- Haskell-Cafe - Haskell.org

Int vs Integer
by Daniel Carrera 19 Dec '05

19 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 19 Dec '05

19 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 19 Dec '05

19 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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c2hs seems to ignore the types I give it when generating foreign import declarations
by Cale Gibbard 19 Dec '05

19 Dec '05

Hello, I've just started writing bindings to the Enlightenment Foundation Libraries, using c2hs. I've run into the following problem with the types in the foreign import declarations generated. In Imlib2.h, there is a function prototype: int imlib_get_visual_depth(Display * display, Visual * visual); In Imlib.chs, I have (among other things) import qualified Graphics.X11.Xlib.Types as Xlib -- ... {# fun imlib_get_visual_depth as getVisualDepth { id `Xlib.Display', id `Xlib.Visual' } -> `Int' #} I run: c2hs -k /usr/local/include/Imlib2.h Imlib.chs and in the resulting Imlib.hs, I get: --- this part is correct: getVisualDepth :: Xlib.Display -> Xlib.Visual -> IO (Int) getVisualDepth a1 a2 = let {a1' = id a1} in let {a2' = id a2} in getVisualDepth'_ a1' a2' >>= \res -> let {res' = cIntConv res} in return (res') --- this part is not: foreign import ccall safe "Imlib.h imlib_get_visual_depth" getVisualDepth'_ :: ((Ptr ()) -> ((Ptr ()) -> (IO CInt))) Why did it choose the type Ptr (), when I gave the type Xlib.Display? I could do an additional cast, but it will get tedious, as I have a lot of functions to import, and there shouldn't be any need for it anyway. As this is, it doesn't typecheck at all. If by hand, I change it to read: foreign import ccall safe "Imlib.h imlib_get_visual_depth" getVisualDepth'_ :: Xlib.Display -> Xlib.Visual -> IO CInt (which is what I expected it to produce in the first place) then it works as expected. Am I making a grievous error, or is this a bug/lack of a feature? - Cale

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About print and side-effects
by Daniel Carrera 18 Dec '05

18 Dec '05

Hi all, The recent responses to my first question (thanks guys!) included the following bit: main = print (fact 42) Now, print is a side-effect. Shouldn't it involve a do-block or a nomad or one of those scary things you hear about when learning about side effects in functional programs? How come 'print' is allowed to exist alone if it's a side-effect? 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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Substring replacements
by Daniel Fischer 18 Dec '05

18 Dec '05

Okay, I have looked up KMP and implemented it. Seems to work -- my first use of QuickCheck, too. It's slower than Bulat's and Tomasz' for Branimir's test :-(, but really fast for my test. Undoubtedly, one can still tune it. Here's the code: module KMP where import Data.Array searchReplace :: String -> String -> String -> String searchReplace "" _ str = str searchReplace src@(c:cs) dst str = process 0 str "" where len = length src pat = listArray (0,len-1) src bord = array (0,len) $ (0,-1):(1,0):[(i+1,boun i (bord!i)) | i <- [1 .. len-1]] boun i j | j < 0 = 0 | pat!i == pat!j = j+1 | otherwise = boun i (bord!j) getBord s n | m < 1 = m | s == pat!m = m | otherwise = getBord s m where m = bord!n process n str _ | n >= len = dst ++ process 0 str "" process _ "" mat = reverse mat process 0 (s:st) _ | s == c = process 1 st [s] | otherwise = s:process 0 st "" process n str@(s:st) mat | s == pat!n = process (n+1) st (s:mat) | otherwise = let j = getBord s n (ret,skip) = splitAt j mat in if j < 0 then reverse mat ++ process 0 str "" else reverse skip ++ process j str ret Cheers, Daniel

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RE: [Haskell-cafe] +RTS -M800M
by Simon Marlow 17 Dec '05

17 Dec '05

On 16 December 2005 10:05, Joel Reymont wrote: > I'm trying to restrict GHC to 800Mb of heap at runtime by passing in > +RTS -M800M, the machine has 1Gb of memory and top shows free > physical memory dropping below 175Mb. I suppose I'm missing something > obvious or paying attention to the wrong statistics, Unix has a good > VM manager after all. Are my runtime options correct, though? -M800m should do more or less the right thing, but it is possible for GHC to exceed this figure by a small percentage. Rather than considering the "worst case" requirements for the next GC, GHC uses a more "average case" estimate, which sometimes ends up being wrong, but in most cases results in better utilisation of the available memory. Cheers, Simon

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Darcs apply error while getting the complete GHC tree
by Joel Reymont 17 Dec '05

17 Dec '05

I thought I would get the _complete_ ghc tree so that I could build ghc with patches of my own. I decided to get the libraries while I'm at it. I got two errors and don't know what to do now. Do I need to get the whole tree again? Can I somehow reapply the patches that did not get applied? Should I just get the CVS trees instead? Thanks, Joel P.S. -- darcs get http://cvs.haskell.org/darcs/ghc.ghc-6.4/ Copying patch 13136 of 13136... done!es.ghc-6.4/ Applying patch 12 of 13136... Unapplicable patch: Thu Jan 11 14:26:13 GMT 1996 partain * [project @ 1996-01-11 14:06:51 by partain] darcs failed: Error applying hunk to file ./ghc/includes/rtsTypes.lh -- and then -- darcs get http://cvs.haskell.org/darcs/libraries.ghc-6.4/ ... Copying patch 3403 of 3403... done! Applying patch 2115 of 3403... Unapplicable patch: Sat Mar 27 14:07:36 GMT 2004 panne * [project @ 2004-03-27 14:07:33 by panne] Moved around the sources and the test examples to the standard places in the hierarchical libs. The NHC98 build is currently untested, but it might work, nevertheless... :-} darcs: ./HUnit/Test: createDirectory: already exists (File exists) -- http://wagerlabs.com/

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