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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 05 Dec '07

05 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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when I am rebuilding a package that has package dependencies
by Galchin Vasili 04 Dec '07

04 Dec '07

Hello, Which document discusses how to first build all dependencies and then finally the target package? I prefer to read relevant documentation so I get fully up to speed rather than take up bandwidth on this newsgroup asking one question at a time. Regards, Vasya

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isSpace
by Ryan Bloor 04 Dec '07

04 Dec '07

hi I am having trouble with a function that is supposed to eliminate spaces from the start of a String and return the resulting string. I reckon a dropWhile could be used but the isSpace bit is causing me problems... words :: String -> String words a = case dropWhile isSpace a of "" -> s:ss -> (s:word) : words rest where (word,rest) = break isSpace ss Thanks Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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