May 2001 - Haskell-Cafe

Re: Announcement: new HOpenGL Tutorial
by Sven Panne 30 May '01

30 May '01

[ redirected to haskell-cafe ] Ronald Legere wrote: > Looks great! I think the community will really > appreciate it. And I extremely appreciate it when other people write documentation for my stuff. :-) Thanks, great job! > I will have to 'give it a whirl' myself, if I can > ever get HOpenGL to compile on my solaris sparc. (I > keep having trouble with 'test' in the makefiles. I > must have a wierd version of test or something. I am no > OS guru :) (If anyone has ever seen this problem, let > me know ) Just a note: HOpenGL has been moved into GHC's fptools repository, and the tar balls on my web page are quite old. I'm not sure if they will work with more recent versions of GHC, so if you're totally stuck, join the bleeding edge and compile GHC from CVS, adding --enable-hopengl to configure's options. This builds an OpenGL package, too, but the resulting GHC doesn't know about it, so you will have to fiddle around with GHC's options a bit. Furthermore, the binding is not yet complete again, I guess about 80% has been ported. Not optimal, but I'm working on it... Alas, other Haskell systems are not yet supported, but given the recent advances in the FFI and library area, this will hopefully change in the near future. Cheers, Sven

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Re: Functional programming in Python
by Marcin 'Qrczak' Kowalczyk 29 May '01

29 May '01

29 May 2001 22:44:38 +0200, Ketil Malde <ketil(a)ii.uib.no> pisze: > Wouldn't > x f g > in a Forth'ish machine mean > g(f,x) -- using "standard" math notation, for a change > rather than > g(f(x)) > ? It depends whether f changes the value at top of the stack or only puts something there. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Functional programming in Python
by Ketil Malde 29 May '01

29 May '01

Jerzy Karczmarczuk <karczma(a)info.unicaen.fr> writes: >> BTW, before I knew Haskell I exprimented with a syntax in which 'x f' >> is the application of 'f' to 'x', and 'x f g' means '(x f) g'. > Hmmm. An experimental syntax, you say... > Oh, say, you reinvented FORTH? Wouldn't x f g in a Forth'ish machine mean g(f,x) -- using "standard" math notation, for a change rather than g(f(x)) ? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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Why is there a space leak here?
by Tom Pledger 29 May '01

29 May '01

David Bakin writes: : | I have been puzzling over this for nearly a full day (getting this | reduced version from my own code which wasn't working). In | general, how can I either a) analyze code looking for a space leak | or b) experiment (e.g., using Hugs) to find a space leak? Thanks! | -- Dave a) Look at how much of the list needs to exist at any one time. | -- This has a space leak, e.g., when reducing (length (foo1 1000000)) | foo1 m | = take m v | where | v = 1 : flatten (map triple v) | triple x = [x,x,x] When you consume the (3N)th cell of v, you can't yet garbage collect the Nth cell because it will be needed for generating the (3N+1)th, (3N+2)th and (3N+3)th. So, as you proceed along the list, about two thirds of it must be retained in memory. | -- This has no space leak, e.g., when reducing (length (foo2 1000000)) | foo2 m | = take m v | where | v = 1 : flatten (map single v) | single x = [x] By contrast, when you consume the (N+1)th cell of this v, you free up the Nth, so foo2 runs in constant space. | -- flatten a list-of-lists | flatten :: [[a]] -> [a] : Rather like concat? Regards, Tom

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Re: Functional programming in Python
by Marcin 'Qrczak' Kowalczyk 29 May '01

29 May '01

Mon, 28 May 2001 10:23:58 +0100, Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> pisze: > It seems that right-associativity is so intuitive that even the > person proposing it doesn't get it right. :-) And even those who correct them :-) >> f x (foldr1 f xs) f x foldr1 f xs > > Wouldn't the rhs actually mean f x (foldr1 (f xs)) in current notation? No: f (x (foldr1 (f xs))) Basically Haskell's style uses curried functions, so it's essential to be able to apply a function to multiple parameters without a number of nested parentheses. BTW, before I knew Haskell I exprimented with a syntax in which 'x f' is the application of 'f' to 'x', and 'x f g' means '(x f) g'. Other arguments can also be on the right, but in this case with parentheses, e.g. 'x f (y)' is a function f applied to two arguments. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Funny type.
by Jay Cox 29 May '01

29 May '01

One day I was playing around with types and I came across this type: >data S m a = Nil | Cons a (m (S m a)) The idea being one could use generic functions with whatever monad m (of course m doesn't need to be a monad but my original idea was to be able to make mutable lists with some sort of monad m.) Anyway in attempting to define a generic show instance for the above datatype I finally came upon: >instance (Show a, Show (m (S m a))) => Show (S m a) where > show Nil = "Nil" > show (Cons x y) = "Cons " ++ show x ++ " " ++ show y which boggles my mind. But hugs +98 and ghci -fallow-undecidable-instances both allow it to compile but when i try >show s on >s = Cons 3 [Cons 4 [], Cons 5 [Cons 2 [],Cons 3 []]] (btw yes we are "nesting" an arbitrary lists here! however structurally it really isnt much different from any tree datatype) we get ERROR - *** The type checker has reached the cutoff limit while trying to *** determine whether: *** Show (S [] Integer) *** can be deduced from: *** () *** This may indicate that the problem is undecidable. However, *** you may still try to increase the cutoff limit using the -c *** option and then try again. (The current setting is -c998) funny thing is apparently if you set -c to an odd number (on hugs) it gives *** The type checker has reached the cutoff limit while trying to *** determine whether: *** Show Integer *** can be deduced from: *** () why would it try to deduce Show integer? Anyway, is my instance declaration still a bit mucked up? Also, could there be a way to give a definition of show for S [] a? Heres my sample definition just in case anybody is curious. >myshow Nil = "Nil" >myshow (Cons x y) = "Cons "++ show x ++ > " [" ++ blowup y ++ "]" > where blowup (x:y:ls) = myshow x ++ "," ++ blowup (y:ls) > blowup (x:[]) = myshow x > blowup [] = "" Thanks, Jay Cox yet another non-academic person on this list.

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Re: Type resolution problem
by Marcin 'Qrczak' Kowalczyk 29 May '01

29 May '01

Mon, 28 May 2001 17:53:38 -0700, Juan Carlos Arevalo Baeza <jcab(a)roningames.com> pisze: > Uncommenting the type expression above clears the error. > But, why can't the compiler deduce it by itself? Monomorphism restriction strikes again. See section 4.5.5 in the Haskell 98 Report. A pattern binding without an explicit type signature is considered monomorphic (must resolve to a simgle non-overloaded type) and points of usage or defaulting rules for numeric types determine the type (neither applies here). IMHO monomorphism restriction should be removed (the case for bindings of the form var = expr without type signature). -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Multithreaded stateful software
by Mark Carroll 29 May '01

29 May '01

Often I've found that quite how wonderful a programming language is isn't clear until you've used it for a non-trivial project. So, I'm still battling on with Haskell. One of the projects I have coming up is a multi-threaded server that manages many clients in performing a distributed computation using a number of computers. So, we care about state, and control flow has some concurrent threads and is partially event-driven. Some possibilities come to my mind: (a) This really isn't what Haskell was designed for, and if I try to write this in Haskell I'll never want to touch it again. (b) This project is quite feasible in Haskell but when it's done I'll feel I should have just used Java or something. (c) Haskell's monads, concurrency stuff and TCP/IP libraries are really quite powerful and useful, and I'll be happy I picked Haskell for the task. Does anyone have any thoughts? (-: I have a couple of symbolic computation tasks too that use complex data structures, which I'm sure that Haskell would be great for, but I want it to be more generally useful than that because, although it's nice to always use the best tool for the job, it's also nice not to be using too many languages in-house. -- Mark

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RE: Multithreaded stateful software
by Simon Peyton-Jones 28 May '01

28 May '01

| (c) Haskell's monads, concurrency stuff and TCP/IP libraries | are really quite powerful and useful, and I'll be happy I | picked Haskell for the task. Definitely (c). See Simon Marlow's paper about his experience of writing a web server (highly concurrent), and my tutorial "Tackling the awkward squad". Both at http://research.microsoft.com/~simonpj/papers/marktoberdorf.htm Haskell is a great language for writing concurrent applications. Simon

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RE: Functional programming in Python
by Peter Douglass 28 May '01

28 May '01

Peter Hancock wrote: > > foo( bar( baz( x ) ) ) > > it's: > > (foo ( bar (baz x) ) ) > > Clearly the outer parentheses are unnecessary in the last expression. > One undeniable advantage of (f a) is it saves parentheses. Yes and no. In ( ( ( foo bar) baz) x ) the parens can be omitted to leave foo bar baz x but in ( foo ( bar (baz x) ) ) You would want the following I think. foo . bar . baz x which does have the parens omitted, but requires the composition operator. --PeterD

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