July 2002 - Haskell - Haskell.org

(no subject)
by MDjekic 25 Jul '02

25 Jul '02

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still random number problem
by Junjie Xu 25 Jul '02

25 Jul '02

Hi there, First of all, I would say thank you very much for all who helped me during the past days. Since I am a beginner , sometime I spent even several hours to solve a very simple problem.So, I still need your help in the future. The problem is: ******************************************************** import Random uni :: IO () -> Float uni = do xs <- newStdGen let m = (head (randoms xs) :: Float ) doubleit :: Float -> Float doubleit n = 2.0*n main = print (doubleit uni) ******************************************************** The result is: bash-2.05$ ghci ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.02.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package std ... linking ... done. Prelude> :cd test Prelude> :l random.ls can't find module `random.ls' Prelude> :l random.hs Compiling Main ( random.hs, interpreted ) random.hs:11: The last statement in a 'do' construct must be an expression Failed, modules loaded: none. Prelude> ******************************************************* What is wrong with it? Thank you very much. Kevin

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Need help
by Junjie Xu 24 Jul '02

24 Jul '02

Hi there, I found I can't generate a random float number(between 0 to 1) use the following method. ***************************************************************** module Main where import System(getArgs) import Random uni :: Float uni = head (randoms g) where g = mkStdGen 77 main = print (show (uni)) ********************************************** once I run it, it will generate a float number. When I run it again, it genetrate the same result. I think maybe the cause is that the seed is 77. To solve this, I think the seed should be replaced by current time .But I don't know how to capture the time. I referenced the relevant library, but I still have no idea. Who know the solution, please give me a hand. Thank you very much. Kevin Xu

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Size �zell....
by seni＠istiyorum.com 24 Jul '02

24 Jul '02

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RE: functional dependencies
by Ashley Yakeley 24 Jul '02

24 Jul '02

At 2002-07-23 09:06, Simon Peyton-Jones wrote: >Dead right! Imagine there was a method in class D: > > class C a b => D a where > op :: a -> b > >The type of 'op' is > > op :: D a => a -> b > >You can't really expect that the 'b' here is determined by 'a'! Agreed. If you were going to allow "D a", they would have to be considered different "b"s. >Still, I suppose that if the methods in class D mention >only 'a', then it is strange to require D to be parameterised over >'b'. Thus, this seems more reasonable: > > class C a b => D a where > op :: a -> a > >Even this seems odd, because we would get the deduction > > D a |- C a b > >(i.e. from a (D a) dictionary you can get a (C a b) dictionary) >and that's only true for a particular 'b'. Well that's OK. From a (D a) dictionary, you get a (C a b) dictionary, where you're given "a", and you can find "b" from the fundep. -- Ashley Yakeley, Seattle WA

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RE: Bug in library report
by Simon Peyton-Jones 24 Jul '02

24 Jul '02

| As near as I can tell, the precedence of the bounds and | indices in an array doesn't matter at all. Simon M's | suggested change to Page 24 is therefore good for | consistency, but doesn't appear to actually have any effect. | Am I missing something? I think you are right, because the bounds are always a tuple, which is in parens by construction. Similarly the list of index value pairs is always enclosed in square brackets. But the consistency appeals to me. And it definitely matters for Ratio. Simon

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Need Help
by David Feuer 23 Jul '02

23 Jul '02

One possible solution under Linux is to use Haskell's lazy file I/O with /dev/urandom (or /dev/random if you're doing cryptography). David

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PADL'03: Final Call (subm. deadline: July 31)
by Dr. Gopal Gupta 23 Jul '02

23 Jul '02

FINAL CALL FOR PAPERS!!! Fifth International Symposium on Practical Aspects of Declarative Languages 2003 (PADL '03) http://www.research.avayalabs.com/user/wadler/padl03/ New Orleans, Louisiana, USA Jan 13-14, 2003 Co-located with POPL 2003 o PAPER SUBMISSION DEADLINE: July 31. o PADL'03 proceedings will be published as Springer Verlag LNCS, o $500 award for best/most-practical paper.

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Re: Bug in library report
by Jan-Willem Maessen 23 Jul '02

23 Jul '02

Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> guesses: > So I guess that means very few people are using Array and Ratio, > or at least we aren't show'ing and read'ing them, or the particular > enclosing precedence value that triggered this bug is very rare. I'd revise that to "very few people are using Ratio". Recent discussion suggests people are at least tinkering with arrays... [That said, when you need Rationals, nothing else really subsitutes.] As near as I can tell, the precedence of the bounds and indices in an array doesn't matter at all. Simon M's suggested change to Page 24 is therefore good for consistency, but doesn't appear to actually have any effect. Am I missing something? -Jan-Willem Maessen

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RE: unsafePerformIO around FFI calls
by Simon Marlow 23 Jul '02

23 Jul '02

> If this is true, then is it equivalently safe to wrap the following > Haskell action in unsafePerformIO: > > myFunc i = > do arr <- newArray (0,255) 0 > mapM_ (\j -> writeArray arr j (i+j)) [0..255] > foo <- newIORef 0 > mapM_ (\j -> readArray arr j >>= modifyIORef foo (+j)) [0..255] > readIORef foo >>= return > > ? (don't call me on syntax errors -- i haven't checked this > at all, but you should get the idea) Yes, but for this particular case you would use the ST monad instead (with STArray and STRef). That way you get to avoid mentioning anything with 'unsafe' in its name, and the compiler does the safety proof for you. Cheers, Simon

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