- Glasgow-haskell-users

Newbie ST/Monad Question
by Carl McTague 24 Aug '01

24 Aug '01

That's so bizarre! It works now and I can't reproduce that strange output at all (and the Union-find code works flawlessly now, too)! I'm sure I'm more baffled than you. I feel like a real idiot. Sorry about that. But thanks! You've exorcized the unnatural daemon from my computer! Carl

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Re: Newbie ST/Monad Question
by Marcin 'Qrczak' Kowalczyk 24 Aug '01

24 Aug '01

Fri, 24 Aug 2001 11:28:56 -0600, Carl McTague <mctague(a)one.net> pisze: > import ST > > three :: Int > three = runST (do v<-newSTRef 0 > writeSTRef v 3 > readSTRef v) > > main = print (three*3) > > when this is compiled and run using ghc, it prints > > carl@exp(monad) ./a.out > <<ST action>> Are you sure? It prints 9 for me. > what need I do to make it print 6? Change one of 3's to 2 :-) -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Newbie ST/Monad Question
by Carl McTague 24 Aug '01

24 Aug '01

Dear Haskell Hackers, Please forgive me if I've sent this is the wrong mailing list. I'm writing a simple union/find implementation using the ST library, but I've run into some kind of conceptual problem that I've been able to isolate in the following: import ST three :: Int three = runST (do v<-newSTRef 0 writeSTRef v 3 readSTRef v) main = print (three*3) when this is compiled and run using ghc, it prints carl@exp(monad) ./a.out <<ST action>> carl@exp(monad) what need I do to make it print 6? Sincerely, Carl

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Re: Haskell on Alpha
by Ken Shan 24 Aug '01

24 Aug '01

Hello, I've finally made a release of my port of GHC 5.00.2 to alpha-dec-osf3. It is at: http://www.eecs.harvard.edu/~ccshan/ghc/ This is my first time building a binary distribution (not to mention my first time participating in GHC development)... Please let me know if you find any problems. Thanks! -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig I saw my inner child once, on a milk carton.

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MPI status?
by Duncan Coutts 24 Aug '01

24 Aug '01

Hi all, I'm doing an internship at Cray Inc this summer and I might get the opportunity of some computer time on a T3E machine. Now I am also a great Haskell fan and have been wondering about the possibility of getting parallel Haskell running on a real super-computer. I've read that someone is working on porting ghc's parallel features to an MPI implementation and also that someone recently got the alpha port working again. In case you don't know a T3E is a distributed memory alpha based super-computer. It supports MPI & pthreads. So my question is, what is the state of the art? What is the state of the MPI port? Does the alpha port work sufficiently well. One potential problem I can see is that I'm not sure gcc is avaliable on the T3E and I'm not sure how dependant ghc is upon the various GNU C extensions. So, if I can get an account, I'll have untill the end of September to test anything anyone can give me. You know they still program these machines in C, C++ & Fortran. Shocking! ;-) Duncan

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Newbie: Mutable Arrays
by Saswat Anand 24 Aug '01

24 Aug '01

Hi, I find two versions of Mutable arrays ie STArray and IOArray. What is the difference between them. Where each of them are supposed to be used? Thanks, Saswat

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RE: ghc --make and memory
by Simon Marlow 23 Aug '01

23 Aug '01

> I am finding that very large compilations fall over with ghc > --make because ghc --make > runs out of heap space. However if I restart them they > manage to continue. Is there a > way of making ghc --make use memory more efficiently? Bashing on GHC's memory use is something we want to do, but it'll be after the 5.02 release. Retainer and lag-drag-void profiling is going in post-5.02 too, which should help us track down any space leaks. Meanwhile, memory is cheap at the moment! Cheers, Simon

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ghc --make and memory
by George Russell 23 Aug '01

23 Aug '01

I am finding that very large compilations fall over with ghc --make because ghc --make runs out of heap space. However if I restart them they manage to continue. Is there a way of making ghc --make use memory more efficiently?

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RE: Newbie: Mutable Arrays
by Simon Marlow 23 Aug '01

23 Aug '01

> I find two versions of Mutable arrays ie STArray and > IOArray. What is the > difference between them. Where each of them are supposed to be used? STArray is for use in the ST monad (see the ST module), and IOArray is used in the IO monad. Otherwise, the behaviour is identical. Also check out MArray for overloaded mutable arrays, which let you abstract away from the actual monad and use the same function names for either - they even let you use strict unboxed arrays just by changing type signatures. Cheers, Simon

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RE: Problems with scoped type variables and existential data constructors
by Simon Peyton-Jones 21 Aug '01

21 Aug '01

| | /tmp/t.hs:8: | | Could not deduce `Silly a1 t' from the context (Silly a t) | | Probable fix: | | Add `Silly a1 t' | | to the the existential context of a data constructor | | arising from use of `Exist' at /tmp/t.hs:8 | | in the definition of function `applyId': Exist (Id x) | | The trouble is that | Exist :: forall a, t. Silly a t => Exist t | | Now this is an ambigous type, Sorry, I was wrong about this. The type should be data Exist a = forall t. Silly a t => Exist t Exist :: forall a, t. Silly a t => t -> Exist a So it's not ambiguous; but my other comment was right. In your example: | > applyId (Exist x) = Exist (Id x) the LHS pattern discharges the constraint (Silly a1 t), but the RHS introduces the constraint (Silly a2 t), and there's no connection between t1 and t2. Why should there be? You could give a type signature applyId :: Exist a -> Exist a and that would force them to be the same S Simon

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