April 2003 - Haskell-Cafe

RE: Super Haddock (was Re: Literate Programming)
by Simon Marlow 25 Apr '03

25 Apr '03

Ketil Z. Malde writes: > > "Simon Marlow" <simonmar(a)microsoft.com> writes: > > > Somebody else¹ wrote: > > >> This seems like a good way of transitioning from .lhs to > >> haddock'd .hs. In the long run, I think we (meaning Simon :-) > >> should to extend haddock to take the place of the .lhs style of > >> documenting code. > > > That's an interesting idea. It's not at all what Haddock > was intended > > for, but that's not to say it couldn't be done! > > I'm not sure I would like this. I guess I'm one of (the apparently > very few?) who are using LaTeX lhs style (using \begin/\end{code}). > Would a Haddock replacement give me the same kind of functionality in > producing a nice printable copy? I definitely have grown attached to > having math, footnotes, page headings, sections, and so on. Haddock isn't (intended to be) a literate programming system, so I don't imagine it will replace full LaTeX literate source any time soon. The suggestion is just that it could produce nicely-formatted source code complete with lightly marked-up comments - which I think is a fine idea, and not too difficult to implement using the existing framework. > ¹) As Exchange doesn't leave useful thread information, I'm not quite > sure who. Simon and others, if you must use Exchange, could you take > care to leave an attribution, prefereably including the message-id? I can get proper In-Reply-To headers by avoiding Exchange altogether, but it requires me to select an option each time I send a message, so I often forget (sorry!). I've reported it as a bug in Exchange, but it might be a generation or two before it gets fixed. Cheers, Simon

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Slow program
by Mark Carroll 25 Apr '03

25 Apr '03

One of my GHC programs is running much more slowly than I'd expect, and I wonder if laziness is getting in the way or if it's getting into an infinite loop. How should I diagnose the problem? Tonight I've started with Hat - it seems to install okay, but doesn't seem to have a Control/Monad/Identity.hx file that I need. ): -- Mark

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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 23 Apr '03

23 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 22 Apr '03

22 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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