May 2008 - Haskell-Cafe

one-way monads
by Zsolt SZALAI 21 May '08

21 May '08

Hi! Now, i'm getting familiar with monads, but there is a little discredit about one-way monads. For example, in monads presented in Monads for Functional Programming by Philip Wadler, they are all two-way monads, internal data can be extracted from the monad, and these data, call them state, are in deed _local_ to a function using the monadic computation, so the state there is just a little magic, and they are really just functions(as in a pure language). An other example is Random. I've figured out, that when initializing a generator, it uses the actual timestamp and cputime to initialize. If it wouldnt happen, two distinct uses of Random in a program would result the same random number sequence. Here comes IO and one-way monads, where the internal state can not be extacted, and seems, that the internal data is global to the program. Hows that could be? Is it just because main::IO() or because the implementation of IO uses external C functions and there are no internal state in the monad itself at all? And why one can not write a function that makes an IO computation and the return type does not include and IO contructor? It is a feature of the language and specific to IO, or anybody could write a monad with this property(one-way), assuming the implementation does not use external languages? Or the one-way property is just that, there is no such functions, that allow extracting internal data? Also, is there any other monad, that has the one-way property? Thanks, -- Zsolt

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are there any good c parsers + ast builders available for haskell?
by Carter Tazio Schonwald 21 May '08

21 May '08

Hello, I'm thinking about having a go at writing some simple static analysis tools for C, and I think using Haskell would make it a happier time coding wise, the question being, are there preexisting parsing tools for C available for haskell? many thanks -Carter

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Performance: MD5
by Andrew Coppin 20 May '08

20 May '08

Hi folks. OK, try this: darcs get http://darcs.orphi.me.uk/MyMD5 cd MyMD5 ghc -O2 --make md5sum md5sum <some large filename> On my PC, it takes 3.5 seconds to compute the MD5 hash of a 10 MB file. For comparison, the md5.exe I downloaded from the Internet does it instantaneously. It's literally so fast I can't even time it. As far as I know, my program always produces the *correct* answer, it just takes far too long to do it. If anybody has any interesting insights as to why my version is so damned slow, I'd like to hear them. (Indeed, a description of the process for tracking the problem down would also be useful!) [Much to my bemusement, when I had a look at the code, I discovered that tucked away in a corner somewhere, it reads a ByteString from disk and instantly converts it to [Word8]. Uh... oops? Anyway, eliminating this changed the numbers I get from the profiler, but it's still far too slow.]

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Link to Hierarchical-libraries documentation generated with haddock 2.0?
by Alfonso Acosta 20 May '08

20 May '08

Hi all, GHC's documentation page [1] only points to hierarchical-libraries documentation generated by haddock-0.8 [2]. I need the 2.0 version in order to get links in the documentation of the project I'm working on (which needs haddock 2.0 due to the use of quite a few GHC extensions). Before I proceed to generate them myself, has anyone done it already? (I guess that must be the case since HackageDB users haddock 2.0 and gets all the links right) I would be more than happy to just click a link instead :) Either way, I think it would be a good idea to provide an alternative link to the 2.0 version. Thanks, Fons [1] http://www.haskell.org/ghc/documentation.html [2] http://www.haskell.org/ghc/docs/latest/libraries.html.tar.gz

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FD Int Constraint Solving Library
by Alexander Fuchs 20 May '08

20 May '08

Hi, I was wondering if someone could point me to a Haskell library for constraint solving over finite domains with integer arithmetic, i.e. something like FaCiLe ( http://www.recherche.enac.fr/log/facile/ ). A Haskell FF interface to such a library would probably be fine as well. It seems to me that such a library/wrapper should exist, but I couldn't find one. Thanks, Alexander

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ByteString.pack behavior
by Dušan Kolář 20 May '08

20 May '08

Hello all, Maybe there is something obvious I can't see, but I have this behavior for 6.8.2 ghci: $ghci ttest1p.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Main ( ttest1p.hs, interpreted ) Ok, modules loaded: Main. *Main> encode' [1..100] Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. [1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,0,10,0,11,0,12,0,13,0,... // deleted *Main> B.pack [0..100] "\NUL\SOH\STX\ETX\EOT\ENQ\ACK\a\b\t\n\v\f\r\SO\SI\DLE\DC1\DC2\DC3\DC4\NAK\SYN\ETB\CAN\EM\SUB\ESC\FS\GS\RS\US !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcd" *Main> B.pack $ encode' [1..100] "*** Exception: divide by zero where ttest1p.hs: import qualified Data.ByteString as B encode' [] = [] encode' (x:xs) = if x==0 then 0:0:encode' xs else (x `mod` 256) : (x `div` 256) : encode' xs What is the difference, except list length and value structure? Where is my error? Thanks for any hint, Dusan

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How does GHC read UNICODE.
by L.Guo 20 May '08

20 May '08

Hi Haskellers: I am a Chinese. Mostly, it is needed to read/write UNICODE charactors. Currently, I can only use the ByteString module in GHC 6, 2007. But I feel it is not an easy method. Does GHC support it now ? or, is there any other way to do this ? Regards -------------- L.Guo 2008-05-20

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question on ghc-pkg database
by Galchin, Vasili 20 May '08

20 May '08

Hello, What is the difference between the following two ghc-pkg database instances: /usr/lib/ghc-6.8.2/package.conf /usr/local/lib/ghc-6.8.2/package.conf ?? Thanks, Vasili

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Experimental compilation of Haskell to Erlang
by Dimitry Golubovsky 20 May '08

20 May '08

Hi, Some time ago I became interested in compilation of Haskell programs (via Yhc Core) to Erlang to be able to run Haskell code in Erlang environment. This experiment seems to have been successful, so I'd like to publish its results for everyone to read and criticize. Results of the experiment are described in this Haskell Wiki article: http://www.haskell.org/haskellwiki/Yhc/Erlang/Proof_of_concept Any feedback is appreciated. Thanks. -- Dimitry Golubovsky Anywhere on the Web

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[tagsoup] is it the expected behaviour ?
by Fernand 20 May '08

20 May '08

Hi, Experimenting with tagsoup (I'm using GHC 6.8.2 and tagsoup-0.6), I found something which appears to me as strange behaviour : when parsing tag's attributes that have spaces enclosing the "=" sign, tagsoup seems to interpret these as empty attributes' names and values. For instance (notice the spaces enclosing the equal sign) : $ ghcii.sh -package tagsoup [...] Loading package tagsoup-0.6 ... linking ... done. Prelude> :m +Text.HTML.TagSoup Prelude Text.HTML.TagSoup> parseTags "<a href = \"what\">uh ?</a>" [TagOpen "a" [("href",""),("","what")],TagText "uh ?",TagClose "a"] Here, am I wrong when expecting [TagOpen "a" [("href","what")],TagText "uh ?",TagClose "a"], or is there some HTML interpretation I don't know on the parsing of attributes ? Sincerely yours, Fernand

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