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scholarship information
by To Chieu Cuong 09 Mar '02

09 Mar '02

Dear Sir or Madam, I'm working at Department of Engineering Mechanics, Ho Chi Minh city University of Technology, Viet Nam as a teaching assistant. I have a M.E. of Civil Engineering and a B.S. of Computer Science. I'm looking for a scholarship. Would you please send me scholarship information of your university? I look forward to hearing from you. With best regards, Cuong. _________________________________________________________ Do You Yahoo!? Get your free @yahoo.com address at http://mail.yahoo.com

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Debugging Haskell
by Jerry, JiJie 09 Mar '02

09 Mar '02

Good day everyone, I was fiddling around with this tiny echo client/server haskell program from 'The Great Language Shootout' site (http://www.bagley.org/~doug/shootout/) and got stuck. The code (attached) has been reformatted with minimal API tweak (mkPortNumber, writeSocket, readSocket) to please my ghc-5.02.2, and all what I get is something stuck forever after the first iteration: $ ./echo 3 Client wrote: Hello there sailor Server recv: Hello there sailor Server read: Hello there sailor Server wrote: Hello there sailor After adding all these print statement, I still don't have a clue what's jammed there. Hope someone here can shred some light. BTW, I'd also like to take this chance to ask how to debug a haskell program in general? Regards, Jerry

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Concurrent Haskell (GHC) and Win32 Applications ?
by Antony Blakey 09 Mar '02

09 Mar '02

Hi, I'm experimenting with GHC as a platform for writing a Win32 GUI application, and I've hit what I think is a brick wall. The problem I think is with the lightweight thread implementation - Win32 calls can (and will) block the OS thread that makes the call, which blocks the entire system. Given that I'm wanting to write a network server with a Win32 GUI, this is obviously a Bad Thing. So, does any know if I missing some vital piece of information? I am using the following reduced test case, which I don't think is clueless. import Win32 import Concurrent endless:: Char -> IO () endless c = do { putChar c; endless c } main = do { task1; task2; endless 'b' } where task1 = forkIO ( do { messageBox nullHANDLE "Hey" "Yeah" mB_OK; return () } ) task2 = forkIO ( endless 'a' ) It does intersperse the 'a' and 'b' output nicely, but stops while the message box is on the screen. I'm not keen on writing a seperate GUI management thread in C++ and linking - the point is to do all of this in Haskell and write an application indistinguishable from a C++ GUI app e.g. imagine Simon Peyton Jones' sample webserver with a realtime GUI monitor. Thanks in advance for any pointers!

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RE: first-class polymorphism beats rank-2 polymorphism
by Ralf.Laemmel＠cwi.nl 08 Mar '02

08 Mar '02

Simon Peyton-Jones wrote: > In fact GHC does "forall-lifting" on type signatures to bring the > foralls to the front. But there's a bug in 5.02's forall-lifting... > ... > Perhaps you can try the 5.03 snapshot release? Certain things work there. In fact, it is fascinating. But now I did a new experiment ... It seems that forall-lifting does not work as expected for type synonyms in 5.03. Here is an example from my upcoming ICFP submission :-) Here is an innocent type synonym for generic functions with three parameters of kind * -> *: type Generic i o m = forall x. i x -> m (o x) This is a good candidate for all the parameters: type Id x = x Now I tried to define sequential composition. In fact, the following type deals with a very restricted case for simplicity: sequ :: forall t. (Generic t t Id) -> (Generic t t Id) -> (Generic t t Id) sequ f g = undefined Looking at the type of sequ, the foralls for t end up at the top. Hence, I have no chance to define sequential composition. Main> :t sequ forall t x1 x11 x. (t x1 -> Id (t x1)) -> (t x11 -> Id (t x11)) -> t x -> Id (t x) Main> Please let me know if this is a bug. Please don't fix it too soon :-) because otherwise I had to rewrite some material which is now based on first-class polymorphism and datatypes for i, o, and m. Such code will probably look better in proper rank-2 style with type synonyms for type constructor parameters. I just realized that type synonyms in GHC seem to be valid arguments even when not completely instantiated? This is wonderful. It is not supported in hugs. How difficult is it to cope with this? Does it make a real difference for the type system? Ralf

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[E-CFP] WFLP2002 - deadline is approaching
by Marco Comini 08 Mar '02

08 Mar '02

Dear Colleague I would like to remind you that the WFLP 2002 paper submission deadline is approaching. WFLP 2002 is the 11th international workshop on functional and (constraint) logic programming. WFLP 2002 aims to bring together researchers interested in functional programming, (constraint) logic programming, as well as their integration. For further details about the workshop please visit the web site http://www.dimi.uniud.it/~wflp2002/ Papers are due by March 20, 2002 You can sumbit papers following the electronic sumbission process at: http://www.dimi.uniud.it/~wflp2002/submit/ We hope that this "Call For Papers" will be useful for your work. Please forward the following to anybody who you think may be interested. Our apologies if you received multiple copy of this. -- Marco.

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pattern-matching with labelled types
by Andre W B Furtado 08 Mar '02

08 Mar '02

If I have: > data MyType = MT { > x :: Int, > y :: Char > } How do I update the Int value of MyType leaving the Char value unaffected? I tryied something like: > MT {x = newValue} but GHC gave me a warning about the Char value and it indeed caused strange effects. Of course, it is possible to do something like > update :: MyType -> Int -> MyType > update mt newValue = MT {x = newValue, y = oldValue} > where oldValue = y mt but this really annoys me when MyType has too many fields. Suggestions? Thanks a lot, -- Andre

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RE: rank-n polymorphism
by Simon Peyton-Jones 08 Mar '02

08 Mar '02

Mark Shields and I are writing a paper. The technical basis is in "Putting type annotations to work" Odersky/Laufer, POPL'96. Simon | -----Original Message----- | From: Artem S Alimarine [mailto:alimarin@cs.kun.nl] | Sent: 07 March 2002 16:27 | To: haskell(a)haskell.org | Subject: rank-n polymorphism | | | Dear all, | | GHC 5.0.3 supports rank-n polymorphism. | Could anyone please point me to a paper that describes type | inference algorithm used. | | Thanks in advance | Artem Alimarine | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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RE: first-class polymorphism beats rank-2 polymorphism
by Simon Peyton-Jones 08 Mar '02

08 Mar '02

| So I would claim that these two types are the same: | | forall x. Class x => (forall y. Class y => y -> y) -> x -> x | (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) | | ...so you should be able to do this: | | combinator :: (forall y. Class y => y -> y) -> (forall x. | Class x => x -> x) | combinator f x = combinator' f x | | but for some reason GHC 5.02.2 complains. I think this is a bug. Indeed the two types are the same. In fact GHC does "forall-lifting" on type signatures to bring the foralls to the front. But there's a bug in 5.02's forall-lifting... it doesn't bring the constraints to the front too. I fixed this in 5.03 a while ago, but didn't back-propagate the fix to 5.02. And indeed, 5.03 is happy with the pure rank-2 program. class Class x where combinator' :: (forall y. Class y => y -> y) -> x -> x combinator :: (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) combinator f = combinator' f It's quite a bit of extra work propagating fixes into the 5.02 branch, so I probably won't do so for this one, since only a small minority of people will trip over it. Perhaps you can try the 5.03 snapshot release? Simon

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rank-n polymorphism
by Artem S Alimarine 08 Mar '02

08 Mar '02

Dear all, GHC 5.0.3 supports rank-n polymorphism. Could anyone please point me to a paper that describes type inference algorithm used. Thanks in advance Artem Alimarine

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labelled types efficiency
by Andre W B Furtado 08 Mar '02

08 Mar '02

Another question about labelled types, this time concerning about efficiency: is there any efficiency differences between functions f and g below? > data RType = R Int Char > data Stype = S {x :: Int, y :: Char} > > f :: RType -> Int > f (R x _) = x > > g :: SType -> Int > g s = x s Thanks again, -- Andre

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