October 2000 - Haskell

hugs/ghc + shared mem access
by xander 19 Oct '00

19 Oct '00

Hi, I'm exploring my options to connect 2 programs (1 Haskell, 1 non-Haskell). I could connect both programs by 1 or 2 sockets. I was wondering whether it's feasible to access shared memory from within hugs as an alternative? Any answers? Thanks, xander van wiggen

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Num class
by Koen Claessen 18 Oct '00

18 Oct '00

Hi all, For years I have wondered why the Num class has the Eq class and the Show class as super classes. Because of this, I cannot make functions an instance of Num (becuase they are not in Eq or Show). Or a datatype respresenting an infinite amount of digits (because Eq would not make any sense). Now I have found out the reason! However, it does not make me happy, it makes me even more sad. It is of the defaulting mechanism of course! The defaulting mechanism works as follows: If there is an unresolved overloading error on a type variable a, which has as an *only* constraint (Num a), then we take a to be the suitable default. If Show were not a super class of Num, the following program would generate an error: main = print 42 If Eq were not a super class, the following program would not work: main = print (if 42 == 42 then "koe" else "apa") These programs are all fixed by inserting Show and Eq as super classes of Num. So that one does not even notice! Until now. I am interfacing to an external library that uses double-precision floating points internally for all numbers. This is to be as general as possible. However, I know that when I put for example an Integer in, I get one out too. Thus, I want to give a Haskell interface that can deal with this by any numeric type. So I define a type class: class Num a => Number a where convertToDouble :: a -> Double convertFromDouble :: Double -> a (somehow the Haskell numerical hierarchy does not even let me define general functions that do this! -- but that is besides the point.) instance Number Int instance Number Integer instance Number Float instance Number Double ... All my library functions now have the shape: libraryFunction :: Number a => ... a ... Where as actually: primLibraryFunction :: ... Double ... And now the bad thing... When I use "libraryFunction" on a numeric constant, such as 42, I get the error: ERROR "library.hs" (line 8): Unresolved overloading *** Binding : main *** Outstanding context : Number b This is really annoying, and it is not clear why the default mechanism works this way. So here are my questions. Why does the default mechanism have this restriction? I know that the default mechanism is already broken (some desirable properties are destroyed) -- what properties will be broken by lifting this restriction? /Koen. -- Koen Claessen http://www.cs.chalmers.se/~koen phone:+46-31-772 5424 mailto:koen@cs.chalmers.se ----------------------------------------------------- Chalmers University of Technology, Gothenburg, Sweden

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pronunciation of >>=
by Scott Turner 18 Oct '00

18 Oct '00

Is there a common way to pronounce ">>=" in discussions or when teaching? I've learned all my Haskell from printed/visual documents. -- Scott Turner p.turner(a)computer.org http://www.ma.ultranet.com/~pkturner

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Mutually recursive structures
by Timothy Docker 17 Oct '00

17 Oct '00

The following problem has been taxing me.... I have a list of pairs that I have parsed from a input file, which represent a hiirarchy, where the first element is the name of the object, and the second is the name of the parent if there is one: type ParseOutput = [(String,Maybe String)] I wish to convert this to a list of "objects", where from each object I can navigate to the parent object (if any), or the children (if any): data Obj = Obj { name::String, parent::(Maybe Obj), children::[Obj] } type Result = [Obj] convert:: ParseOutput -> Result In a language with mutable references, this would be a relatively straightforward. I would just create a dictionary mapping from name to Obj, and then iterate over them, filling in the parents and children where appropriate. odict = {} for (name,parent) in parseOutput: odict[name] = Obj() for (name,parent) in parseOutput: if parent: parent = odict[parent] child = odict[name] child.parent = parent parent.children.append( child ) This gives away my background! How can I do this in Haskell? If I don't have mutable references, I figure that I must need to use laziness in some way, perhaps similar to how I would build an infinite structure. A hint or two would be great. Tim

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Haskell function with String arguments in DLL
by Christian Lescher 17 Oct '00

17 Oct '00

I'm trying to build a DLL with a Haskell function of type String -> String, that should be called from outside (e.g. VBA), but it still doesn't work. I'm afraid I need an example or some kind of 'step by step instruction'. Who can help me? (I use GHC 4.08.1.) Christian

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Haskell puzzle
by Rishiyur S. Nikhil 16 Oct '00

16 Oct '00

In Haskell, is the following transformation always legal? \x->\y->e transforms to \x y->e After answering this question, please scroll down about 75 lines for a follow-up question. What if the two lambda-bound variables are the same? I.e., the lhs is \x->\x->e Please refer to Section 3.3 of the Haskell 98 Report, para 3 (one line para).

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NHC98 and GHC 4.08.1 differ on monad related functions
by José Romildo Malaquias 14 Oct '00

14 Oct '00

Hello. While porting Haskore to NHC98 I got an error I am not understanding. I have attached a test module that shows the error message: $ nhc98 -c Test.hs ==================================== Error after type deriving/checking: No default for Monad.MonadPlus at 7:1.(171,[(2,209)]) No default for Monad.MonadPlus at 6:1.(174,[(2,208)]) GHC 4.08.1 and Hugs98 accepts the code without complaining. Any hints? Romildo -- Prof. José Romildo Malaquias <romildo(a)iceb.ufop.br> Departamento de Computação Universidade Federal de Ouro Preto Brasil

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Binary files and NHC98
by José Romildo Malaquias 14 Oct '00

14 Oct '00

Hello. In order to experiment with the Binary module distributed with nhc98, I wrote the attached program which writes a binary file and then reads it. When executed, I got an extra byte (8) that I cannot explain: [65,66,67,68,8] Any clues why it appears? Regards, Romildo -- Prof. José Romildo Malaquias <romildo(a)iceb.ufop.br> Departamento de Computação Universidade Federal de Ouro Preto Brasil

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Haskore and nhc98
by José Romildo Malaquias 14 Oct '00

14 Oct '00

Hello. I am just curious whether anybody has already tried Haskore (http://www.haskell.org/haskore/) with the NHC98 Haskell compiler. I am trying to do it, while GHC 4.08.1 is non functional in my RH Linux 7.0 box. Romildo -- Prof. José Romildo Malaquias <romildo(a)iceb.ufop.br> Departamento de Computação Universidade Federal de Ouro Preto Brasil

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(no subject)
by Luciano Caixeta Moreira 13 Oct '00

13 Oct '00

unsubscribe haskell(a)haskell.org

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