- Haskell-Cafe - Haskell.org

Parsec on TeX
by Ross Boylan 05 May '08

05 May '08

I am new to Haskell and Parsec, and am trying to understand both. I tried to follow the example of how to use Parsec to parse TeX begin/end groups, but can't get it to run. I'm using HUGS -98 on Debian. When I copied the code I got errors about unknown terms (reserved and braces). I've tried to get them from the lexer, but now get this error :load grammar.hsl ERROR "grammar.hsl":21 - Type error in explicitly typed binding *** Term : envEnd *** Type : String -> GenParser Char a [Char] *** Does not match : String -> Parser () Can anyone help me understand what the problem is? Here's the code the caused the above error; I believe the part after --TeX example is verbatim from the Parsec documentation. I picked haskell as the language for to lexer "arbitrarily." import Text.ParserCombinators.Parsec import qualified Text.ParserCombinators.Parsec.Token as P import Text.ParserCombinators.Parsec.Language(haskell) reserved = P.reserved haskell braces = P.braces haskell -- TeX example environment = do{ name <- envBegin ; environment ; envEnd name } <|> return () envBegin :: Parser String envBegin = do{ reserved "\\begin" ; braces (many1 letter) } envEnd :: String -> Parser () envEnd name = do{ reserved "\\end" ; braces (string name) }

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Data structure to manage collection of sets with efficient lookup, intersection?
by Patrick Surry 05 May '08

05 May '08

New to Haskell, with a mental block about how to represent this situation efficiently: I have an unknown function f which is defined on subsets of some universal set (say integers 1...N). I know the values of f for some subsets, and using those can infer values on other subsets. So what I need is a way to manage a collection of subsets s_i (and the associated values f(s_i)) so that I can efficiently (a) check whether a subset s is already 'known' in my collection, and (b) find all subsets t in the collection that intersect s. In a "traditional" language, I'd likely create a dictionary with keys s_i containing the f(s_i) as values, along with a separate dictionary keyed on the elements of the universal set (1...N) in which each entry is a list of all s_i containing the given element of the universal set. Together they let me check, given a subset s, whether I know f(s), and also get the list of all known subsets intersecting s (by the union of the lists of sets containing each element of s). I can't quite wrap my head around how to do this efficiently in Haskell, maybe because of the way the above is duplicating information about the subsets across two different data structures? Any thoughts? Thanks, Patrick ________________________________________________________________________ DISCLAIMER: This e-mail is intended only for the addressee named above. As this e-mail may contain confidential or privileged information, if you are not the named addressee, you are not authorised to retain, read, copy or disseminate this message or any part of it. If you received this email in error, please notify the sender and delete the message from your computer.

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Cabalizing darcs
by Gwern Branwen 05 May '08

05 May '08

So recently I spent a bit of time working on a cabalization of Darcs. It works well for me, and is reasonably easy to apply (attached are three files; do a 'darcs get --lazy http://darcs.net' with Darcs-2 to get the latest, and copy the files into it, the usual autoconf, and it should then work as normal; if this doesn't work for you, I'd appreciate knowing). My question for y'all is: would you find an unofficial version of Darcs - one with the cabalization added and uploaded to Hackage - useful? Or would it just be frustrating and redundant and Not Good? (If yes, any suggestions for a package name? 'darcs' is out, but 'darcs-cb' and 'darcs-cabalized' strike me as horribly clunky or obscure.) -- gwern Threat WANK football Talent anarchy RIT interception Hutsul forschung ISS

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unapplying function definitions?
by PR Stanley 05 May '08

05 May '08

Hi What on earth is unapplying function definitions? The following is taken from chapter 13 of the Hutton book: "...when reasoning about programs, function definitions can be both applied from left to right and unapplied from right to left." Cheers Paul

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Question related to Multi Param Type Classes
by Sai Hemanth K 05 May '08

05 May '08

Hi, I declared a multi param type class as below: *class MyString m c where zIndex :: m -> Int -> c zLength :: m -> Int zPack :: [c] -> m zEquals :: c ->c -> Bool zWrap :: [c] -> (m,AnExistingDataType)* In the end I did not needed it for my program, but for those few minutes I played with the idea, I came across a zillion questions. Can some body help me here please? When I defined a function like below, \begin{code} compareStr::(MyString m c) => (m,Int) ->(m,Int)->Int->Int compareStr (s1,pos1) (s2,pos2) soFar | (pos1 < zLength s1) && (pos2 < zLength s2) = let c1 = zIndex s1 pos1 c2 = zIndex s2 pos2 in if(zEquals c1 c2) then compareStr (s1,(pos1 + 1)) (s2, (pos2 + 1)) (soFar + 1) else soFar | otherwise = soFar \end{code} when I loaded it on ghci ( invoked with -XMultiParamTypeClasses ), I got an essay in greek (or is it latin?), which started something like below: Could not deduce (MyString m c) from the context (MyString m c4) arising from a use of `zLength' at GenericZAlgo.lhs:42:21-31 Possible fix: add (MyString m c) to the context of the type signature for `compareStr' In the second argument of `(<)', namely `zLength s1' In the first argument of `(&&)', namely `(pos1 < zLength s1)' In a pattern guard for the definition of `compareStr': (pos1 < zLength s1) && (pos2 < zLength s2) And if I let the type inference run the show, the type it shows is : compareStr :: (MyString t c2, MyString m c2, Num a, MyString t1 c2, MyString t c, MyString t1 c1) => (t, Int) -> (t1, Int) -> a -> a And puzzlingly, if I try the same thing myself and reload it on the ghci,I still get similar error. I can go on about other 'puzzles'. Can some one please tell me whats going on here? Apologies, if it is something that's been documented left to right, top to bottom, but I could not get anywhere readily. Any pointers to the right documentation will be much appreciated. On the surface it looks like something that n00bs like yours truly better stay away. But I have a feeling that this one may throw some more light on type inference. Many thanks

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Grokking monads by not overlooking concatMap
by Achim Schneider 05 May '08

05 May '08

guard True = return () guard False = [] -- the textbook example f = do x <- [1..4] y <- [2..8] guard (x*y==8) return (x,y) -- the translation. f' = [1..4] >>= (\x -> [2..8] >>= (\y -> (if (x*y == 8) then return () else []) >> return (x,y))) -- another one. note that we can move the final -- return out of the parens. It would even work without it, but we'd -- get a list containing many empty lists: concat fixes that. f'' = [1..4] >>= (\x -> [2..8] >>= (\y -> if (x*y == 8) then [(x,y)] else [] >>= return )) -- bind desugared. Actually, it seems to be impossible to -- write this in monadic notation and I'm getting quite confused. f''' = concatMap (\x -> concatMap (\y -> if x*y == 8 then [(x,y)] else []) [2..8]) [1..4] -- That's the one I've been looking for. Remember that -- return e = [e] f'''' = concatMap (\x -> concatMap (\y -> concatMap (\_ -> [(x,y)]) (if x*y == 8 then [()] else [])) [2..8]) [1..4] Morale: Premature term elimination is the root of all misunderstanding. Haskell wins the wickedness of design contest by using [()] and [] as truth values. -- (c) this sig last receiving data processing entity. Inspect headers for past copyright information. All rights reserved. Unauthorised copying, hiring, renting, public performance and/or broadcasting of this signature prohibited.

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A common pattern
by Andrew Coppin 05 May '08

05 May '08

I've found myself writing code like this several times now. Is there a better way? read_args h = do line <- hGetLine h case line of "." -> return [] ('#':y) -> do ys <- read_args h return (y:ys)

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ANN: ReviewBoard 0.1 bindings
by Adam Smyczek 05 May '08

05 May '08

This package is part of a development tool designed to monitor code changes, analyze dependencies etc. Actually, we are still in process to develop the tool and this binding is the first functional ready package others might be interested in as well. The package contains a basic set of API calls to ReviewBoard server and a small demo app, a command line tool that makes submitting new code review requests as easy as: svn diff | mkrr -r [reviewers] (ReviewBoard does not support darcs) :( For details see haddock documentation. The ReviewBoard project page: http://code.google.com/p/reviewboard/ And the bindings on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ ReviewBoard-0.1 A thanks to this list for help with this project and help in getting started with Haskell by following many good threads. Adam

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Ann: Win32-notify 0.1
by Niklas Broberg 05 May '08

05 May '08

Hi all, I wanted to use hinotify, which gives notifications on file system changes, but found (quite naturally) that it wouldn't run on my windows machine (since it's a binding to the linux kernel). So I started writing a library that would give similar functionality on Windows. This is the first release of that, more a proof-of-concept than anything useful at this stage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Win32-notify-0.1 Still, feel free to test it, and I would love feedback on everything from desired API to suggestions on how to implement the low-level bindings more efficiently. Cheers, /Niklas

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sound synthesis
by Thomas Girod 05 May '08

05 May '08

Hi there. Following this advice (http://reddit.com/info/6hknz/comments/c03vdc7) I'm posting here. Recently, I read a few articles about Haskell (and FP in general) and music/sound. I remember an article ranting about how lazy evaluation would be great to do signal processing, but it was lacking real world example. I tried to do a little something about it, even though I'm still an haskell apprentice. So, here I come with a small bit of code, waiting for your insights to improve it. The task is to generate a sine wave and pipe it to /dev/dsp on my linux box. There is probably a nicer way to make some noise, like using SDL audio API bindings, but I didn't take the time to poke around this yet. So here it is : > module Main where > import qualified Data.ByteString as B > import Data.Word > import IO (stdout) > rate = 44100 > sinusFloat :: [Float] > sinusFloat = map (\t -> (1 + sin (t*880*2*pi/rate)) / 2) [0..44099] > sinusWord :: [Word8] > sinusWord = map (\s -> floor (s * max)) sinusFloat > where max = 255 > byte = B.pack sinusWord > main = B.hPut stdout byte It is supposed to generate a 880hz sine wav playing for one second, by typing ./bin > /dev/dsp, assuming your soundcard has a 44100hz samplingrate. /dev/dsp is supposed to receive its audio flux as an unsigned byte stream, that's why I'm converting my sine from [-1;1] to [0;1] and then to [0;255] Word8. However, I must miss something because the sound does not have the right frequency and is played too long. I guess the default sound format is 44100hz 16bits stereo, which would explain why it doesn't behave as expected. I'm wondering how I could convert a [Word16] to ByteString, and how I could use lazy evaluation to generate an infinite sine that stops with the interupt. Thomas

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