- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] First steps in Haskell
by Simon Peyton-Jones 20 Dec '05

20 Dec '05

| (mild) culture shock here. It is typical for people in the Haskell | community to view things in a rather principled way. A language | tutorial is supposed to introduce /the language/. If you want to know | how to compile or execute a Haskell program, well then, look at the | appropriate tutorial on the /implementation/. At first this may appear | like deliberately creating hurdles, but it isn't, it's merely the way | many (though not all) Haskell people tend to think. They take it for | granted that a new user is at least educated enough to be aware of the | difference between the language itself, and its concrete implementation | in the form of an interpretation or a compilation system. I, for one, don't take it for granted! Furthermore, I think the Haskell community is pretty friendly; for example, a great deal of entirely non-condescending advice is given to newcomers on Haskell Café. I'm certain there are hurdles, but I think on the whole they are there by accident rather than design. (Apart from functional programming itself, which often is a bit of a hurdle, but that's the reason we are here!) It turns out that there are a couple of introductory Wiki pages already: http://www.haskell.org/hawiki/HaskellNewbie http://www.haskell.org/hawiki/HaskellDemo It'd be great if someone would like to improve them in the light of this thread. (Anyone can do this.) Ideally there should be a single Wiki page that the Haskell.org web site can point to; perhaps the editing could bear that in mind? Simon

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Haskell trickery
by Joel Reymont 20 Dec '05

20 Dec '05

Folks, How is one to interpret the following? I'm particularly interested in the "IO $ \ s -> " notatoin as I have never seen that before. allocaBytes :: Int -> (Ptr a -> IO b) -> IO b allocaBytes (I# size) action = IO $ \ s -> case newPinnedByteArray# size s of { (# s, mbarr# #) -> case unsafeFreezeByteArray# mbarr# s of { (# s, barr# #) -> let addr = Ptr (byteArrayContents# barr#) in case action addr of { IO action -> case action s of { (# s, r #) -> case touch# barr# s of { s -> (# s, r #) }}}}} Lemmih suggested that this is unrolling the code (manual inlining?) but how much speedup is that buying you? Last but not least, what is case action addr of { IO action -> case action s of { (# s, r #) -> Thanks, Joel -- http://wagerlabs.com/

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Array performance is killing me
by Joel Reymont 20 Dec '05

20 Dec '05

Folks, I managed to reduce my memory consumption but I have a performance problem now. This is a profiling report from my app. -O was not supplied when compiling the library and the relevant modules are at http://wagerlabs.com/array.tgz. COST CENTRE MODULE %time %alloc byteArrayFromPtr Script.Array 39.1 43.9 readBits Script.Array 23.6 25.5 emptyByteArray Script.Array 7.6 6.0 endian#2 Script.Endian 4.4 4.2 bytearray Script.Pickle 3.9 3.4 copyMArray Script.Array 2.8 3.4 sequ Script.Pickle 2.7 2.0 withByteArray Script.Array 2.3 2.1 num#2 Script.Pickle 1.3 1.7 wstr#5 Script.Endian 1.1 0.6 byteSize Script.Pickle 1.1 0.5 I run into trouble with packets of ~50K that consist of a header (a few bytes) and compressed data. The compressed data has to be uncompressed and then parsed again. Overall, this takes a copy from array (I use hGetArray) into Ptr Word8 to be supplied to ZLib's compress and then another copy from Ptr Word8 into a byte array for parsing. readBits is supposed to extract numbers from a byte array and can be found in Array.hs. uncompress is in ZLib.hs and the endian bits in Endian.hs. The code should compile without problems. Please help me optimize this! Thanks, Joel -- http://wagerlabs.com/

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comprehension generators from IO [a]'s ?
by Steve Harris 20 Dec '05

20 Dec '05

[reposted from haskell mailing list where I got no bites :) ] Folks, I'm new to using monads, and I'd like some help improving a function definition that I wrote and which works, but for which I think there should be a clearer way to write it. What I'm after is something like: -- (psuedo-code) [(b,c,d) | b <- getDirectoryContents a_dir, c <- getDirectoryContents (a_dir ++ "/" ++ b), d <- getDirectoryContents (a_dir ++ "/" ++ b ++ "/" ++ c) ], ie. where the generators feed from IO actions instead of lists, but I gather this comprehension style isn't supported, which is too bad because it's really easy to read. (Is this what was meant by "monad comprehensions" that I've heard reference to?) Here's how I actually wrote it, using nested folds: import System.IO -- Load directory entries 3 levels deep under a_dir, as list of tuples (b,c,d) load3DirLevels :: FilePath -> IO [(String,String,String)] load3DirLevels a_dir = do bs <- getDirectoryContents a_dir foldM (\tups b -> do cs <- getDirectoryContents (a_dir ++ "/" ++ b) foldM (\tups' c -> do ds <- getDirectoryContents (a_dir ++ "/" ++ b ++ "/" ++ c) foldM (\tups'' d -> do return $ (b, c, d) : tups'' ) tups' ds ) tups cs ) [] bs This function isn't so clear at a glance, and yet what it's doing seems like a pretty common thing to want to do: are there any library functions for monads (or IO in particular) that make this sort of thing easier, or should I to try and write my own function? Looks not too difficult to write but I think I might miss something important if I didn't ask first... How would you do it? Thanks Steve

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Help with division
by Daniel Carrera 20 Dec '05

20 Dec '05

Hello all, Playing around with Haskell... I'm writing a 'choose' function: --//-- fac :: Int -> Int fac 0 = 1 fac n = n*fac(n-1) choose :: Int -> Int -> Int choose n k = fac(n) / (fac(k) * fac(n-k)) --//-- I'm having problems with the last line. --//-- Prelude> :l test.hs Compiling Main ( test.hs, interpreted ) test.hs:13:20: No instance for (Fractional Int) arising from use of `/' at test.hs:13:20 Probable fix: add an instance declaration for (Fractional Int) In the definition of `choose': choose n k = (fac (n)) / ((fac (k)) * (fac (n - k))) Failed, modules loaded: none. --//-- The problem is that it doesn't like the / sign. For example, if I replace the / by * the function is compiled just fine. Does anyone know what I'm missing? Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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Int vs Integer
by Daniel Carrera 20 Dec '05

20 Dec '05

Hello all, I found a good Haskell tutorial (second link on the Tutorials column) (now that I know how to run the programs in it). I have a question. What's the difference between the types Int and Integer? Likewise, what's the difference between the types Float and Double? Are those just synonims? Thanks for the help. Cheers, Daniel. -- /\/`) http://oooauthors.org /\/_/ http://opendocumentfellowship.org /\/_/ \/_/ I am not over-weight, I am under-tall. /

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Using MonadError within other Monads
by Karl Grapone 20 Dec '05

20 Dec '05

Hi, I'm having trouble making use of MonadError within another Monad, in this case IO. I've blundered around for a while, trying various combinations of things, but I don't think I've fully cottoned-on to nesting of monads. Following is some code which does not compile, but hopefully shows you what my intentions are. I'd appreciate it if someone could show me how to use the nested monads in this situation. Thanks. module Test where import Control.Monad.Error import Data.Char import System.IO instance Error Int where noMsg = 1 strMsg = length f :: IO (Either String String) f = do n <- readLn if n == 2 then return $ throwError "I don't like strings with 2 characters." else do s <- mapErrs2 (g n) putStrLn s g :: Int -> IO (Either String String) g 0 = throwError "I won't do zero length strings." g 1 = do c <- getChar c' <- mapErrs (h c) return [c'] g n = do c <- getChar cs <- g (n-1) c' <- mapErrs (h c) return (c':cs) h :: Char -> Either Int Char h c | isUpper c = throwError 200 | otherwise = return $ toUpper c mapErrs :: Either Int Char -> Either String Char mapErrs (Right c) = Right c mapErrs (Left 200) = Left "The String contained uppercase characters." mapErrs (Left i) = Left ("Unrecognised failure in h, code = " ++ (show i)) mapErrs2 :: Either String String -> Either String String mapErrs2 (Right s) = Right s mapErrs2 (Left e) = Left ("g Error: " ++ e)

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RE: [Haskell-cafe] First steps in Haskell
by Simon Peyton-Jones 20 Dec '05

20 Dec '05

Daniel is right, by definition. He is a new user. He had difficulty. That much is incontrovertible. While he may seem unusual, perhaps he is only unusual in that he's told us about his experience rather than trying Perl instead. For which, much thanks, Daniel! In any case, I think it'd be great if he, or anyone else, started a Wiki page on "very first steps in Haskell", along the lines he suggests. Maybe one exists already? Once it does, it'd be good to point to it from haskell.org. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Wolfgang Jeltsch | Sent: 19 December 2005 09:50 | To: haskell-cafe(a)haskell.org | Subject: Re: [Haskell-cafe] First steps in Haskell | | Am Sonntag, 18. Dezember 2005 17:25 schrieb Daniel Carrera: | > [...] | | > This is a real problem for Haskell. I expect that a lot of people try | > Haskell and give up because they can't even write the simplest function. | | Hello Daniel, | | honestly, I have to say that during my years with Haskell, this seems to be | the first time that I see somebody trying to enter a Haskell program via the | command line of an interactive environment. So I suppose that it's not "a | lot of people" who experience the problems you do. | | > It's hard not to be put off by this. I love the theory behind Haskell, | > but the practice of it seems to be a real problem. | | Did you have a look at the Hugs or GHCi documentation? I'm very sure, it will | tell you that what you may enter at the command line are (merely) expressions | to evaluate and special commands like :t for displaying the type of an | expression, and that the Haskell declarations (normally) have to be read from | a file. | | > I hope someone will show me how to make this program work. Even better, | > I hope someone will fix the compilers and interpreters if they need | > fixing, or fix the documentation if that's what needs fixing. | | I suppose that neither the compilers and interpreters nor the documentation | need(s) fixing, although I might be wrong with the documentation. | | > Best, | > Daniel. | | Best wishes, | Wolfgang | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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XML Schema for Haskell parsing?
by Dimitry Golubovsky 19 Dec '05

19 Dec '05

GCCXML defines some XML schema for the results of a C(++) program parsing. Does there exist any agreed-upon XML schema to represent the results of Haskell program parsing? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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RE: [Haskell-cafe] +RTS -M800M
by Simon Peyton-Jones 19 Dec '05

19 Dec '05

| Is there any chance there is a bug in recent GHCs (or perhaps Linux?) | affecting this? I'm fairly sure I've lately seen Haskell programs | consuming significantly larger amounts of memory than they should be | allowed to, and I can try to isolate a test case if it is of | interest. New bugs are always possible! And in the last few months, Simon has been doing fairly major surgery on parts of the runtime system, as part of getting multiprocessor-GHC to play nicely. So if you can find a test case where GHC is generating code that uses a lot more memory than it used it, it would be very valuable to us. Thanks! Simon

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