October 2007 - Haskell-Cafe

SOE/GLFW compilation problems
by Joel Stanley 07 Oct '07

07 Oct '07

Hi folks, I've recently been able to get the SOE/GLFW code up and running from http://haskell.org/soe/software1.htm and things are working well under GHCi under Mac OS X. However, when attempting to compile a simple graphics script using ghc, I'm getting some unexpected link errors: $ ghc -o mygfx mygfx.hs -iSOE/src compilation IS NOT required /usr/bin/ld: Undefined symbols: _SOE_Blue_closure _SOE_Red_closure _SOE_closeWindow_closure _SOE_drawInWindow_closure _SOE_ellipse_closure _SOE_getKey_closure _SOE_openWindow_closure _SOE_polyline_closure _SOE_runGraphics_closure _SOE_withColor_closure collect2: ld returned 1 exit status I'm quite new to Haskell, so I'm not sure if there was something special I had to do in order to get the code for the closures generated by ghc. Any ideas? Thanks in advance. Joel

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Extract source code from literate Haskell (LHS) files
by Peter Verswyvelen 06 Oct '07

06 Oct '07

This is of course very easy to do manually, but does a command line tool exist for extracting source code from literate Haskell files? Thanks, Peter

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GLFW for WinHugs
by Peter Verswyvelen 06 Oct '07

06 Oct '07

The latest version of SOE comes with a wrapper for a nice GLFW library. This library comes with a demo of a 3D bouncing "Amiga" ball so it must be the best library in the world ;-) ;-) Since I'm letting my students play with WinHugs, I would prefer to have a WinHugs compatible version of that library. I tried to convert it, but I got stuck when ffihugs complained about not finding RTS.h, which seems to be a GHC-only include file. Would it be possible to convert this library to WinHugs? I guess similar work has been done for other libraries, so any hints are welcome. Thanks, Peter

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Puzzled
by Peter Verswyvelen 06 Oct '07

06 Oct '07

The following code, when compiled with GHC 6.6.1 --make -O gives a stack overflow when I enter 1000000 as a command line argument: (please don't look at the efficiency of the code, it can of course be improved a lot both in time performance and numeric precision...) import System leibnizPI :: Integer -> Double leibnizPI n = sum (map leibnizTerm [0..n]) where leibnizTerm n = let i = fromIntegral n in 4 * (((-1) ** i) / (2*i+1)) main = do args <- getArgs let n = read (head args) print (leibnizPI n) However, if I replace main = print (leibnizPI 1000000) is does not stack overflow. Now, if I leave the original main, but replace sum in leibnizPI by mysum xs = aux 0 xs where aux s (x:xs) = aux (s+x) xs aux s [] = s then I don't get a stack overflow. However, I do get a stack overflow when I compile it without -O, in all cases. This puzzles me. I don't see any non-tail calls in my code... I guess it has to do with strictness? http://www.haskell.org/haskellwiki/Performance/Strictness Why isn't it possible to annotate strictness on the type signature in Haskell as in Clean? Is this on the TODO list? Many thanks, Peter

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more basic articles on functional programming using haskell
by Vikrant 06 Oct '07

06 Oct '07

part I -> http://www.ias.ac.in/resonance/August2007/p27-48.pdf part II -> http://www.ias.ac.in/resonance/September2007/p40-63.pdf == Vikrant

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Hackathon underway
by Don Stewart 05 Oct '07

05 Oct '07

Just to let everyone know , the Haskell Hackathon is underway (the 2nd this year), in Freiburg, Germany, and library and infrastructure code is being worked on furiously -- Cabal and related tools in particular. You can follow all the action from planet.haskell.org, and from the hackathon site, including photos. http://www.haskell.org/haskellwiki/Hac_2007_II#Photos_.2F_blogs -- Don

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Re: [Haskell-cafe] Curry and uncurry
by PR Stanley 05 Oct '07

05 Oct '07

No, still no idea! I tried curry f where f :: (Num a) => (a, a) -> a and it didn't like it. For some reason I'm finding this a little chalenging. Thanks, Paul At 17:03 03/10/2007, you wrote: >On 10/3/07, PR Stanley ><<mailto:prstanley@ntlworld.com>prstanley(a)ntlworld.com> wrote: >I didn't even know about the curry and uncurry functions. I'm not >looking for the answer but some guidance would be much appreciated. >Thanks, Paul > > >You can look at the types without seeing the implementation, too. >Just start up GHCI and type: > > :t curry > >or > > :t uncurry > >Justin

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Space and time leaks
by Ronald Guida 05 Oct '07

05 Oct '07

I need some help with space and time leaks. I know of two types of space leak. The first type of leak occurs when a function uses unnecessary stack or heap space. GHCi> sum [1..10^6] *** Exception: stack overflow Apparently, the default definition for "sum" has a space leak. I can define my own sum in terms of strict foldl ... > sum' xs = foldl' (+) 0 xs ... and it doesn't overflow the stack. GHCi> sum' [1..10^6] 500000500000 (0.27 secs, 112403416 bytes) GHCi> sum' [1..10^7] 50000005000000 (2.73 secs, 1161223384 bytes) GHCi> sum' [1..10^8] 5000000050000000 (27.83 secs, 11645261144 bytes) I think there's still a space leak; I don't understand why GHCi using 10^8, 10^9, 10^10 bytes of memory for these calculations. The other type of space leak I know of is when I have a chunk of data in memory that I no longer need, such that the data is still being referenced somewhere. Since, in theory, I might still access the data, the garbage collector can't reclaim the memory. I'm not sure how to construct an example though. Regarding time leaks, I only know of one kind of leak. If I have a calculation that accumulates data over time, and I don't ask for any results until the end, then, due to laziness, that calculation might accumulate a chain of unevaluated thunks. When I get the the end and "demand" the final result, I have to wait for it while the RTS evaluates a long chain of thunks. The chain of unevaluated thunks is a space leak. The time leak occurs because the capture process and the accumulate process are supposed to be interleaved, such that I perform some computations after I capture each piece of data. If I have to wait around at the end, then it means the capture process and the accumulate process happened in sequence. As far as I know, every time leak has a companion space leak; it's not possible to create a time leak without a space leak to go with it. Is this really true? Now for the hard questions. 1. How do I go about detecting space and time leaks? 2. Once I find a leak, how do I fix it? 3. Are there any programming techniques I can use to avoid leaks? -- Ron

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The "Exp -> Term a" problem (again), how to dynamically create (polymorphic) typed terms in Haskell ??
by Pasqualino 'Titto' Assini 04 Oct '07

04 Oct '07

Hi, I am trying to write an interpreter for a little functional language but I am finding very problematic to dynamically create a typed representations of the language terms. I have googled around and found a few solutions but none seem to solve the problem. This is the example code: > {-# OPTIONS -fglasgow-exts #-} > module Eval where These are my untyped terms (what I get from my parser): > data Exp = EDouble Double | EString String | EPrim String | EApp Exp Exp deriving (Show) And these are the typed terms: > data Term a where > Num :: Double -> Term Double > Str :: String -> Term String > App :: Term (a->b) -> Term a -> Term b > Fun :: (a->b) -> Term (a->b) The problem is to write a function that converts between Exp and Term t as in: > test :: Term Double > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) So this is the conversion function: > class TypeCheck t where > typecheck :: Exp -> Term t A few primitives: > instance TypeCheck (String->String) where > typecheck (EPrim "rev") = Fun reverse > typecheck (EPrim "show") = Fun show > instance TypeCheck (Double->Double) where > typecheck (EPrim "inc") = Fun ((+1) :: Double -> Double) > instance TypeCheck (Double->String) where > typecheck (EPrim "show") = Fun show > instance TypeCheck Double where > typecheck (EDouble x) = Num x > instance TypeCheck String where > typecheck (EString x) = Str x The problem arises in the conversion of the function application (EApp). It does not seem to be possible to define "typecheck" on EApp in a generic way and is also not possible to distinguish between the different cases: > typecheck (EApp f a) = App (typecheck f :: Term (String->String)) (typecheck a:: Term String) The following pattern overlaps the previous one: > typecheck (EApp f a) = App (typecheck f :: Term (Double->String)) (typecheck a:: Term Double) To avoid this problem I could split my untyped terms in different data types as in: data EDouble = EDouble Double data App a b c = App a b c ... and define TypeCheck separetely on every data type. However, in that case what would be the type of my parser?? parser :: String -> ?? Any suggestion woud be very welcome indeed, titto

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Re: [Haskell] reading from stdin
by Stefan O'Rear 04 Oct '07

04 Oct '07

On Thu, Oct 04, 2007 at 05:46:09PM +0100, Axel Simon wrote: > Hi, > > I'm trying to continuously output data to a file handle while reading > single characters from the user to adjust the speed at which things are > output. I'm interested to get this to work in Hugs on Windows. > I successfully used the following function ghci under Mac OS: > > {{{ > getUserInput :: IO (Maybe Char) > getUserInput = do > hasInput <- hReady stdin > if hasInput then liftM Just (hGetChar stdin) else return Nothing > }}} > > This function returns a character to me if there's one available. > If anybody could give me a hint how to get this working in Hugs under > Windows, please tell me. Something in the 'win32' package, or maybe the FFI. MSDN will be very handy. > P.S.: Sorry to post to haskell@..., but nothing else seemed to match. This is a thread for haskell-cafe@. Stefan

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