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Haskell Help
by Gyslain Giguère 25 Apr '03

25 Apr '03

Hi! I'm a computer science student whose last assignment (due next wednesday) of the semester is to present and explain a Haskell 98 program I found on the internet. Unfortunately, I can't find anything written in Haskell 98 that can be explained in about 20 minutes, and doesn't use any graphic libraries (since we haven't studied them). So if you have any examples (course exercises, handouts), I would be very grateful! Thanks to all! Gyslain Giguère, Student, Cognitive Computer Science, Université du Québec à Montréal giguere.gyslain(a)courrier.uqam.ca

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Re: Extracting values from several data constructors
by Dean Herington 24 Apr '03

24 Apr '03

[redirected to haskell-cafe] On 24 Apr 2003, Steffen Mazanek wrote: > Hello. > I am sure this question is not new. Even so, please > give me the solution or point me to an old thread. > > How can I implement something like: > > data Test = T1 Int | T2 Int > test::Test->Int > test (_ x) = x > > Or is this impossible? In case of yes, why? > This would save a lot of case differentiations in > my application. Or is there an appropriate compiler option? It can be done, but only for labeled fields. For example: data Test = T1 { test :: Int } | T2 { test :: Int } See section 3.15 of the Haskell report. Dean

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

Derek Elkins <ddarius(a)hotpop.com> wrote : <snip> > Yes, I've tried it. I didn't mean GHCi doesn't EVER validate the > constraints on input, just that it doesn't when you use :t. So do you mean to use :t & cut & paste the definition? Interesting. A fair bit of work (more than just adding a parameter () to the definition), but I guess it'd work. <snip> Jon Cast

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Is this a bug in multithreading code?
by Nick Name 23 Apr '03

23 Apr '03

Hi all, does someone know why this program works ------------------------ import Concurrent loopM = sequence_ . repeat main = do v <- newMVar 0 forkIO (loopM (modifyMVar_ v (return . (1+)))) -- forkIO (loopM (modifyMVar_ v (return . (1+)))) loopM (readMVar v >>= print) ------------------------- but uncommenting the commented line makes the program fail with "stack overflow" ? Thanks for help Vincenzo

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > On Tue, 22 Apr 2003, Jon Cast wrote: > > When you have two identical methods like this, you can try removing > > the type from one and seeing what hugs or ghci thinks the type is. > > Then, you can specialize that if you want. > Unfortunately, in this case, with that function in isolation I get, > Ambiguous type variable(s) `s', `m' > in the constraint `MonadState s m' > arising from use of `get' .... Ah, yes the good old monomorphism restriction. Should have thought about that. The way around this is to make the declaration a function: > next_random_probability () = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability This will typecheck, giving most general type (MonadState s m, Num a, Random a, RandomGen s) => () -> m a, according to ghci. Then, you can take off the argument type to get the resulting type. <snip> Jon Cast

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

Derek Elkins <ddarius(a)hotpop.com> wrote: > On Tue, 22 Apr 2003 23:05:09 +0100 > Alastair Reid <alastair(a)reid-consulting-uk.ltd.uk> wrote: > > > > > > Unfortunately, in this case, with that function in isolation I get, > > > > > Ambiguous type variable(s) `s', `m' in the constraint > > > `MonadState s m' arising from use of `get' .... > > > > An easy way to get round this is to give a completely bogus type > > signature like: > > > > foo :: a > > > > The compiler will report a mismatch and the error message will tell > > you what it thinks the type is. > > Or the straightforward and simple answer: in GHCi or Hugs, just do :t > f. E.g. :t get ==> (MonadState s m) => m s. GHCi (and I imagine Hugs > too) doesn't try to validate the constraints. Have you tried this? GHCi does indeed typecheck the input (``validate the constraints'' as you put it). It has to, to be a Haskell 98-conforming interpreter. Jon Cast

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Re: Writing for both State and StateT
by Jon Cast 23 Apr '03

23 Apr '03

> On Tue, 22 Apr 2003, Derek Elkins wrote: > > I had a similar problem, but that was because I needed to use the > > constructors. MonadState has the methods you are using just write > > your code requiring MonadState as both State and StateT are > > instances. > > next_random_probability :: ( ..., MonadState g m) => m a > (snip) > Works perfectly, thanks very much! That must be the only combination I > didn't try. (-: When you have two identical methods like this, you can try removing the type from one and seeing what hugs or ghci thinks the type is. Then, you can specialize that if you want. > No doubt it'll all be clearer when I better understand what's going on! Jon Cast

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Duda
by Daniela 23 Apr '03

23 Apr '03

Estamos escribiendo un programa en el cual debemos definir la estructura de expresiones regulares y no sabemos como hacerlo en este lenguaje. Alguna idea? Gracias.

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Re: Writing for both State and StateT
by Derek Elkins 22 Apr '03

22 Apr '03

I had a similar problem, but that was because I needed to use the constructors. MonadState has the methods you are using just write your code requiring MonadState as both State and StateT are instances. next_random_probability :: ( ..., MonadState g m) => m a >From: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> >To: haskell-cafe(a)haskell.org >Subject: Writing for both State and StateT >Date: Tue, 22 Apr 2003 07:48:57 -0400 (EDT) > >At the moment I have code like, > > >next_random_probability :: (RandomGen g, Num a, Random a) => State g a > >next_random_probability = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability > >next_random_probabilityT :: (RandomGen g, Num a, Random a, Monad m) => >StateT g m a > >next_random_probabilityT = > do old_rng <- get > let (random_probability, new_rng) = randomR (0, 1) old_rng > put new_rng > return random_probability > > >Because these just use get and put which are both from MonadState, is it >possible to write just one next_random_probability with a more general >type signature, maybe mentioning MonadState, or at least two wrappers that >call the same worker function, so that the same code is good for both >State and StateT? > >-- Mark > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963

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Writing for both State and StateT
by Mark T.B. Carroll 22 Apr '03

22 Apr '03

At the moment I have code like, next_random_probability :: (RandomGen g, Num a, Random a) => State g a next_random_probability = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability next_random_probabilityT :: (RandomGen g, Num a, Random a, Monad m) => StateT g m a next_random_probabilityT = do old_rng <- get let (random_probability, new_rng) = randomR (0, 1) old_rng put new_rng return random_probability Because these just use get and put which are both from MonadState, is it possible to write just one next_random_probability with a more general type signature, maybe mentioning MonadState, or at least two wrappers that call the same worker function, so that the same code is good for both State and StateT? -- Mark

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