May 2002 - Haskell - Haskell.org

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by ＠Traduire 24 May '02

24 May '02

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Prob with numeric values
by Jos Mistiaen 23 May '02

23 May '02

Hello, I m having some difficulties with the following thing: I've made a class which looks somehow like this: class A a where inc::a->a .... now, when I want to make a instance of it with type Int and define the function inc like this: instance A Int where inc a = a + 1 I can't make a call to it when I enter the folowing inc 5 (When I type inc 5::Int it does the job perfectly, because I explicitly say 5 is an Integer, but how can I tell the program 5 is an Int?????ithout saying it explicitly in the prompt) What s being the problem here?? Am I overlooking something here? If you can help me... don't hesitate to contact me Thx, a haskell-lover _________________________________________________________________ Verzend en ontvang Hotmail via je mobieltje: http://mobile.msn.com

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(no subject)
by matush23＠netscape.net 21 May '02

21 May '02

Hi. Im a new haskell user, I already have gfl library, and I need instructions to manage it, and some examples, as prim, djikstra, kirchkoff. Thanks __________________________________________________________________ Your favorite stores, helpful shopping tools and great gift ideas. Experience the convenience of buying online with Shop@Netscape! http://shopnow.netscape.com/ Get your own FREE, personal Netscape Mail account today at http://webmail.netscape.com/

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Web-based Interface for Experimenting with System I
by Geoff Washburn 21 May '02

21 May '02

We would like to announce the availability of a web-based interface to software for experimenting with automated type inference for System I, a type system for the pure lambda calculus with intersection types and the new technology of _expansion_variables_. System I is the starting point for our current and future research on compositional program analysis. We believe it is a good approach to compositional analysis because it has the _principal_typings_ property (which is much stronger than and not to be confused with the property of ML-like languages often referred to by the name of "principal types"). The web interface can be found at: http://types.bu.edu/modular/compositional/experimentation-tool/ More information on our related research agenda and software for download are available at: http://types.bu.edu/modular/compositional/ Geoff Washburn, Assaf Kfoury, Joe Wells (just a few of the members of the Church Project)

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Class Multiplicity
by Ashley Yakeley 21 May '02

21 May '02

I have a curious Haskell design pattern. It's called "one class per function". It's strange, but I find that as I need more and more generality, I end up with classes that look like this: class (Monad m) => MonadGettableReference m r where { get :: forall a. r a -> m a; }; class (Monad m) => MonadSettableReference m r where { set :: forall a. r a -> a -> m (); }; ...and then I'll have a bunch of "joining" classes. Here's a joining class: class ( MonadGettableReference m r, MonadSettableReference m r ) => MonadFixedReference m r; instance ( MonadGettableReference m r, MonadSettableReference m r ) => MonadFixedReference m r; Sooner or later, for maximum generality they're all going to look like this: class Foo a where { foo :: a; }; class Bar a where { bar :: a; }; class ( Monad m, forall a. Foo (a -> m a), -- pending appropriate extension forall b. Bar (m b) ) => FooBar m; instance ( Monad m, forall a. Foo (a -> m a), forall b. Bar (m b) ) => FooBar m; I'm not sure if this is a good thing or a bad thing or what. -- Ashley Yakeley, Seattle WA

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Equivalence in class laws
by Wolfgang Jeltsch 21 May '02

21 May '02

Hello, instances of classes are required to fulfill certain laws. These laws are usually given in the form t = t' where t and t' are terms. My question is what the exact meaning of the = is. Does it mean that evaluation of the terms must lead to equal results regarding (==)? But if this is the case, what about terms of types which aren't instances of Eq? Furthermore, wouldn't equivalence be undefined if at least one of the two terms is _|_. Or does the = mean that the results of both terms must have exactly the same structure. But if this is true, what would it mean that two functions have the same structure? Does the = even include that all free variables occuring in t and/or t' must be evaluated the same way? In the following example, for instance, f x and g x wouldn't be equivalent according to the last definition although they would yield the same result for every x: f :: () -> () f _ = undefined g :: () -> () g () = undefined When evaluating f x, nothing of x would be evaluated which wouldn't be the case when evaluating g x. So x is not evaluated the same way in f x and g x but both terms always yield the same result (always _|_). Consider replacing each of the two occurences of undefined above by (). The difference in the evaluation of x would now result in different results of f x and g x if x = undefined. f x would be () and g x would be _|_. Where difference in free variable evaluation causes difference in term results there requirement of result equivalence of course includes requirement of "free variable evaluation equivalence". In these cases the second and the third definition approach from the beginning of this mail would be equivalent. Now let's consider the Monad/MonadPlus law m >>= \x -> mzero = mzero and the monadic instances of []. Here we have the case that evaluation behaviour of m is not the same for both terms. (The second term doesn't even contain m and therefore does no evaluation of m at all which is of course not true for the first term.) And here the different evaluation behaviour of m results in both terms not having the same result for every m. For example undefined >>= \x -> mzero is not mzero but _|_. You may also take a term like [1 ..] >>= \x -> mzero which maybe looks harmless but also has _|_ as result. Does this mean that [] doesn't satisfy the Monad/MonadPlus laws? I'm interested in your answers regarding all these questions. Wolfgang

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Negative literals and the meaning of case -2 of -2 -> True
by Thomas Hallgren 20 May '02

20 May '02

Hi, The Haskell report seems pretty clear about the meaning of numeric literals and negation in expressions: -2 should be interpreted as negate (fromInteger 2). That is, negated literals are not treated specially, the general rule -(e) ==> negate (e) applies. (See section 3.2 and 3.4 of the Haskell 98 report [1].) The mening of numeric literals in patterns also seems clear (section 3.17.2 says " The interpretation of numeric literals is exactly as described in Section 3.2), but the report does not say anything explicit about negated literals, although negated numeric literals is a special case in the syntax. Presumably, numeric literals should be understood as including negated numeric literals in the sections defining the meaning of pattern matching? To find out how Haskell implementations treat negated literals, I tested the following program: ------------------------------------------------ main = print (minusTwo,trueOrFalse) minusTwo = -2::N trueOrFalse = case minusTwo of -2 -> True _ -> False data N = Negate N | FromInteger Integer deriving (Eq,Show) instance Num N where negate = Negate fromInteger = FromInteger ------------------------------------------------- The result is: * hugs Dec 2001: main outputs: (FromInteger (-2),True) * ghc 5.02.2: main outputs: (FromInteger (-2),True) * hbc 0.9999.5b: main outputs: (Negate (FromInteger 2),False) * nhc98 1.12: compiler outputs: Fail: What? matchAltIf at 7:13 From this I conclude that hbc is the only Haskell implementation that treats negated literals in expressions in accordance with the report, but it treats negated literals in patterns differently. Hugs and ghc treat expressions and patterns consistently, but they disagree with the report. Nhc98 appears to be buggy. Perhaps this is not a serious problem, but wouldn't it be nice if the report and the implementations agreed on this point? I bet Simon PJ will suggest that the report is changed to match GHC, rather than vice versa, as usual :-) I think the report is right, but the meaning of negated literals in patterns could be clearer... References [1] Haskell 98 report http://research.microsoft.com/~simonpj/haskell98-revised/haskell98-report-h…

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WAAAPL 2002, final call for papers
by Ralf Hinze 20 May '02

20 May '02

Apologies if you receive multiple copies... ============================================================================ FINAL CALL FOR PAPERS ACM SIGPLAN WAAAPL 2002 [Deadline for submission: 3rd June 2002] Workshop on Algorithmic Aspects of Advanced Programming Languages Part of PLI'02 Pittsburgh, PA, USA Monday, October 7 http://www.cs.uni-bonn.de/~ralf/waaapl02.{html,pdf,ps,dvi,txt} ============================================================================ Scope ----- WAAAPL (pronounced "wapple") seeks papers on all aspects of the design, analysis, evaluation, or synthesis of algorithms or data structures in the context of advanced programming languages, such as functional or logic languages, where traditional algorithms or data structures may be awkward or impossible to apply. Possible topics include (but are not limited to): o new algorithms or data structures, o empirical studies of existing algorithms or data structures, o new techniques or frameworks for the design, analysis, evaluation, or synthesis of algorithms or data structures, o applications or case studies, o pedagogical issues (language aspects of teaching algorithms or algorithmic aspects of teaching languages). A previous WAAAPL workshop has been held in Paris (1999). Submission details ------------------ Deadline for submission: 3rd June 2002 Notification of acceptance: 1st July 2002 Final submission due: 1st August 2002 WAAAPL Workshop: 7th October 2002 Authors should submit papers of at most 12 pages, in postscript format, formatted for A4 paper, to Ralf Hinze (ralf(a)cs.uni-bonn.de) or Chris Okasaki (Christopher.Okasaki(a)usma.edu) by 3rd June 2002. The proceedings will be published by ACM and will appear in the ACM digital library. Programme committee ------------------- Richard Bird Oxford University Michael Hanus University of Kiel Ralf Hinze University of Bonn (co-chair) Zhenjiang Hu University of Tokyo Haim Kaplan Tel Aviv University Chris Okasaki United States Military Academy (co-chair) Melissa O'Neill Harvey Mudd College ============================================================================

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announce: hatchet version 0.1, a type checking/inference tool for Haskell 98
by Bernard James POPE 20 May '02

20 May '02

Hi Haskell friends, This is an announcement for Hatchet version 0.1 http://www.cs.mu.oz.au/~bjpop/hatchet.html Hatchet is a type checking and inference tool for Haskell 98. It is based on "Typing Haskell in Haskell" by Mark Jones (http://www.cse.ogi.edu/~mpj/thih/) It is incomplete, hence version 0.1, but (I think) usable. I'm making it available in the hope that it might be of use to other people. Documentation is available on the Hatchet web page. Please feel free to help make Hatchet better. Regards, Bernie.

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by giftcaravan＠usa.com 19 May '02

19 May '02

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