May 2005 - Haskell-Cafe

Int vs Integer?
by Daniel Carrera 05 May '05

05 May '05

Hello, What's the difference between "Int" and "Integer"? How can I tell Haskell which one I want? I'm having trouble with a function: ints = 0 : map (1+) ints prng = map iter ints where iter 0 = [0,0] iter n = [i,j] where i = mod n 256 j = ints !! i (eventually 'iter' will be more a more interesting function). When I try to load this module in Hugs I get the error: ERROR "./PRNG.hs":24 - Type error in list *** Expression : [i,j] *** Term : j *** Type : Integer *** Does not match : Int Line 24 is the 'prng' line. I've spent all day trying to fix this, but for the life of me I can't figure out why it happens or how to make it stop. Haskell just doesn't seem to like the 'iter !! i' construct. It's interesting that if you replace 'i' by (say) 3, Haskell runs the program happily. But, if you replace 'i = mod n 256' by 'i=3' you get the same error. Any words of wisdom would be appreciated. Cheers, Daniel.

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Where do you use Haskell?
by Daniel Carrera 05 May '05

05 May '05

Hi all, Again, I'm the new guy slowly learning this "fuctional programming" thing. :-) I've been reading, and I'm really liking the elgance of Haskell, and FP as a whole. But I wonder about the range of applicability. You see, one of the things about FP is that there are no side-effects and the same function on the same parameters always returns the same value. But any program that interacts with the real world cannot meet those properties. The function ask_user_name() will not return the same value each time. If write a chess program, the function get_user_move() will be different each time. And every time you update the screen, you're having a side-effect. So, I figure that to do these tasks you heed that "do ... <-" work around. But that kills the whole point of using FP in the first place, right? So, I'm tempted to conclude that FP is only applicable to situations where user interaction is a small part of the program. For example, for simulations. Now, I'm sure I'm not the first person to have this train of thought. And I'm sure there is a good answer why I'm wrong. :-) I'm eager to hear what that might be. Cheers, Daniel.

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Conal Elliott's Vertigo, anyone get it to run?
by Atwood, John Wesley 04 May '05

04 May '05

I tried to run Conal Elliott's Vertigo demos, on 3 diffferent XP boxes; it ran on none. The error is "Common Language Runtime Services - Appication has generated an exception that could not be handled. Process id=xxxx, Thread id=xxxx". Any suggestions? http://conal.net/Vertigo/ John Atwood

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Comparison with Clean?
by Daniel Carrera 04 May '05

04 May '05

Hi all, Anyone here familiar with the Clean programming language? http://www.cs.ru.nl/~clean/ It looks /very/ similar to Haskell, both in functionality and syntax. I would be grateful for any sort of comparison. I'm trying to decide which language I should try to learn. Cheers, Daniel.

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Speed comparison?
by Daniel Carrera 04 May '05

04 May '05

Hi all, Thank you for all the information on my previous question. I learned a lot, and good pointers to more info. My next question is about speed. How fast would you consider Haskell? (say, for computational work). How would you compare it to C, Python and Ruby? I suggest C, Python and Ruby as a basis of comparison because: 1) C is what I'd use if I wanted something fast. 2) Python or Ruby is what I'd use if I wanted clean code. I know that Haskell can be compiled, thanks to ghc. Ruby is interpreted and I'm pretty sure Python is quasi-interpreted (it's "compiled" on the fly to Java-style bytecode and a VM runs the bytecode). Furthermore, the Haskell page says that ghc produces fast programs. So I would guess that Haskell is faster than Python, but not as fast as C. Would that be correct? For computational work (e.g. a simulation, factoring primes, whatever) does it approach C speed? I would be happy to hear a comparison with any other language you find interesting.:-) As well as any other information you'd like to share. Cheers, Daniel.

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Re: [Haskell-cafe] Speed comparison?
by John Hughes 04 May '05

04 May '05

>>Furthermore, the Haskell page says that ghc produces fast programs. >>So I would guess that Haskell is faster than Python, but not as fast >>as C. >> >>Would that be correct? >> >> > >Usually yes. > > > In a sense. I think it's important to remember that what matters is performance of a whole application, not of highly tuned benchmark code, and in writing applications with good performance, then the quality of profiling tools, and (especially) the amount of time the programmer has available for tuning, can be far more important than the compiler. There have certainly been occasions where Haskell entries to the ICFP programming contest have beat OCaml or even C/C++ entries on performance, by a wide margin. Likewise Ericsson's ATM switch, which is controlled by a million lines plus of Erlang (running on a byte code interpreter!), performs better than all its competitors. Benchmarks give a very one-sided view, ignoring the large effect that ease of programming can have on the final system's performance. John Hughes

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RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Simon Peyton-Jones 04 May '05

04 May '05

I'm afraid so, as of today. It's the kind of thing that could get fixed if enough people wanted it, but at the moment it's pretty low on the list. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Henning Thielemann | Sent: 04 May 2005 10:31 | To: Simon Peyton-Jones | Cc: Haskell Cafe | Subject: RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes,instances | | | On Wed, 4 May 2005, Simon Peyton-Jones wrote: | | > hi-boot files can't contain class declarations. This should be checked, | > but isn't, hence puzzling message. | > | > GHC 6.4 has a better setup. Actually, it still doesn't check for | > absence of class decls, but it will shortly. | | This means it is impossible to declare mutually dependent classes where | each class resides in a separate module? | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Comparison with Clean?
by Krasimir Angelov 04 May '05

04 May '05

If you want to compare Haskell and Clean then take a look at Object I/O library. It is ported to Haskell and you can learn a lot comparing both implementations. I spent a lot of time translating Clean functions to Haskell and my own impression is that Haskell's do notation is much more easier that uniqueness typing. In Clean you have to explicitly maintain the state while in Haskell it is hidden. In order to preserve uniquenes property of state, quite often you have write uggly functions like: getXYZ :: State -> (XYZ, State) getXYZ st = (f st, st) while in Haskell the above function should be: getXYZ :: State -> XYZ getXYZ st = f st It is more clean and from the type signature you can see that this is readonly access to the state. Cheers, Krasimir On 5/4/05, Daniel Carrera <dcarrera(a)digitaldistribution.com> wrote: > Hi all, > > Anyone here familiar with the Clean programming language? > > http://www.cs.ru.nl/~clean/ > > It looks /very/ similar to Haskell, both in functionality and syntax. > > I would be grateful for any sort of comparison. I'm trying to decide > which language I should try to learn. > > Cheers, > Daniel. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Simon Peyton-Jones 04 May '05

04 May '05

hi-boot files can't contain class declarations. This should be checked, but isn't, hence puzzling message. GHC 6.4 has a better setup. Actually, it still doesn't check for absence of class decls, but it will shortly. Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Henning Thielemann | Sent: 04 May 2005 10:10 | To: Haskell Cafe | Subject: [Haskell-cafe] Mutual recursive modules with GHC: Classes, instances | | | I tried to set up mutual recursive modules according to section 4.6.10. of | the manual of GHC-6.2.2. I illustrate my problems with a simple fictitious | example. | | ---- A.hi-boot ---- | | module A where | | | class GHC.Num.Num a => C a where | answer :: a | | f :: B.T -> GHC.Num.Int | | | ---- A.hs ---- | | module A where | | import B | | class Num a => C a where | answer :: a | | instance C Int where | answer = 42 | | f :: B.T -> Int | f (B.Cons x) = x | | | ---- B.hs ---- | | module B where | | import {-# SOURCE #-} A | | | data T = Cons Int | | newAnswer :: Int | newAnswer = A.answer + 1 | | -------------- | | | $ ghc --make A.hs B.hs | Chasing modules from: A.hs,B.hs | Compiling B ( B.hs, B.o ) | | B.hs:1: | Failed to find interface decl for `A.$dmanswer' | from module `A' | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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[Haskell-cafe] Mutual recursive modules with GHC: Classes, instances
by Henning Thielemann 04 May '05

04 May '05

I tried to set up mutual recursive modules according to section 4.6.10. of the manual of GHC-6.2.2. I illustrate my problems with a simple fictitious example. ---- A.hi-boot ---- module A where class GHC.Num.Num a => C a where answer :: a f :: B.T -> GHC.Num.Int ---- A.hs ---- module A where import B class Num a => C a where answer :: a instance C Int where answer = 42 f :: B.T -> Int f (B.Cons x) = x ---- B.hs ---- module B where import {-# SOURCE #-} A data T = Cons Int newAnswer :: Int newAnswer = A.answer + 1 -------------- $ ghc --make A.hs B.hs Chasing modules from: A.hs,B.hs Compiling B ( B.hs, B.o ) B.hs:1: Failed to find interface decl for `A.$dmanswer' from module `A'

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