December 2004 - Haskell-Cafe

Re: [Haskell-cafe] Why no IO transformer monad?
by Udo Stenzel 20 Dec '04

20 Dec '04

Henning Sato von Rosen <henning.von.rosen(a)gmail.com> schrieb am 18.12.04 18:31:11: > For each basic monad there seems to be a corresponding transformer, > e.g. 'StateT' for 'State' and so on. To be useful every transformer has a "run" method of some kind. For a hypothetical IO transformer it would look like runIOT :: Monad m => IOT m a -> m a ...which is basically the same as unsafePerformIO and defeats the purpose of having an IO Monad at all. If you want to retain the IO-type in the result, then you don't need IOT at all, layering any transformer on top of IO is exactly what's needed. Udo. __________________________________________________________ Mit WEB.DE FreePhone mit hoechster Qualitaet ab 0 Ct./Min. weltweit telefonieren! http://freephone.web.de/?mc=021201

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FFI woes!
by Sebastian Sylvan 20 Dec '04

20 Dec '04

Hi! I've been trying to use the C library FMOD in Haskell for about an hour now. The first test was the simple FSOUND_Init(int,int,uint) function which I got working by doing the following: foreign import ccall "fmod.h FSOUND_Init" fsound_Init :: CInt -> CInt -> CUInt -> IO CChar Which appeared to work just fine (the call went through, and I got sensible return values). Then I wanted to play some music so I tried to bring in the function FMUSIC_LoadSong which has the following C prototype (copy-pasted from the API): FMUSIC_MODULE * F_API FMUSIC_LoadSong( const char *name ); By doing this in Haskell: data MusicModule = MusicModule foreign import ccall "fmod.h FMUSIC_LoadSong" fmusic_LoadSong :: CString -> IO ForeignPtr MusicModule) I assume that this is how the ForeignPtr is meant to be used (with a dummy data type). Now if I compile this (under windows) with: ghc -fglasgow-exts --make FMODTest.hs -o test.exe -Llib -Iincludes -lfmod I get the following error: ./FMOD.hs:9: Unacceptable result type in foreign declaration: IO (ForeignPtr MusicModule) When checking declaration: foreign import ccall safe "static fmod.h &FMUSIC_LoadSong" fmusic_LoadSo ng :: Ptr CChar -> IO (ForeignPtr MusicModule) That's not a very helpful error message! Any ideas? /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862

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Why no IO transformer monad?
by Henning Sato von Rosen 19 Dec '04

19 Dec '04

Hi all! For each basic monad there seems to be a corresponding transformer, e.g. 'StateT' for 'State' and so on. I guess the reason lies in the fact that if there were an IO Transformer monad, one might run it and get the 'Universe' out of the IO monad. But, I am working on understanding IO magic, on a much more concrete level, so I am am looking for a really good explanation of the above. (And the IO monad in general.) Thanks! /Henning

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code example
by armin langhofer 19 Dec '04

19 Dec '04

hi all i'm going to write an exam in haskell tomorrow (monday) and i therefore i have to understand what this does: repeat_until::(a->Bool)->(a->a)->a->a repeat_until crit change x |crit x = x --zufrieden |otherwise = repeat_until crit change (change x) --weiter fix::(Float->Float)->Float->Float fix f = repeat_until pred f where pred x = (abs(f x -x) <= 0.01) square::Float->Float square x = x*x this is the use: ================ Prelude> :l e:\haskell\burstall.hs Main> fix square 0.01 0.01 (56 reductions, 80 cells) Main> fix square 0.1 0.01 (92 reductions, 128 cells) Main> fix square 1 1.0 (54 reductions, 77 cells) Main> Main> it seems that i dont have a clue how it works. maybe some of you could explain it to me that i can pass the exam tomorrow, thanx and best regards, armin. -- armin langhofer snail: fischer von erlachstr. 29 snail: a-5020 salzburg phone: +43 699 11648495 email: office(a)langhofer.at gnupg: http://langhofer.at/pubkey.txt

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Seeking comments on this IO proposal
by John Goerzen 17 Dec '04

17 Dec '04

Hello, One of my favorite features from Python is its notion of "file-like objects". For those of you unfamiliar with Python, here's a quick backgrounder: anything can present a file-like interface. It could be a file, socket, an interface to a gzip/gunzip processor, character set codecs, in-memory buffers, etc. File-like objects are used in Python for everything from gzip files to certain network data. The memory buffers are great for unit testing. As I've started working on some gzip stuff in Haskell, I've developed a prototype for a similar concept in Haskell. One of my main goals is to keep is as compatible with standard Haskell as I possibly can. To that end, a Handle is still acceptable as a parameter to any of the functions I'm proposing. My proposal is here: http://www.complete.org/~jgoerzen/t/MissingH.IO.HVIO.html I'm aware that others have been working on IO proposals; specifically, Simon Marlow's here: http://www.haskell.org/~simonmar/io/System.IO.html I'm especially interested in their feedback on my proposal, and I'm wondering: * Do you think HVIO has merit? * Would I be better advised to try to implement some existing ideas instead? * Are there any other implementations of these things that are ready to use? (With code) One note: I'm trying to keep the interface and behavior as identical to the normal System.IO interface as possible. So I'm still using Strings, raising the same exceptions, etc. In fact, the Handle instance of these classes looks like: instance Handle HVIOReader vGetChar = hGetChar ... -- John

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Question regarding theis mailing list and Google Groups
by Arjun Guha 16 Dec '04

16 Dec '04

Does anybody out there use Google Groups Beta to post to the `fa.haskell' group? I am directly subscribed to the email list, however I was trying some new things and it seemed like `fa.haskell' was vaguely related to the Haskell Café list. However, my post has not shown up on Haskell Café. Does anyone here know what's going on? I posted the following code to the `Haskell problem please help' thread: > rotations:: [a] -> [[a]] > rotations xs = rotate (xs ++ xs) len where > len = length xs > rotate _ 0 = [] > rotate ys n = (take len ys):(rotate (tail ys) (n - 1)) Did anyone receive this? -Arjun

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Re: Haskell problem please help
by Christian Maeder 16 Dec '04

16 Dec '04

N.B. if you want a faster answer such questions are best posted to haskell-cafe(a)haskell.org My answer: I asume that your function all_rotations somehow needs to be called recursively for the rest of the input list xs. Cheers Christian imranazad wrote: > Hi, > > im not very good with haskell, i barely know the basics, my coursework > requires me to genereate a vigenere square....... > well anyway at the moment im trying to define a functin all_rotations > which for any list returns the list of all its rotations, so far i've made > an attempt but it only rotates once....any ideas? i dont want the complete > solution just help thats all, much appreciated. > > all_rotations :: [a] -> [[a]] > all_rotations [] = [] > all_rotations (x:xs) = reverse (x:xs) : [] >

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Re: [Haskell-cafe] FFI woes!
by Udo Stenzel 16 Dec '04

16 Dec '04

Robert Dockins <robdockins(a)fastmail.fm> schrieb am 16.12.04 14:56:49: > This is exactly the guarantee you _don't_ have. The only guarantee you > have is that it _won't_ be run if the Ptr _is_ being referenced. >From the GHC library documentation: newForeignPtr :: FinalizerPtr a -> Ptr a -> IO (ForeignPtr a) [...] The only guarantee is that the finaliser runs before the program terminates. So it seems, finalization *is* guaranteed, just not promptness of finalization. > My concern is that this approach is a little too non-deterministic. > Because playing sound involves HW resources, [...] If I understand correctly, the OP is dealing with library resources (mainly memory) while the library is managing the hardware ressources. Udo. __________________________________________________________ Mit WEB.DE FreePhone mit hoechster Qualitaet ab 0 Ct./Min. weltweit telefonieren! http://freephone.web.de/?mc=021201

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Parse text difficulty
by Douglas Bromley 15 Dec '04

15 Dec '04

Hi Everyone My first post to the mailing list is a cry for help. Apologies for that. I've seen an example of how this is done in the archives but I'm afraid I'm a bit more behind than the person who seemed to understand the answer so if someone could help me?? The problem is this: I've show(n) a particular data type and it shows up as: [([2,6],"British"),([1],"Charles"),([1,8],"Clarke"),([2,6],"Council"),([2],"Edinburgh"),([1],"Education"),([4],"Increasingly")] What I want to do is format that nicely into a table. The best way of doing (I thought) was to: Remove the first "[(" and final ")]" Then replace "),(" with a newline(\n) Which would give: [2,6],"British" [1],"Charles" [1,8],"Clarke" [2,6],"Council" ......etc I get the impression I may find it easier adding newlines earlier on in my program but I thought this may be the easiest way. I'll include all the code for the whole program in case it helps to see where I'm coming from. It takes an input file of text and outputs an index to an output file. My soul question and drive is to lay out the index in a nicely formatted fashion. Any help would be very much appreciated. module TextProc where -- import Prelude hiding (Word) import IO import List type Word = String -- define types type Line = String type Doc = String start :: IO () start = do putStrLn "******** Enter Choice *********" putStrLn "1. Enter Input and Output files" putStrLn "2. Exit" putStrLn "*******************************" choice <- getLine if (choice == "1") then ( do putStrLn "Type input file name:" fileNameI <- getLine text <- readFile fileNameI putStrLn "Type output file name:" fileNameO <- getLine writeFile fileNameO (makeIndex text) ) else ( do return() ) makeIndex :: Doc -> Doc -- changed so output can be written to file makeIndex = show . shorten . -- [([Int], Word)] -> [([Int], Word)] amalgamate . -- [([Int], Word)] -> [([Int], Word)] makeLists . -- [(Int, Word)] -> [([Int], Word)] sortLs . -- [(Int, Word)] -> [(Int, Word)] allNumWords . -- [(Int, Line)] -> [(Int, Word)] numLines . -- [Line] -> [(Int, Line)] splitUp -- Doc -> [Line] splitUp :: Doc -> [Line] splitUp [] = [] splitUp ls = takeWhile (/='\n') ls : -- first line (splitUp . -- split up other line dropWhile (=='\n') . -- delete 1st newLine(s) dropWhile (/='\n')) ls -- other lines numLines :: [Line] -> [(Int, Line)] numLines lines -- list of pairs of = zip [1 .. length lines] lines -- line no. & line splitWords :: Line -> [Word] -- split up lines into words splitWords [] = [] splitWords line = takeWhile isLetter line : -- first word in line (splitWords . -- split other words dropWhile (not.isLetter) . -- delete separators dropWhile isLetter) line -- other words where isLetter ch = ('a' <= ch) && (ch <= 'z') || ('A' <= ch) && (ch <= 'Z') || ('-' == ch) numWords :: (Int, Line) -> [(Int, Word)] -- attach line no. to each word numWords (number, line) = map addLineNum (splitWords line) -- all line pairs where addLineNum word = (number, word) -- a pair allNumWords :: [(Int, Line)] -> [(Int, Word)] allNumWords = concat . map numWords -- doc pairs sortLs :: [(Int, Word)] -> [(Int, Word)] sortLs [ ] = [ ] sortLs (a:x) = sortLs [b | b <- x, compare b a] -- sort 1st half ++ [a] ++ -- 1st in middle sortLs [b | b <- x, compare a b] -- sort 2nd half where compare (n1, w1) (n2, w2) = (w1 < w2) -- 1st word less || (w1 == w2 && n1 < n2) -- check no. makeLists :: [(Int, Word)] -> [([Int], Word)] makeLists = map mk -- all pairs where mk (num, word) = ([num], word) -- list of single no. amalgamate :: [([Int], Word)] -> [([Int], Word)] amalgamate [ ] = [ ] amalgamate [a] = [a] amalgamate ((n1, w1) : (n2, w2) : rest) -- pairs of pairs | w1 /= w2 = (n1, w1) : amalgamate ((n2, w2) : rest) | otherwise = amalgamate ((n1 ++ n2, w1) : rest) -- if words are same grow list of numbers shorten :: [([Int], Word)] -> [([Int], Word)] shorten = filter long -- keep pairs >4 where long (num, word) = length word > 4 -- check word >4

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convergence of functions of Complex variables
by William Lee Irwin III 15 Dec '04

15 Dec '04

This does not work as expected on Complex numbers due to some odd typechecking hassles apparently associated with abs. How do I get this to typecheck for both real (e.g. Double) and Complex arguments? \begin{code} module Jacobi (sn, cn, dn, sd, cd, nd, cs, ds, ns, sc, dc, nc) where scd x k | abs k < 1e-14 = (sin x, cos x, 1) | otherwise = ((1+m)*s/(1+m*s^2), c*d/(1+m*s^2), (1 - m*s^2)/(1+m*s^2)) where k' = cos $ asin k m = -tanh(log(k')/2) (s, c, d) = scd (x/(1+m)) m sn x k = let (s,_,_) = scd x k in s cn x k = let (_,c,_) = scd x k in c dn x k = let (_,_,d) = scd x k in d sd x k = (sn x k)/(dn x k) cd x k = (cn x k)/(dn x k) nd x k = 1/(dn x k) cs x k = (cn x k)/(sn x k) ds x k = (dn x k)/(sn x k) ns x k = 1/(sn x k) sc x k = (sn x k)/(cn x k) dc x k = (dn x k)/(cn x k) nc x k = 1/(cn x k) \end{code}

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