July 2005 - Haskell-Cafe

polymorphic type
by wenduan 06 Jul '05

06 Jul '05

Dear all, Suppose we have defined two functions as below: case :: (a -> c,b -> c) -> Either a b -> c case (f, g) (Left x) = f x case (f, g) (Right x) = g x plus :: (a -> b, c -> d) -> Either a b -> Either c d plus (f, g) = case(Left.f, Right.g) My question is regarding to the function signature of 'plus' , in its signature, does the 'a' in 'a -> b' and in 'Either a b', must be instantiated to the same object when the function is applied?E.g.,Either a b is instantiated to 'Either Char b', will the 'a' in 'a -> b' be instantiated to 'Either Char b'?Furthermore, are the two bs in 'a -> b' and 'Either a b' not conflicting?What I thought at first the signature of plus should be: plus :: (a -> c, b -> d) -> Either a b -> Either c d?Anyone know where I was wrong? -- X.W.D

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Cabal with Alex and and Happy
by Brian Smith 06 Jul '05

06 Jul '05

Is there an example of how to build a Cabal package that has a lexer generated with Alex and a parser generated with Happy? As far as I can tell, the way to do this is to add "Other-Modules: Module.Name.Of.Parser.y Module.Name.Of.Lexer.x" to each executable/library stanza. But, when I try this, I get: Could not find module `GHC.Exts': it is a member of package base-1.0, which is hidden The generated parser code contains: #if __GLASGOW_HASKELL__ >= 503 import GHC.Exts #else import GlaExts #endif Also, I don't see any way of passing options to preprocessors using Cabal. In particular, how do you pass options to Happy? I am using GHC and Cabal from the CVS head, built about two weeks ago. Thanks, Brian

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can't build module with ffi 'wrapper' by cabal (or ghc): unknown symbol
by Evgeny Chukreev 06 Jul '05

06 Jul '05

Hello! http://toril.ru/hmhash/hmhash-0.1.tar.bz2 ###### ./Setup.hs clean; ./Setup.hs configure; ./Setup.hs build -v5 Configuring HMhash-0.1... configure: searching for ghc in path. configure: found ghc at /usr/bin/ghc configure: looking for package tool: ghc-pkg near compiler in /usr/bin/ghc configure: found package tool in /usr/bin/ghc-pkg configure: Using install prefix: /usr/local configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.4 configure: Using package tool: /usr/bin/ghc-pkg configure: Using haddock: /usr/bin/haddock configure: Using happy: /usr/bin/happy configure: Using alex: /usr/bin/alex configure: Using hsc2hs: /usr/bin/hsc2hs configure: Using cpphs: /usr/bin/cpphs configure: Reading installed packages... configure: Dependency base-any: using base-1.0 Preprocessing library HMhash-0.1... /usr/bin/hsc2hs -osrc/Codec/Mhash.hs src/Codec/Mhash.hsc Building HMhash-0.1... /usr/bin/ghc -package-name HMhash -odir dist/build/src -hidir dist/build/src --make -isrc -ffi -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances -fallow-incoherent-instances -Wall -O0 -package base-1.0 Codec.Mhash -v Glasgow Haskell Compiler, Version 6.4, for Haskell 98, compiled by GHC version 6.2.2 Using package config file: /usr/lib/ghc-6.4/package.conf Using package config file: /home/akshaal/.ghc/i386-linux-6.4/package.conf Hsc static flags: -static *** Chasing dependencies: Chasing modules from: Codec.Mhash Stable modules: *** Compiling Codec.Mhash ( src/Codec/Mhash.hs, interpreted ): compile: input file src/Codec/Mhash.hs *** Checking old interface for Codec.Mhash: Compiling Codec.Mhash ( src/Codec/Mhash.hs, dist/build/src/Codec/Mhash.o ) *** Parser: *** Renamer/typechecker: <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' <no location info>: Warning: Defined but not used: `k' *** Desugar: Result size = 6135 *** Simplify: Result size = 6847 Result size = 5654 Result size = 5564 *** Tidy Core: Result size = 5564 *** CorePrep: Result size = 6911 *** Stg2Stg: *** CodeGen: *** CodeOutput: *** C Compiler gcc src/Codec/Mhash_stub.c -o /tmp/ghc29101.s -DDONT_WANT_WIN32_DLL_SUPPORT -v -S -Wimplicit -O -D__GLASGOW_HASKELL__=604 -ffloat-store -I src/Codec -I /usr/lib/ghc-6.4/include Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.5/specs Configured with: ../src/configure -v --enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib --enable-nls --without-included-gettext --enable-__cxa_atexit --enable-clocale=gnu --enable-debug --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i486-linux Thread model: posix gcc version 3.3.5 (Debian 1:3.3.5-8) /usr/lib/gcc-lib/i486-linux/3.3.5/cc1 -quiet -v -I src/Codec -I /usr/lib/ghc-6.4/include -D__GNUC__=3 -D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=5 -DDONT_WANT_WIN32_DLL_SUPPORT -D__GLASGOW_HASKELL__=604 src/Codec/Mhash_stub.c -quiet -dumpbase Mhash_stub.c -auxbase-strip /tmp/ghc29101.s -O -Wimplicit -version -ffloat-store -o /tmp/ghc29101.s GNU C version 3.3.5 (Debian 1:3.3.5-8) (i486-linux) compiled by GNU C version 3.3.5 (Debian 1:3.3.5-8). GGC heuristics: --param ggc-min-expand=81 --param ggc-min-heapsize=95593 ignoring nonexistent directory "/usr/i486-linux/include" #include "..." search starts here: #include <...> search starts here: src/Codec /usr/lib/ghc-6.4/include /usr/local/include /usr/lib/gcc-lib/i486-linux/3.3.5/include /usr/include End of search list. *** Assembler gcc -Isrc/Codec -c /tmp/ghc29101.s -o dist/build/src/Mhash_stub.o *** Assembler gcc -Isrc/Codec -c /tmp/ghc29100.s -o dist/build/src/Codec/Mhash.o *** Deleting temp files Deleting: /tmp/ghc29101.s /tmp/ghc29100.s Upsweep completely successful. *** Deleting temp files Deleting: link(batch): upsweep (partially) failed OR Main.main not exported; not linking. *** Deleting temp files Deleting: /usr/bin/ar qv dist/build/libHSHMhash-0.1.a dist/build/src/Codec/Mhash.o /usr/bin/ar: creating dist/build/libHSHMhash-0.1.a a - dist/build/src/Codec/Mhash.o ###### ./Setup.hs install Installing: /usr/local/lib/HMhash-0.1 & /usr/local/bin HMhash-0.1... Registering HMhash-0.1... Reading package info from ".installed-pkg-config" done. building GHCi library /usr/local/lib/HMhash-0.1/HSHMhash-0.1.o... done. Saving old package config file... done. Writing new package config file... done. ###### ghci -package HMhash ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Loading package HMhash-0.1 ... linking ... ghc-6.4: /usr/local/lib/HMhash-0.1/HSHMhash-0.1.o: unknown symbol `CodecziMhash_d7Ew' ghc-6.4: unable to load package `HMhash-0.1' ------------------ I know how to fix it by hands: rm dist/build/libHSHMhash-0.1.a /usr/bin/ar qv dist/build/libHSHMhash-0.1.a dist/build/src/Codec/Mhash.o dist/build/src/Mhash_stub.o But, how to do it with cabal? -- ... WBR, Evgeny Chukreev ...

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Fork & execve
by yin 05 Jul '05

05 Jul '05

Hello! Is there some way how to fork a process in haskell? And excuting an another proces, waiting until it exit and colecting his stdout? Matej 'Yin' Gagyi PS: I'm sorry for my English!

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New-bee question about seq in Parsec.Pos
by Christoph Bauer 04 Jul '05

04 Jul '05

Hi, I try to understand, how Parsec works. In Pos.hs I found these definition of SourcePos: data SourcePos = SourcePos SourceName !Line !Column deriving (Eq,Ord) My poor haskell knowledge says me, that Line and Column is always strict. So, what does the following function? forcePos :: SourcePos -> SourcePos forcePos pos@(SourcePos name line column) = seq line (seq column (pos)) thanks for an enlightenment, Christoph Bauer -- let () = let rec f a w i j = Printf.printf "%.20f\r" a; let a1 = a *. i /. j in if w then f a1 false (i +. 2.0) j else f a1 true i (j +. 2.0) in f 2.0 false 2.0 1.0

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haskell parser for java bytecode?
by Kimberley Burchett 03 Jul '05

03 Jul '05

I'm interested in using haskell to do static analysis of java bytecode. To my surprise, I wasn't able to find any existing haskell libraries for parsing java .class files. Is anyone aware of such a library? I've already started writing my own implementation, which I intend to make available as a reusable library once it's ready, but if someone else has already done the work then I'd love to avoid having to repeat it. Kimberley Burchett http://www.kimbly.com/

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pair (f,g) x = (f x, g x)?
by wenduan 02 Jul '05

02 Jul '05

I came across a haskell function on a book defined as following: pair :: (a -> b,a -> c) -> a -> (b,c) pair (f,g) x = (f x,g x) I thought x would only math a single argument like 'a', 1, etc....,but it turned out that it would match something else, for example, a pair as below: square x = x*x pair (square.fst,Char.toUpper.snd) (2,'a') (4,'A') The type declaration of pair is what confused me, pair :: (a -> b,a -> c) -> a -> (b,c),it says this function will take a pair of functions which have types of a->b,a->c,which I would take as these two functions must have argument of the same type, which is a,and I didn't think it would work on pairs as in the above instance,but surprisingly it did,can anybody enlighten me? -- X.W.D

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Re: haskell parser for java bytecode?
by Frank-Andre Riess 01 Jul '05

01 Jul '05

Hi there, keeping out of the previous tiff, I'd just like to mention that I, too, am interested in such a project - I don't require a parser for Java bytecode, but I think I'd be a good experience and at least it would get me to know the JVM a bit better... maybe :) By the way, there once was some discussion about having Haskell actually -target- the JVM (I think, it was on the Glasgow Haskell Users list). That might as well be an interesting thing to do. Best regards, Frank-Andre Riess

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Conversion between MonadPlus instances
by Gracjan Polak 01 Jul '05

01 Jul '05

Hi all, A simple question for advanced Haskellers, but I still have some problems bending my mind over it. Example: I have some function, that can return multiple results. Currently I need only the first one, but in the spirit of NotJustMaybe, I try to be as general as possible. If I code it like this: reduction :: (MonadPlus m) => [Rule] -> Expr -> m Expr reduction expr = do rule <- rules reduction rule expr Variable m gets unified with []. But I want m to stay as general as possible here. This version works, but I somehow do not like the "smell" of it reduction expr = do let listmonad = do rule <- rules reduction rule expr msum (map return listmonad) Is there a better way how to "embed" MonadPlus in other "MonadPlus"? -- Gracjan

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RE: ANNOUNCE: GHC survey results
by Simon Marlow 01 Jul '05

01 Jul '05

On 28 June 2005 14:11, Bulat Ziganshin wrote: > Tuesday, June 28, 2005, 1:58:13 PM, you wrote: > >> http://www.haskell.org/ghc/survey2005-summary.html >> There's a lot to take in, but it's an interesting read. Enjoy! > > thank you all, processing all the 600 answers was not easy work :) > > i have several comments regarding results of this survey: > > 1) GHCi compiles to bytecode several times faster than GHC makes > unoptimized compilation. can unoptimized GHC compilation just create > bytecode (as Ocaml does)? Do you really mean "several times faster"? My impression is that it's a few percent faster, if at all. Some measurements would be good. We've thought about compiling to persistent byte code in the past, but the gains never seemed to be worth the effort: we'd need a byte-code serialiser and loader. > 2) is your plans to support x86-64 platform includes Windows (and all > other sorts of Unix), or it's for Linux only? Definitely Windows too. I hear the cygwin/mingw tools are still unstable on Windows x64, though. > 3) many users complaining about non-compatibility between GHC > versions. if they mean library interfaces changes then how about using > Pesco's library versioning scheme? (see > http://www.haskell.org/tmrwiki/EternalCompatibilityInTheory) I've read that article, and I think it's an interesting idea. I can't disagree with the arguments put forward, but somehow, the cure seems a bit painful. Cheers, Simon

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