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compiling under osx
by Eli Jordan 21 Apr '05

21 Apr '05

hi im a first year computer science student, and we are currently learning haskell. We have recently begun to learn the IO functions and to compile programs. The problem i have is that when i execute the command ghc -o <exe-name> <name>.hs it is supposed to produce two files, when i do it <exe-name> is not produced. i am running under osx 10.3.9, and do not understand why this is happening, any help would be greatly appreciated. cheers Eli Jordan

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profiling and GHC6.4
by Ketil Malde 21 Apr '05

21 Apr '05

Hi, I have some problems getting profiling to work with a ghc-6.4 that I compiled from source. Everything seems to work all right, but the output doesn't contain any times; in time profiling (+RTS -p) times are all zeros, with heap profiling, I never get any 'samples' output, just an empty one. I can go back to see if there's anything I configured incorrectly, but this sounds weird enough that I thought I'd ask the list if this is a known problem (preferably with a known solution). Is it? (ghc 6.4 compiled from source with ghc 6.2.2 on Ubuntu 5.04, Linux 2.6.12-rc2) -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Variable arity function (VarArg)
by ChrisK 21 Apr '05

21 Apr '05

And while I'm posting to the list, I'll send something I wish I had found earlier. I had wanted to write show several things, and writing show 10 times was not clever. And so I initially created an infix operator to put between everything to do the showing, which was not much better. But this http://okmij.org/ftp/Haskell/types.html#polyvar-fn was the clever technique. {-# OPTIONS -fglasgow-exts #-} -- Techinque from http://okmij.org/ftp/Haskell/types.html#polyvar-fn -- canShowList and ssum will take a variable number of arguments data CanShow where { CanShow :: Show a => a -> CanShow ; CSLit::String->CanShow} instance Show CanShow where show (CanShow a) = show a show (CSLit s) = s -- The initial type is accumulator, here a simple list class (Show a) => ShowList a r where canShowList :: [CanShow] -> a -> r -- After accumulating last argument, you can apply a function, e.g. reverse instance (Show b) => ShowList b [CanShow] where canShowList l x = reverse $ (CanShow x):l -- Get next argument instance (Show a,ShowList b r) => ShowList a (b->r) where canShowList l x = canShowList ((CanShow x):l) -- Could eat initial fully typed arguments and make tuple with [] sL :: (ShowList a r) => a -> r sL = canShowList [] pio :: [CanShow]->IO() pio = putStr . unlines . (map show) eatFirstClass :: (forall a r.(ShowList a r)=>(a->r))->[CanShow] eatFirstClass s = s "and more" -- Using the same technique to do more work -- This example needs the "r->a" FunDep to work: class (Num a)=>ScaledSum a r | r->a where ssum' :: (a,a) -> a -> r instance ScaledSum Double Double where ssum' (s,t) x = (s*(t+x)) -- As an added bonus we get "context sensativity" instance ScaledSum Double Int where ssum' (s,t) x = floor (s*(t+x)) instance (ScaledSum a p) => ScaledSum a (a->p) where ssum' (s,t) x = ssum' (s,t+x) ssum a x = ssum' (a,0) x -- This fails since ::[Int] applies to (print $ build 1 2 3) --main = print $ build 1 2 3 ::[Int] -- Use parenthesis to control ::[Int] syntax main = do let empty :: [String] empty = [] full = [1,2,3] efc = eatFirstClass (sL empty full) eol = CSLit "\n" ssI = (ssum 12) 1 (1/2) (-3.5) :: Int ssD = (ssum 12) 1 (1/2) (-3.5) :: Double pio (sL "Hi!" (17,'a') eol (1,CSLit ['a']) empty (CSLit "ping\n") full efc ("ssum",ssI,ssD) "Bye!") let other = sL "Terminate with ::[CanShow]" full 1 eol 2 eol 3 ::[CanShow] putStrLn $ show other === GHCI === *Main> main "Hi!" (17,'a') (1,a) [] ping [1,2,3] [[],[1,2,3],"and more"] ("ssum",-24,-24.0) "Bye!" ["Terminate with ::[CanShow]",[1,2,3],1, ,2, ,3]

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RE: [Haskell] the wonders of lazy IO
by Simon Marlow 20 Apr '05

20 Apr '05

On 19 April 2005 06:30, Johannes Waldmann wrote: > Thanks for the comments. Indeed I knew the solution - > once I isolated the problem. > > Note that ghc(i) behaves differently when running main2 below > (it says "file is locked"). (hugs does not complain.) > > import System > > main1 = do > system "echo A > foo" > a <- readFile "foo" > system "echo B > foo" > b <- readFile "foo" > print (a == b) > > main2 = do > writeFile "foo" "A" > a <- readFile "foo" > writeFile "foo" "B" > b <- readFile "foo" > print (a == b) GHC is correct to complain about main2, FWIW. The Haskell report specifies single-writer/multiple-reader locking on files. Cheers, Simon

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readline troubles
by Arjan van IJzendoorn 19 Apr '05

19 Apr '05

Hello all, Using GHCi 6.2.2 and HaXml-1.12 gives you the output below on Windows with Cygwin. I've seen this message on several other occasions and I finally managed to fix this using info at: http://www.haskell.org/pipermail/glasgow-haskell-users/2004-November/007370… (thank you, Sigbjorn!) However, this is no trivial job and everyone who wants to compile a project using readline on Windows needs to patch this. My question is: has this been solved in GHC 6.4 and if not, can this easily be solved? Maybe by including some version of readline in the Windows GHC installation? Cheers, Arjan van IJzendoorn ----------------------- GHCi output [...header...] Loading package base ... linking ... done. Loading package lang ... linking ... done. Loading package haskell98 ... linking ... done. Loading package concurrent ... linking ... done. Loading package QuickCheck ... linking ... done. Loading package readline ... linking ... c:/apps/ghc/HSreadline.o: unknown symbol `_rl_redisplay_function' ghc.exe: unable to load package `readline'

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Re: [Haskell] the wonders of lazy IO
by Cale Gibbard 19 Apr '05

19 Apr '05

> Well, not quite. ($!) (like `seq`, out of which it's built) forces > evaluation only to "weak head normal form": essentially enough to > determine the top-level constructor. Here, for String, that means > only the first character need be evaluated, which in practice means > only the first bufferful of the file is read. You need to use ($!!) > or something like it to force evaluation of the entire file contents. > [See http://www.mail-archive.com/haskell@haskell.org/msg15819.html.] > > Dean > Ah, good point, I missed that.

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the wonders of lazy IO
by Johannes Waldmann 19 Apr '05

19 Apr '05

it took me quite a while to isolate the following. what does this program print? certainly "A" (written by the first system call) is different from "B"? import System main = do system "echo A > foo" a <- readFile "foo" system "echo B > foo" b <- readFile "foo" print (a == b) best regards, -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- ---- http://www.imn.htwk-leipzig.de/~waldmann/ -------

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Y in haskell?
by Lloyd Allison 18 Apr '05

18 Apr '05

Is it possible to define Y in Haskell (pref' H98) -- and get it to pass type checking? -- E.g. As in SML let datatype 'a rt = recrt of ('a rt) -> 'a; fun Y G = let fun Ggg (recrt g) n = G(g (recrt g))n in Ggg (recrt Ggg) end; fun F f n = if n=0 then 1 else n*f(n-1); in Y F 3 end -- I've tried various things, some of which *seem* to put the checker in a loop? -L --

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big integer in haskell
by ting wang 16 Apr '05

16 Apr '05

Hello all, I am a new learner, and i am trying to write a function to calculate factorial of a natural number: (I use ghc 6.4, window xp) fac :: Int -> Int fac n | n == 0 = 1 | n > 0 = fac (n-1) * n | otherwise = 0 this works when n < 14 and result is 1278945280, when n > 14 the answer is not right n = 15 result is 2004310016. I know it is maybe because of overflow. So i calclate 2^32, 2^64, 2^128 the result correct and is much bigger as 14!,Why? 2^n use some big integer or my implementation of n! is not correct, I'm really confused. Someone can help me? Thanks Ting Wang

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Control.Monad.Writer as Python generator
by ChrisK 16 Apr '05

16 Apr '05

Hi, I was thinking to myself: What in Haskell would give me a "yield" command like a Python generator? And the answer was "tell" in Control.Monad.Writer -- and I wrote some simple examples (see below). Most Python code using yield would be translated to something much more idiomatic in Haskell than using Writer, or to something more complicated if it needed IO. I thought this interesting enough to put on the haskell mailing list and wiki since it seemed to be in neither place (I searched, but your searching may be better than mine). If there are no objections then I'll put this example on the wiki; any suggestions where on wiki to place it (e.g. MonadWriter)? === CUT HERE === import Control.Monad.Writer -- Some type signatures would need -fglasgow-exts to compile -- We only care about the Writer output, not the function return value asGenerator :: Writer [a] v -> [a] asGenerator writer = values where (_,values) = runWriter writer --yield :: (MonadWriter [a] m) => a -> m () yield x = tell [x] -- This allows several items to be yielded with one command --yieldMany :: (MonadWriter [a] m) => [a] -> m () yieldMany = tell zeros :: [Integer] zeros = asGenerator (do yield 0 yield 0 yield 0) zerosInf :: [Integer] zerosInf = asGenerator zeros' where zeros' = (yield 0 >>zeros') -- The Collatz sequence function foo :: (Integral a) => a -> a foo x = case (x `mod` 2) of 0 -> x `div` 2 1 -> (3*x+1) -- Uses "return ()" to end the list when 1 is reached --collatzW :: (MonadWriter [a] m, Integral a) => a -> m () collatzW x = do yield x case x of 1 -> return () _ -> collatzW (foo x) -- Keeps going, will repeat "1,4,2,1,.." if 1 is reached --collatzInfW :: (MonadWriter [a] m, Integral a) => a -> m t collatzInfW x = do yield x collatzInfW (foo x) --collatzGen :: (MonadWriter [a] (Writer [a]), Integral a) => a -> [a] collatzGen x = asGenerator (collatzW x) --collatzInfGen :: (MonadWriter [a] (Writer [a]), Integral a) => a -> [a] collatzInfGen x = asGenerator (collatzInfW x) -- And these can be combined collatz1 x = asGenerator (collatzW x >> yield 0 >> collatzW (x+1)) === CUT HERE === *Main> zeros [0,0,0] *Main> take 10 zerosInf [0,0,0,0,0,0,0,0,0,0] *Main> collatzGen 13 [13,40,20,10,5,16,8,4,2,1] *Main> take 100 $ collatzInfGen 13 [13,40,20,10,5,16,8,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2, 1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2, 1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1] *Main> collatz1 12 [12,6,3,10,5,16,8,4,2,1,0,13,40,20,10,5,16,8,4,2,1]

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