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Overloaded function and implicit parameter passing
by José Romildo Malaquias 27 Oct '00

27 Oct '00

Hi. While experimenting with the implicit parameter extension to Haskell 98, implemented in GHC 4.08.1 and latest Hugs, I came accross a difference among those implementations regarding overloading functions with implicit parameters. As a test consider the program ------------------------- cut here module Main where class C a where f :: (?env :: Integer) => a -> Integer instance C Integer where f x = ?env + x main = putStrLn (show (f (45::Integer) with ?env = 100)) ------------------------- cut here Hugs accepts this program and outputs 145, as expected. But GHC 4.08.1 refuses to compile it, emitting the message $ ghc -fglasgow-exts Test1.hs -o test1 Test1.hs:7: Unbound implicit parameter `env_rJX :: Integer' arising from use of `env_rJX' at Test1.hs:7 In the first argument of `+', namely `env_rJX' In the right-hand side of an equation for `f': env_rJX + x Compilation had errors Would anybody comment on what is going on with GHC? I am willing to use implicit parameters in the software I am developing, but I have the need to overload functions with implicit parameters. While Hugs is good for development, its performance may rule it out when the final product is ready. So I will need a good Haskell compiler to compile my system. Any comments? Romildo -- Prof. José Romildo Malaquias <romildo(a)iceb.ufop.br> Departamento de Computação Universidade Federal de Ouro Preto Brasil

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class instance with nested types
by Matthias Höchsmann 27 Oct '00

27 Oct '00

Hello, I have the following problem: basic datatypes > type Sequence a = [a] > data Tree a = N a (Forest a) deriving (Ord,Eq,Show) > type Forest a = Sequence (Tree a) i want to construct a class Xy > class Xy s a where > test :: s a -> a and make an instance for list of characters > instance Xy [] Char where > test [a] = a this works, and an instance for a forest and tried something like this > instance ([] Tree) Char where > test x@(N a xs):txs = a I get illegal type errors. Is it possible to use nested types in a class ? Hope you can help me Matthias

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RE: .net and haskell
by Nigel Perry 27 Oct '00

27 Oct '00

At 10:27 am -0700 26/10/00, Simon Peyton-Jones wrote: >| I was reading some .net stuff (ducks) on microsoft, and they >| mentioned haskell as one of the languages someone was >| targetting for it. >| Anyone know anything about this project? > >I know of several stabs in this direction, none completed. > >- There is most of a Java back end for GHC > >- There are various pieces of a C# back end for GHC, > compiling via in intermediate generic OO language > called GOO. Nigel Perry is working on this (actively > I think) This is indeed being worked on. Currently user code compiles (as far as it has been tested) but there is no prelude yet so it doesn't run too well ;-) The code generator was designed for research and supporting scripting, which is a kind way of saying it doesn't produce blazingly fast code. > >- Don Syme and Reuben Thomas are working on a back end for > GHC that compiles to a polymorphically-typed IL for .NET > that is Don's baby. A lot of this works, but it's not complete. > >Maybe others are doing stuff too? It's a pity that there's nothing >that's usable yet, but I hope that'll change. Cheers, Nigel

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RE: .net and haskell
by Simon Peyton-Jones 26 Oct '00

26 Oct '00

| I was reading some .net stuff (ducks) on microsoft, and they | mentioned haskell as one of the languages someone was | targetting for it. | Anyone know anything about this project? I know of several stabs in this direction, none completed. - There is most of a Java back end for GHC - There are various pieces of a C# back end for GHC, compiling via in intermediate generic OO language called GOO. Nigel Perry is working on this (actively I think) - Don Syme and Reuben Thomas are working on a back end for GHC that compiles to a polymorphically-typed IL for .NET that is Don's baby. A lot of this works, but it's not complete. Maybe others are doing stuff too? It's a pity that there's nothing that's usable yet, but I hope that'll change. Simon

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cpp superior to ghc . . .
by George Russell 26 Oct '00

26 Oct '00

Why does the Haskell language not allow "type" declarations to appear in the declaration parts of where and let clauses? I've just been writing a huge functions which requires lots and lots of repetitive internal type annotations (to disambiguate some complicated overloading) but I can't abbreviate them with "type" because they depend on things only in scope inside the function. In the end I abbreviated them with a few #define's but I don't really think it should be that way . . .

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Re: Haskell Programming Environment
by senganb＠ia.nsc.com 25 Oct '00

25 Oct '00

> Hello, > > I'm writing my master thesis. Its subject is 'Haskell Programming > Environment'. It is (or rather will be) an extended text editor working i= > n > graphical (XFree86) environment designed for Haskell programmers. It will= > be > implemented using Fudgets library. > I'm wondering what features would you like to find in such environment. W= > hat > should be neccessary, what would help, what would make writing programs > easier, etc. > I have some concepts, but I would like to hear some suggestions from you. > > Thanks for all answers. a) the ablility to highlight an area of code and get its type (be it a function, or some well-formed chunk of code b) the ability to highlight a function and get its definition in another area (think multiple text editing in vim) c) interaction with hugs/stg-hugs so that just written code can be pasted into a "hugs window" for evaluation. d) Debug mode which automatically adds "deriving show" to all datatypes which are not showable/adds exporting of all Datatypes as non-abstract for use in hugs to just allow things to be tried out. e) Debug mode which invisibly replaces functions such as "fromJust" with error making versions (... fromJust' "the file and line at which I'm invoked" ...) to make it easier to find the cause of the error (fromJust Nothing just comes up with an error telling you that it's fromJust that failed. Last time that happened, I hacked hugs to dump the evaluation stack, from which I guessed which possible fromJusts it could have been). f) Use ghc's .hi file to allow strictness of arguments to appear if you leave the mouse over an argument. g) For bonus points (harder, but really useful when stuck): given an expression, show me (possibly using daVinci) how it gets evaluated: Lazyness behaviour is not always obvious, I'd like to see it. I've been wanting to code one of these myself, but have had no time. Try and see if stg-hugs is useable yet since that would be a much better environment to do it in. Sengan

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group theory. Reply
by S.D.Mechveliani 25 Oct '00

25 Oct '00

Hi, all, To Eric Allen Wohlstadter's (wohlstad(a)cs.ucdavis.edu) : Are there any Haskell libraries or programs related to group theory? I am : taking a class and it seems like Haskell would be a good programming : language for exploring/reasoning about group theory. What I had in mind : was perhaps you could have a function which takes a list(set) and a : function with two arguments(binary operator) and checks to see whether or : not it is a group. I think it might be a fun exercies to write myself but : I'd like to see if it's already been done or what you guys think about it. Marc van Dongen <dongen(a)cs.ucc.ie> writes > I think Sergey Mechveliani's docon (algebraic DOmain CONstructor) > has facilities for that. Have a look at: > > http://www.cs.bell-labs.com/who/wadler/realworld/docon.html Sorry, DoCon (<http://www.botik.ru/pub/local/Mechveliani/docon/2.01/>) really supports the Commutative Rings, but provides almost nothing for the Group theory. For example, for the domain (Integer,Integer) it would set automatically (IsGroup,Yes) for the Additive semigroup and (IsGroup,No) for the Multiplicative semigroup. For the additive case, it would also set the group generator list [(1,0),(0,1)]. In both cases, it would also set cardinality = Infinity. Similar attributes are formed for the constructors of Permutation, Vector, Matrix, Polyninomial, Fraction, ResidueRing. And that is all. It does not provide so far any real algorithmic support for the Group theory, except some operations on permutations. But one may develop the program by adding the needed algorithms and introducing new attributes. : What I had in mind : was perhaps you could have a function which takes a list(set) and a : function with two arguments(binary operator) and checks to see whether or : not it is a group. I think it might be a fun exercies to write myself but : I'd like to see if it's already been done or what you guys think about it. I never programmed this. It looks like some exercise in algorithms. There are also books on the combinatorial group theory, maybe, they say something about efficient procedures for this. Regards, ------------------ Sergey Mechveliani mechvel(a)botik.ru

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Re: mapM/concatMapMy
by John Hughes 24 Oct '00

24 Oct '00

Sengan Baring-Gould writes: >Is >>= not lazy? since no experts have answered yet, this newbie will answer. I think it's strict. Well, it depends. (>>=) is an overloaded operator, with a different implementation for every monad -- when you define a monad, you give the implementation of (>>=). If your implementation is strict (presumably in the first operand), then (>>=) is strict *at that type*. If your implementation is lazy, then it isn't. The same goes for (+): at most types (+) is strict, but if you define your own kind of number with a lazy addition, then on that type (+) will be lazy. For many monads, (>>=) *is* strict, which fits with the intuition that it is a `sequencing' operator. But by no means for all. The simplest counter-example is the identity monad: newtype Id a = Id a instance Monad Id where return = Id Id x >>= f = f x where m>>=f is strict in m only if f is a strict function. A more interesting example is the state transformer monad: newtype ST s a = ST (s -> (a,s)) instance Monad (ST s) where return x = ST (\s -> (x,s)) ST h >>= f = ST (\s -> let (a,s') = h s ST h' = f a in h' s') where once again, the implementation of (>>=) is strict only if f is a strict function. Hence `lazy state' makes sense! John Hughes

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LOOKING FOR B-TREES MODULE
by Franck Delaplace 24 Oct '00

24 Oct '00

I am looking for an haskell module which implements balanced trees ? Can somebdody help me Thank you -- Franck Delaplace @w3:http://taillefer.lami.univ-evry.fr:8090/~delapla/ La.M.I-U.M.R C.N.R.S Université d'Evry Val d'Essonne Cours Monseigneur Roméro 91025 Evry CEDEX (France)

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CFP: Eight International Conference on Computer Aided Systems Theory
by Juan J. Quintela 23 Oct '00

23 Oct '00

The following message is a courtesy copy of an article that has been posted to comp.lang.ml,comp.lang.functional as well. Dear Colleagues, I would be most grateful if you would distribute the appended Call For Papers to your colleagues (and/or any mailing lists you see appropriate). Any help in distributing the Call For Papers would be most appreciated. Kindest regards, Juan Quintela CALL FOR PAPERS --------------- Eight International Conference on Computer Aided Systems Theory Formal Methods and Tools for Computer Science See the webpage at: http://azuaje.ulpgc.es/congresos/eurocast2001/ The topics for the workshop are: Workshop "FP" Contributions addressing to the following and similar issues: 1.Concurrency. Distributed and web applications. 2.Verification:tools and methods. 3.Persistence. 4.Typing and theoretical foundations. You only have to send one extended abstract before the: 31 October. -- In theory, practice and theory are the same, but in practice they are different -- Larry McVoy

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