- Haskell-Cafe - Haskell.org

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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lambdabot on GHC 6.8.2
by Tony Morris 20 May '08

20 May '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 So after some effort, I managed to get lambdabot installed on a ubuntu machine using GHC 6.8.2, however, it fails when evaluating an expression: > 1 + 1 setResourceLimit: invalid argument (Invalid argument) Googling around brings up a bug (#2038) against GHC 6.8.2. Am I hitting this bug? If so, can I get around it to get a working lambdabot? Thanks for any tips. - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIMnFnmnpgrYe6r60RAp7DAJ9JhFsWwL05eyrcECf6C9Z/ML+h0gCgt5/I 3E7eOjVMm+RSXumbYiCsgR8= =bpjW -----END PGP SIGNATURE-----

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Newbie: State monad example questions
by Luke Palmer 19 May '08

19 May '08

Hi Dmitri. I'm just going to ramble on about what I know and how I think of things, and maybe you'll pick something up :-) On 5/19/08, Dmitri O.Kondratiev <dokondr(a)gmail.com> wrote: > getAny :: (Random a) => State StdGen a > getAny = do g <- get > (x,g') <- return $ random g > put g' > return x > > Questions: > return (random g) = State (\s -> ((a,g), s)) > > Is it correct? Yes, where a is a random value and g is a new generator. > 2) What x and g' will match to in: > do ... > (x,g') <- return $ random g > > which, as I understand equals to: > do ... > (x,g') <- State (\s -> ((a,g), s)) > > What x and g' will match to in the last expression? Well, I also suggest working through the desugared version of >>= as the other poster suggested: State (\s -> ((a,g), s)) >>= (\(x,g') -> ...) The question is, what values do x and g' have in '...'. Expand out the definition and see. State is kind of complicated, but you will get an answer. But I can provide some intuition aside from the formality. x <- action "Does" the action and names the return value "x". Your action is "return $ random g", namely it's an action that doesn't have any effects and just returns a value. So: (x,g') <- return $ random g Is equivalent to: let (x,g') = random g Nothing particular to State is happening here. > 3) In general, in do expression (pseudo): > do { x <- State (\s -> (a, s)); ...} > > What x will refer to? Will x stand for a whole lambda function: \s -> (a, s) > ? Here x will be equal to a, and the implicit state will be unchanged after the binding. > 4) How 'g <- get' works in this function (getAny) ? The representation of a state action is "State f", where f takes the current state as an argument and returns the result of the computation along with the new state. "get" is defined as: get = State $ \s -> (s,s) So its function takes a state, and returns the current state as the return value as well as the next state; that is to say, it does not update the state and it also returns it. So: x <- get ... Gives the name "x" to the implicit state at the time "get" was called. > 5) Why we need 'put g'? Otherwise this function would not modify the implicit state, and you'd always get the same random number back when you called it. Something to keep in mind: If you have an "action :: m a" for some monad m, then in: x <- action x will name a value of type Foo. Luke

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My experience setting up Haskell up for GUI development
by Ben Arnold 19 May '08

19 May '08

I'm running Windows Vista and I've been trying to set up an environment for writing GUI applications. A Google search pointed me at two major GUI toolkits for Haskell: gtk2hs and wxHaskell. I started with gtk2hs. The installation instructions were straightfoward and easy to follow, but they just didn't work. The installer did nothing. It opened a command window and immediately terminated. When I ran it from a command window it terminated immediately, apparently doing nothing at all. Running as Administrator didn't help. There was no error message, the log file option didn't generate a log file and Googling for a solution didn't come up with anything. So I gave up on gtk2hs. WxHaskell was a little trickier but more successful. There didn't appear to be any installation instructions on the website, but it did imply that I needed to install wxWidgets first. I did that, and made sure the paths didn't have spaces in them (yawn). And after restarting the PC eventually I got a working Hello World dialogue box. Which is nice, except that when I call "main" twice in succession from GHCi, ghc.exe crashes. So I have no confidence in the wxHaskell libraries either. I don't want to invest time in programming with libraries that perform illegal operations on my operating system. I like Haskell; I really do. I've toyed around with it before a few times and I want to learn more about it, but I can't confine myself to command-line applications forever and for better or worse I run a Windows box. So, in practice, do other people write GUI apps with Haskell on Windows? And if they do, how do they do it? I feel I've got to the stage where I need a concrete recommendation from the experts. My installation: - Windows Vista Home Premium 32-bit 6.0.6001 First attempt - GHC 6.6.1 - gtk2hs-09.12.exe Second attempt - GHC 6.8.2 - wxMSW 2.6.3 - wxHaskell 0.10.3 Ben

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