+1]; /* R/3 system number */ SAP_UC sysId[8+1]; /* R/3 system name */ SAP_UC client[3+1]; /* Client */ SAP_UC user[12+1]; /* User */ SAP_UC language[2+1]; /* Language */ ... continued }}RFC_ATTRIBUTES, *P_RFC_ATTRIBUTES; I would like to create a Haskell Storable counterpart of this structure. Using http://therning.org/magnus/archives/tag/hsc2hs as an starting point, I could create Storable for structures containing fields with basic types. Before working on the real data structures I wrote a more simple example to play with: -- File: ArrayStruct.h -- typedef struct _ArrayStruct { char a[10+1]; char b[20+1]; char c[30+1]; } ArrayStruct, *P_ArrayStruct; -- File: ArrayStruct.c -- #include "ArrayStruct.h" void print_array_struct(ArrayStruct *f) { printf("%s\n", __FUNCTION__); printf("f->a: %s\n", f->a); printf("f->b: %s\n", f->b); printf("f->c: %s\n", f->c); } -- File: HArrayStruct.hsc {-# OPTIONS -ffi #-} module Main where import Foreign import Foreign.C.Types #include "ArrayStruct.h" data HArrayStruct = HArrayStruct { a :: String, b :: String, c :: String } type HarrayStructPtr = Ptr HArrayStruct foreign import ccall "static ArrayStruct.h print_array_struct" f_print_array_struct :: ArrayStructPtr -> IO () instance Storable HArrayStruct where sizeOf _ = (#size ArrayStruct) alignment _ = alignment (undefined :: CInt) peek _ = error "peek is not implemented" poke ptr (HArrayStruct a' b' c') = do (#poke ArrayStruct, a) ptr a' (#poke ArrayStruct, b) ptr b' (#poke ArrayStruct, c) ptr c' printArrayStruct as = with as f_print_array_struct main = printArrayStruct $ HArrayStruct { a="some", b="test", c="data" } -- End of files Of course it won't work as HArrayStruct in file HArrayStruct.hs uses Strings and String is not an instance of Storable. Ideally I would need some sort of Storable array of char. Is Data.Storable.Array the type I'm looking for? Could someone point me to some code using the same kind of structures? Thanks, Olivier.

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GHC API: how to get the typechecked AST?
by Gerbo Engels 09 May '08

09 May '08

Hello all, In the GHC Commentary [1] I can read that the type checker adds type information to the syntax tree. So I would expect that there is a way to get a syntax tree of type HsModule Id. The GHC API provides functions checkModule and checkAndLoadModule which both return a CheckedModule that contains the parsedSource :: Located (HsModule RdrName) and typecheckedSource :: LHsBinds Id. But it doesn't include anything of type HsModule Id. I don't see how to unify the parsedSource and typecheckedSource to get the syntax tree parametrised by Id instead of RdrName (and containing the type information), and the API doesn't seem to provide such functionality (or did I miss something?). Can anyone help? Or indicate where in the parsedSource the LHsBinds should be placed? Regards, Gerbo Engels [1] http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/HsSynType#Deco…

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"resource exhausted"
by Galchin, Vasili 09 May '08

09 May '08

Hello, I am running some monadic code that I have written. I double checked my code and it seems to be ok (no guarantee though). I am getting a "resource exhausted (Message too long)". I just did a google on "resource exhausted" and saw a few posts on the number of open files which is obviously not my case. Any ideas? Kind regards, Vasili

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REMINDER: Haskell Communities and Activities Report
by Janis Voigtlaender 09 May '08

09 May '08

Dear Haskellers, It is not yet too late to contribute to the 14th edition of the HC&A Report. The deadline is this weekend, Saturday, 10 May 2008. If you haven't already, please write an entry for your new project or update your old entry. Please mail your entries to <hcar at haskell dot org> in plain text or LaTeX format. More information can be found in the original Call for Contributions at http://www.haskell.org/pipermail/haskell/2008-April/020378.html We look forward to receiving your contributions. Thanks a lot, Andres and Janis. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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A Cabal problem
by Mario Blazevic 08 May '08

08 May '08

I have a problem with Cabal: it doesn't seem to pick up the "Main-is:" option from the configuration file. Here's my "scc.cabal" file. Note the "Main-is:" line: > Name: scc > Version: 0.1 > Cabal-Version: >= 1.2 > Build-Type: Simple > License: GPL > License-file: LICENSE.txt > Copyright: (c) 2008 Mario Blazevic > Author: Mario Blazevic > Maintainer: blamario(a)yahoo.com > Synopsis: Streaming component combinators > > Executable shsh > Main-is: Shell.hs > Other-Modules: Foundation, ComponentTypes, Components, Combinators > Build-Depends: base, containers, process, readline, parsec > > Library > Exposed-Modules: Foundation, ComponentTypes, Components, Combinators > Build-Depends: base, containers And below are the outputs of Cabal configure and build commands. Note the absence of "--main-is" ghc command-line option. I'm doing this on Ubuntu 8.04 Linux, but I get the same problem with Gentoo. Is there something wrong with my configuration file, or is this a bug in Cabal? > ~/scc/pipe$ runhaskell Setup.lhs configure --verbose --prefix=~ > Configuring scc-0.1... > Dependency base-any: using base-3.0.1.0 > Dependency containers-any: using containers-0.1.0.1 > Dependency process-any: using process-1.0.0.0 > Dependency readline-any: using readline-1.0.1.0 > Dependency parsec-any: using parsec-2.1.0.0 > Using compiler: ghc-6.8.2 > Using install prefix: ~ > Binaries installed in: ~/bin > Libraries installed in: ~/lib/scc-0.1/ghc-6.8.2 > Private binaries installed in: ~/libexec > Data files installed in: ~/share/scc-0.1 > Documentation installed in: ~/share/doc/scc-0.1 > No alex found > Using ar found on system at: /usr/bin/ar > No c2hs found > No cpphs found > No ffihugs found > Using ghc version 6.8.2 found on system at: /usr/bin/ghc > Using ghc-pkg version 6.8.2 found on system at: /usr/bin/ghc-pkg > No greencard found > Using haddock version 0.8 found on system at: /usr/bin/haddock > No happy found > No hmake found > Using hsc2hs version 0.66-ghc found on system at: /usr/bin/hsc2hs > No hscolour found > No hugs found > No jhc found > Using ld found on system at: /usr/bin/ld > No nhc98 found > No pfesetup found > Using pkg-config version 0.22 found on system at: /usr/bin/pkg-config > Using ranlib found on system at: /usr/bin/ranlib > Using tar found on system at: /bin/tar > > ~/scc/pipe$ runhaskell Setup.lhs build --verbose > Creating dist/build (and its parents) > Creating dist/build/autogen (and its parents) > Preprocessing library scc-0.1... > Preprocessing executables for scc-0.1... > Building scc-0.1... > Building library... > Creating dist/build (and its parents) > Creating dist/build (and its parents) > /usr/bin/ghc -package-name scc-0.1 --make -hide-all-packages -i > -idist/build/autogen -idist/build -i. -Idist/build -odir dist/build > -hidir dist/build -stubdir dist/build -package base-3.0.1.0 -package > containers-0.1.0.1 -package process-1.0.0.0 -package readline-1.0.1.0 > -package parsec-2.1.0.0 -O Foundation ComponentTypes Components Combinators > [1 of 4] Compiling Foundation ( Foundation.hs, > dist/build/Foundation.o ) > [2 of 4] Compiling ComponentTypes ( ComponentTypes.hs, > dist/build/ComponentTypes.o ) > [3 of 4] Compiling Components ( Components.hs, > dist/build/Components.o ) > [4 of 4] Compiling Combinators ( Combinators.hs, > dist/build/Combinators.o ) > Linking... > /usr/bin/ar q dist/build/libHSscc-0.1.a dist/build/Foundation.o > dist/build/ComponentTypes.o dist/build/Components.o dist/build/Combinators.o > /usr/bin/ar: creating dist/build/libHSscc-0.1.a > /usr/bin/ld -x -r -o dist/build/HSscc-0.1.o.tmp dist/build/Foundation.o > dist/build/ComponentTypes.o dist/build/Components.o dist/build/Combinators.o > Building executable: shsh... > Creating dist/build/shsh (and its parents) > Creating dist/build/shsh/shsh-tmp (and its parents) > Creating dist/build/shsh/shsh-tmp (and its parents) > /usr/bin/ghc -o dist/build/shsh/shsh --make -hide-all-packages -i > -idist/build/autogen -idist/build/shsh/shsh-tmp -i. > -Idist/build/shsh/shsh-tmp -odir dist/build/shsh/shsh-tmp -hidir > dist/build/shsh/shsh-tmp -stubdir dist/build/shsh/shsh-tmp -package > base-3.0.1.0 -package containers-0.1.0.1 -package process-1.0.0.0 > -package readline-1.0.1.0 -package parsec-2.1.0.0 -O ./Shell.hs > [1 of 5] Compiling Foundation ( Foundation.hs, > dist/build/shsh/shsh-tmp/Foundation.o ) > [2 of 5] Compiling ComponentTypes ( ComponentTypes.hs, > dist/build/shsh/shsh-tmp/ComponentTypes.o ) > [3 of 5] Compiling Components ( Components.hs, > dist/build/shsh/shsh-tmp/Components.o ) > [4 of 5] Compiling Combinators ( Combinators.hs, > dist/build/shsh/shsh-tmp/Combinators.o ) > [5 of 5] Compiling Shell ( ./Shell.hs, > dist/build/shsh/shsh-tmp/Shell.o ) > Warning: output was redirected with -o, but no output will be generated > because there is no Main module. Name: scc Version: 0.1 Cabal-Version: >= 1.2 Build-Type: Simple License: GPL License-file: LICENSE.txt Copyright: (c) 2008 Mario Blazevic Author: Mario Blazevic Maintainer: blamario(a)yahoo.com Synopsis: Streaming component combinators Executable shsh Main-is: Shell.hs Other-Modules: Foundation, ComponentTypes, Components, Combinators Build-Depends: base, containers, process, readline, parsec Library Exposed-Modules: Foundation, ComponentTypes, Components, Combinators Build-Depends: base, containers

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Re: [Haskell] [Haskell-cafe] Help with polymorphic functions
by Reinier Lamers 08 May '08

08 May '08

Op Thursday 08 May 2008 21:10:08 schreef Wei Yuan Cai: > shift is defined as "a -> Int -> a" It's not. It's defined as "(Bits a) => a -> Int -> a" or something along those lines. So there is a restriction that the type a must be a member of the Bits typeclass. Because "test" is essentially just "shift", its type must also be "(Bits a) => a -> Int -> a". Reinier

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