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Question: Database access in Haskell
by yang dianne 07 Mar '02

07 Mar '02

Dear Sir/Madam, I am considering implement a system using Haskell. I need to use database to manage data there. How can I access the data records in database from Haskell? Thanks for your advice! Best Regards, Dianne __________________________________________________ Do You Yahoo!? Try FREE Yahoo! Mail - the world's greatest free email! http://mail.yahoo.com/

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new version of Parser Combinators and Syntax Macros's (beta)
by S. Doaitse Swierstra 07 Mar '02

07 Mar '02

At: http://www.cs.uu.nl/groups/ST/Software/UU_Parsing you will find the latest/greatest version of our combinators, that are: - faster (faster than Parsec) - correct much faster - compute results lazily, and produce error messages online in the IO monad while parsing (using unsafeInterleavedIO) - are compatible with the syntax macro mechanism we have implemented (beta): http://www.cs.uu.nl/~arthurb/index.html Doaitse -- -- __________________________________________________________________________ S. Doaitse Swierstra, Department of Computer Science, Utrecht University P.O.Box 80.089, 3508 TB UTRECHT, the Netherlands Mail: mailto:doaitse@cs.uu.nl WWW: http://www.cs.uu.nl/ tel: +31 30 253 3962 fax: +31 30 251 3791 mobile: +31 6 2880 1680 __________________________________________________________________________

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Rectification Hugs bug
by Rijk J. C. van Haaften 07 Mar '02

07 Mar '02

Dear all, After intensive investication of several people here at Utrecht University, these are the results 1. The very latest Hugs version doesn't have the bug 2. All before-december-2001 versions don't have the bug I were using a version downloaded some weeks ago. After installing the current distribution, the problems disappeared Therefore, I suspect there have been one or more bug-fix updates after December 2001, fixing this problem. However, I couldn't find that documented on the Hugs site. Can anyone confirm such an update? Where can I find last- minute distribution information? Thanks, Rijk-Jan

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Rectification Hugs bug
by Rijk J. C. van Haaften 07 Mar '02

07 Mar '02

Dear all, After intensive investication of several people at Utrecht University, these are the results: 1. The very latest Hugs version doesn't have the bug 2. All before-december-2001 versions don't have the bug I were using a version downloaded some weeks ago. After installing the current distribution, the problems disappeared. Therefore, I suspect there have been one or more bug-fix updates after December 2001, fixing this problem. However, I couldn't find that documented on the Hugs site. Can anyone confirm such an update? Where can I find last-minute distribution information? Thanks, Rijk-Jan

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class hierarchies inmature in hugs
by Ralf Laemmel 07 Mar '02

07 Mar '02

I just saw someone "misusing" haskell(a)haskell.org for bug reports. I am going to join especially - it is a funny one, - the bug hasn't been fixed since Sep. 2001, - it is not even put in the list of known bugs since then. --------------------------------------------------------------- The following code works with ghc but not with hugs (incl. Dec. 2001 release) import Monad class Monad m => C1 m x -- Monad m is implied by C1 but test diverges if constraint not present class (C1 m x) => C2 m x where c2 :: x -> m x instance Monad m => C1 m Bool instance C2 Maybe Bool where c2 = return test :: Maybe Bool test = c2 True --------------------------------------------------------------- So you see, C1 is a kind of derived from Monad. C2 is derived from C1 and hence implicitly from Monad. This is a kind of minimal example. Just attempt to evaluate "test". hugs will loop. There is a way to help hugs. Add Monad m as a further class constraint to C2. Then it will work. Please note both programs do type check, and this is in fact fine. More generally, I get these looping programs whenever I have non-trivial class hierarchies with three or more layers. I guess more people should have suffered from that? Ralf -- Dr.-Ing. Ralf Laemmel CWI & VU, Amsterdam, The Netherlands http://www.cwi.nl/~ralf/ http://www.cs.vu.nl/~ralf/

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Bug in Hugs Dec 2001: type inferencer incorrect!
by Rijk J. C. van Haaften 07 Mar '02

07 Mar '02

Hello everybody, Although I didn't manage to reproduce the bug with a minimal example, I think it is still important enough to tell you (and especially the Hugs writers and maintainers). Yesterday evening, I tried to add some correct (!) code (by-hand-verifyable; by GHC accepted; just using plain Haskell 98) to one of the projects I'm working on. Hugs however said this: Type checking ERROR "TypeInferDecls.hs":102 - Type error in application *** Expression : checkedUnify (snd newTyEnv) inferredHpPtr explicitHpPtr (\t1 t2 ue -> ([l],ExplicitError n t1 t2 ue)) *** Term : checkedUnify *** Type : NonGenerics -> HpPtr -> HpPtr -> ErrorFunction -> TI Error () *** Does not match : a -> b -> c -> d -> e *** Because : types do not match I've read this ten times, checked my code looking whether I were doing strange things, but I can only conclude: This is, even without a minimal example, clearly a bug: the types do match. Moreover, the bug is reported about a completely different part of the file (relative to the location I edited), in code almost unrelated to the code I added (about line 300). I have explicit types everywhere in that file, so if it were wrong, the error should be reported about line 300. So the reporting-place is wrong (but I don't expect Hugs to report good error-placement). Though I didn't manage to write a minimal example, I hope our Hugs experts are able to find the bug soon. As a last point: the file is rather big: 726 lines of code, 27KB. Rijk-Jan van Haaften

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07 Mar '02

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Re: first-class polymorphism beats rank-2 polymorphism
by Ashley Yakeley 07 Mar '02

07 Mar '02

At 2002-03-06 06:13, Ralf Laemmel wrote: >This is not a bug, Isn't it? Consider: class Class x where combinator' :: (forall y. Class y => y -> y) -> x -> x This gives: combinator' :: forall x. Class x => (forall y. Class y => y -> y) -> x -> x What happens when you pass something of type (forall y. Class y => y -> y)? s :: forall x. Class x => x -> x s = combinator' id; So I would claim that these two types are the same: forall x. Class x => (forall y. Class y => y -> y) -> x -> x (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) ...so you should be able to do this: combinator :: (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) combinator f x = combinator' f x but for some reason GHC 5.02.2 complains. I think this is a bug. Apparently 5.03 has rank-N polymorphism so maybe this is fixed too. -- Ashley Yakeley, Seattle WA

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ANNOUNCE: HXML 0.2, XML parser for Haskell
by Joe English 06 Mar '02

06 Mar '02

Announcing HXML version 0.2, a non-validating XML parser written in Haskell. It is designed for space-efficiency, taking advantage of lazy evaluation to reduce memory requirements. HXML is available at: <URL: http://www.flightlab.com/~joe/hxml > The current version is 0.2, and is pre-beta quality. HXML has been tested with GHC 5.02, NHC 1.10, and various recent versions of Hugs 98. Changes in version 0.2: + New Arrow-based combinator library + Added support for CDATA sections + New function parseDocument recognizes (and ignores) the document prolog (XML and DOCTYPE declarations) + Several data structures and public functions have been renamed + Space fault in comment parsing fixed Please contact Joe English <jenglish(a)flightlab.com> with any questions, comments, or bug reports. --Joe English jenglish(a)flightlab.com

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first-class polymorphism beats rank-2 polymorphism
by Ralf Laemmel 06 Mar '02

06 Mar '02

Suppose you want to define a combinator like this: combinator :: (forall y. Class y => y -> y) -> (forall x. Class x => x -> x) This is a function combinator which turns a polymorphic function into another polymorphic function. As you can see, there is Class constraint. So in fact we would like to define the combinator by class overloading on the x in the result type. So how do we get our hands on the x while it is explicitly quantified by forall. Well, we try with implicit quantification: class Class x where combinator' :: (forall y. Class y => y -> y) -> x -> x So far so, good: We still need to define combinator in terms of combinator'. What is the trick to do this? Sounds like a homework question? Well, I know how to do it for first-class polymorphism. So let's restart the example. We define a function type to capture the kind of functions from above: data Bar = Foo (forall x. Class x => x -> x) Hence, the rank-2 type of combinator looks a bit different: combinator :: Bar -> Bar The class looks as follows: class Class x where combinator' :: Bar -> x -> x And here is how to define combinator in terms of combinator'. combinator f = Foo (combinator' f) This works both with GHC and hugs. In the pure rank-2 setting above, one would like to define combinator accordingly: combinator f = combinator' f But GHC 5.02.2 tells me: rank2.hs:11: Could not deduce (Class x) from the context () Probable fix: Add (Class x) to the type signature(s) for combinator arising from use of `combinator'' at rank2.hs:11 In the definition of `combinator': combinator' f This is not a bug, I guess although the probable fix proposal sends you in the wrong direction I claim. I tried a few other tricks but I can't get it done although my first-class polymorphic experience makes me think that it should be possible. Anyway, can this problem be solved? Any hint appreciated. Ralf -- Dr.-Ing. Ralf Laemmel CWI & VU, Amsterdam, The Netherlands http://www.cwi.nl/~ralf/ http://www.cs.vu.nl/~ralf/

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