December 2007 - Haskell-Cafe

a positive challenge for the Haskell effort .....
by Galchin Vasili 05 Dec '07

05 Dec '07

http://code.enthought.com/enthon/ .. how do Haskell libraries/packages stack up against this challenge? Kind regards, Vasya

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An unusual prime number generator...
by Andrew Coppin 05 Dec '07

05 Dec '07

Hi guys. I just wrote this prime number generator. It's... unusual. It seems to go increadibly fast though. Indeed, I asked it to write the first 1,000,000 prime numbers to a text file, and scanning the bit array seemed to take longer than constructing the bit array! Odd... Anyway, for your amusement: module Primes2 (compute_primes) where import Data.Word import Data.Array.IO seive :: IOUArray Word32 Bool -> Word32 -> Word32 -> IO () seive grid size p = mapM_ (\n -> writeArray grid n False) [p, 2*p .. size] next_prime :: IOUArray Word32 Bool -> IO Word32 next_prime grid = work grid 2 where work grid n = do p <- readArray grid n if p then return n else work grid (n+1) copy :: Word32 -> IOUArray Word32 Bool -> Word32 -> IO (IOUArray Word32 Bool) copy p grid size = do let size' = size * p grid' <- newArray (1,size') False mapM_ (\n -> do b <- readArray grid n if b then mapM_ (\x -> writeArray grid' (n + size*x) True) [0..p-1] else return () ) [1..size] return grid' init_grid :: IO (IOUArray Word32 Bool) init_grid = do grid <- newArray (1,6) False writeArray grid 1 True writeArray grid 5 True return grid compute_primes :: Word32 -> IO [Word32] compute_primes n = do g0 <- init_grid ps <- work n g0 6 return (2:3:ps) where work top grid size | size >= top = do p <- next_prime grid putStrLn $ "Found prime: " ++ show p if p > top then return [p] else do seive grid size p ps <- work top grid size return (p:ps) | otherwise = do p <- next_prime grid putStrLn $ "Seiving prime: " ++ show p let size' = p*size grid' <- copy p grid size seive grid' size' p ps <- work top grid' size' return (p:ps) -- Debug... show_grid :: IOUArray Word32 Bool -> IO () show_grid grid = do (b0,b1) <- getBounds grid mapM_ (\n -> do x <- readArray grid n; if x then putChar '-' else putChar '#') [b0..b1] putStrLn ":"

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SingHaskell slides
by Martin Sulzmann 05 Dec '07

05 Dec '07

Slides (in pdf) are now available online: http://taichi.ddns.comp.nus.edu.sg/taichiwiki/SingHaskell2007 http://www.comp.nus.edu.sg/~sulzmann/singhaskell07/index.html - Tom & Martin

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unification would give infinite type
by Rafael 05 Dec '07

05 Dec '07

Hi... I give this error using hugs for the code: --------------------------------------------------- f = foldl (\x y -> add x y) 0 [1,2,3] add x y = return (x + y) --------------------------------------------------- I try: f = foldl (\x y -> counter x y) (return 0) [1,2,3] but it dont solve, and with ghci: " Occurs check: cannot construct the infinite type: b = m b Expected type: b Inferred type: m b In the expression: add x y In a lambda abstraction: \ x y -> add x y " thnks. att Rafael

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building HUnit and other packages on Windows cygwin ...
by Galchin Vasili 05 Dec '07

05 Dec '07

Hello, I believe that HUnit has absolutely not other package dependencies. When I do a "runhaskell Setup.hs build", I get the following error message: gcc: installation problem, cannot exec `cc1': No such file or directory. I am not sure what "cc1" is? A pass/phase of the gnu gcc compiler? Kind regards, Vasya

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foild function for expressions
by Carlo Vivari 05 Dec '07

05 Dec '07

Hi! I'm a begginer in haskell and I have a problem with an exercise, I expect someone could help me: In one hand I have a declaration of an algebra data, like this: data AlgExp a = AlgExp { litI :: Int -> a, litB :: Bool -> a, add :: a -> a -> a, and :: a -> a -> a, ifte :: a -> a -> a -> a} (being ifte an 'ifthenelse' expresion...) What I want to do is to write a fold function for expressions, something like this: foldExp :: AlgExp a -> Exp -> a foldExp alg (LitI i) = litI alg i foldExp alg (LitB i) = litB alg i foldExp alg (add exp1 exp2) = ¿¿¿??? foldExp alg (and exp1 exp2) = ¿¿¿??? foldExp alg (ifte exp1 exp2 exp3) = ¿¿¿??? ..ETC the fact is that I have no idea of what to do with the other expresions (add, and, and ifte)... I really don' t understand how to do this... It's clear that a fold function should colapse in one valour, but how can I espress it in the terms of the exercise? For further information about the problem after this, it's suposed that I have to rewrite some functions for expresions but in terms of foldexp (the one I should write before) Thank you very much!!!! -- View this message in context: http://www.nabble.com/foild-function-for-expressions-tf4932877.html#a141190… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell] Re: Haskell Digest, Vol 52, Issue 1
by Simon Marlow 05 Dec '07

05 Dec '07

Taral wrote: > On 12/4/07, Simon Marlow <simonmarhaskell(a)gmail.com> wrote: >> do >> x <- newIVar >> let y = readIVar x >> writeIVar x 3 >> print y >> >> (I wrote the let to better illustrate the problem, of course you can inline >> y if you want). Now suppose the compiler decided to evaluate y before the >> writeIVar. What's to prevent it doing that? > > Look at the translation: > > newIVar >>= (\x -> let y = readIVar x in writeIVar x 3 >> print y) > > y can't be floated out because it depends on x. y doesn't need to be floated out, just evaluated eagerly. Cheers, Simon

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Expert systems
by Joel Reymont 05 Dec '07

05 Dec '07

Is there an expert system implemented in Haskell, or a library perhaps? A CLIPS/RETE implementation? The main stumbling point, from my perspective, is how to implement a knowledge base and check whether patterns with a certain "shape" have been asserted. It's much easier to do this in a dynamically-typed language. Am I mistaken? Thanks, Joel -- http://wagerlabs.com

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Re: [Haskell] Nested guards?
by Neil Mitchell 04 Dec '07

04 Dec '07

Hi > server text > | Just xs <- parse text = let > x | "field1" `elem` xs = error "... do one thing ..." > | "field2" `elem` xs = error "... do something else ..." > in x > server _ = error "... invalid request ..." This now has the wrong semantics - before if parse text returned Just [] the error invalid request branch was invoked, now its a pattern match failure. I haven't used pattern guards that much (but will once Haskell' standardises them, or they get implemented in Hugs!), but their syntax seems quite natural. This extension seems to make it harder to understand them, and gives some nasty , | parsing issues for a human at least - quite possibly for a compiler too. Perhaps if you gave a little grammar for extended pattern guards (compared to the original) it would be easier to see how naturally they fit in. Thanks Neil

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Looking for smallest power of 2 >= Integer
by Dan Piponi 04 Dec '07

04 Dec '07

Is there anything in any of the interfaces to Integer that will allow me to quickly find the highest bit set in an Integer? If not, does anyone have any recommendations for how to do it efficiently. There are some obvious things that come to mind but which might involve quite a bit of useless copying of data internally by the implementation of Integer. -- Dan

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