May 2003 - Haskell - Haskell.org

Hugs: Instance storage space exhausted
by Elke Kasimir 20 May '03

20 May '03

Hi! Does anybody now how to deal with the hugs message Reading file "...XYZ.hs" - Instance storage space exhausted ? Any hint appreciated, Elke. -- Elke Kasimir EsPresto AG ----------------------------------------------------------------- - Software Development- Breite Str. 30-31 Tel/Fax: +49-30-90 226-750/-760 10178 Berlin/Germany

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Summary of responses to
by Brian McBride 20 May '03

20 May '03

I'd like to thank those folks who responded to my request for an Isabelle like system in Haskell. Whilst no one was able to point me to exactly what I was looking for, I did get helpful pointers to: http://www.cs.kun.nl/~janz/yarrow/ http://www.cs.yale.edu/homes/cc392/report.html http://www.math.chalmers.se/~rjmh/QuickCheck/ Brian

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Re: (no subject)
by l14881＠alunos.uevora.pt 20 May '03

20 May '03

> > `return` takes a single argument. You've given it 3 and 2 arguments in > your two uses. > > Dean > Thanks Dean it help me a lot. i'm not used to program in Haskell i usualy program in Java, C, Prolog and Delphi. I'm implementing a Prolog Interpreter in Haskell in a University Discipline. that's why i have this newby questions about Haskell. p.s.- sorry about my english i don't write in english for a while.

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Combining IO and state monads
by Graham Klyne 19 May '03

19 May '03

I've written and tested a collection of Haskell functions, and now I want to put them together into a "proper" program. The pattern of evaluation that I envisage is a program which maintains a workspace, and performs a sequence of read and write operations that reference and update this workspace, eventually returning some value based on the final state of the workspace. The workspace would seem to be appropriately manipulated using a form of state monad. And the I/O operations would be performed through an IO monad. What I'm unsure about is the best way to combine these so that the real-world state (IO) and workspace state are updated (threaded?) in parallel. I think I can imagine a solution that defines a new monad consisting of an IO paired with a state monad, and then implementing the monadic operators and access functions to manage the combined I/O and state updates. I haven't thought through the details, but my intuition is that it might be a fair amount of additional code. I notice the XML toolbox implements a similar idea [1], though in that case the state is not a separate monad, just combined with IO in a new monad type. [1] http://www.fh-wedel.de/~si/HXmlToolbox/hdoc/MonadStateIO.html I imagine that this is a common requirement, for which there exists an appropriately packaged solution. Is there a standard solution I should look to for this kind of functionality? Or is there some completely different approach that I've overlooked? #g ------------------- Graham Klyne <GK(a)NineByNine.org> PGP: 0FAA 69FF C083 000B A2E9 A131 01B9 1C7A DBCA CB5E

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(no subject)
by l14881＠alunos.uevora.pt 19 May '03

19 May '03

data Wam = Wam { falha::String, h::Int, s::Int, p::Int, heap::Array Int Pares, registos::Array Int Pares, e::Int, b::Int, mode::Char, cp::Int, code::Array Int Int, trail::Array Int Int, tr::Int, hb::Int } deriving Show deref a c = let (tag,value) = ((heap c)!(a)) in if ((tag=="REF") && (value /= a)) then return deref value c else return a c I would like to do something like that but it gives me an error. error: ERROR "trab2.hs":373 - Type error in function binding *** Term : deref *** Type : Int -> Wam -> Int *** Does not match : Wam -> Int Can anyone help, please?? thanks

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Diagrammatic representation of Haskell datatypes
by Bas van Dijk 19 May '03

19 May '03

Hi, I'm desining a visual programming language (just for fun...). The language is pretty much based on "Visual Haskell": http://ptolemy.eecs.berkeley.edu/~johnr/papers/visual.html But it uses another approach to model higher-order functions: http://citeseer.nj.nec.com/dami96higherorder.html The "functional side" of the language is pretty much finished so what's left is the "datatype side". With Object-Oriented languages such as C++ and Java you can use diagram languages such as UML to model the data. I was wondering if there exists diagrammatic languages for modelling Haskell datatypes? It would be great if someone can point me to some research papers. Thanks, Bas van Dijk.

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Blowfish
by Dominic Steinitz 18 May '03

18 May '03

I have been unable to track down the email of the author (Doug Hoyte) of the blowfish code on http://sourceforge.net/forum/forum.php?forum_id=263682. I'd like to include it the Crypto library. I think I can under the terms of the licence (GNU GPL) provided I reproduce the copyright information. Does this sound ok? Or alternatively does someone have Doug's email address so I can check it's ok with him directly. Many thanks, Dominc.

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A Haskell News Reposter
by Steffen Mazanek 17 May '03

17 May '03

Hello, after a reinstallation of our newsserver we had problems to import some vitally old articles due to its incompatibility. So I have written a little Haskell program which converts news articles into expect scripts, which repost them. This is really dirty. Is there a Haskell library which allows direct communication like telnet? It would be quite cool to have a library which provides e.g. a kind of monad for the conversation with such servers, which takes care for errors and the protocol, so that you can roughly write: do s<-connectNews server groups<-listOfGroups s haskell<-searchFor "haskell" groups articles<-listOfArticles haskell article<-searchFor "obfuscated" articles reply article (Art "Steffen.Mazanek(a)UniBw-Muenchen.de" (reSubj article) haskell "These programms are really cool") disconnect s Is there such a library? Can someone provide an example of its usage? Ciao, Steffen Mazanek -- Haskell: that's where I just curry until fail, unwords any error, drop all undefined, maybe break, otherwise in sequence span isControl and take max $, id: (d:[])

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Re: Fixing a type variable inside a function
by oleg＠pobox.com 16 May '03

16 May '03

> foo :: MyTypeClass resultType => resultType > foo = fromString (toString (undefined :: resultType)) > but I can't seem to do this -- GHC's scoped type variables don't seem > to help either, or at least I couldn't get it working. In GHC, type variables in the signature bear no relationship to (local) type variables in the body of the function. Simon Peyton Jones and Mark Shields explain that decision in their paper: http://research.microsoft.com/~simonpj/papers/scoped-tyvars They argue that a type signature does not have to immediately precede the body of the function. A type signature may be far away. In that case, relying in the body of the function something that was bound far earlier in the code did not seem reasonable. Incidentally, Chameleon took an opposite decision. Thus, if you want to use resultType in the body of the function, you have to bind it somewhere in the body of the function: foo :: MyTypeClass resultType => resultType foo::rt = fromString (toString (undefined :: rt)) Omitting the type signature, as in foo = fromString $ toString undefined works too. This is almost the same example that has been explained in the thread "Solved: Typing problems with polymorphic recursion and typeclasses" on this thread two weeks ago.

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Fixing a type variable inside a function
by Andre Pang 16 May '03

16 May '03

Hi all, I'm having some trouble trying to get a particular function working. Let's say I have a type class, and I have two functions -- fromString and toString -- which can convert a string to an instance of that type class, and convert an instance of that type class to a string, respectively. e.g. module SillyFreeTypeVariables where class MyTypeClass c where toString :: c -> String fromString :: String -> c data Instance1 = Instance1 instance MyTypeClass Instance1 where toString _ = "Instance1" fromString "Instance1" = Instance1 data Instance2 = Instance2 instance MyTypeClass Instance2 where toString _ = "Instance2" fromString "Instance2" = Instance2 Now, there's a function foo: foo :: MyTypeClass c => c foo generates things things which belong to MyTypeClass. I want to use it as follows: SillyFreeTypeVariables> foo :: Instance1 Instance1 SillyFreeTypeVariables> foo :: Instance2 Instance2 My problem is: how can foo be written? I've tried: foo = fromString (toString undefined)) but this gives back an error message: Ambiguous type variable `a' in the top-level constraint: `MyTypeClass a' arising from use of `toString' at SillyFreeTypeVariables.hs:18 Which is understandable, because the type of '(toString undefined)' is ambiguous, since there's a free type variable there ("toString undefined" doesn't actually return a type, only a type class constraint, which the type checker complains about). What I'd like is that to be able to fix the type variable in MyTypeClass for "undefined", so that it is the same type as the result type of foo, e.g. foo :: MyTypeClass resultType => resultType foo = fromString (toString (undefined :: resultType)) but I can't seem to do this -- GHC's scoped type variables don't seem to help either, or at least I couldn't get it working. I'm pretty sure that fixing the free type variable would work because if I change foo to look like foo :: MyTypeClass resultType => resultType foo = fromString (toString (undefined :: Instance1)) then the typechecker is happy and it compiles and runs, although now I can obviously only get foo to return something with a type of Instance1, thus rendering it useless :). I don't want to pass a parameter to foo to get this to work. Is it possible? Thanks muchly! -- % Andre Pang : trust.in.love.to.save

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