- Haskell-Cafe - Haskell.org

MonadCont, MonadFix
by Ashley Yakeley 09 Mar '02

09 Mar '02

Are there any useful monads that are instances of both MonadCont and MonadFix? I can't make the two meet. Perhaps this is because continuations have no fixed point, or something. Very annoying. -- Ashley Yakeley, Seattle WA

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Debugging Haskell
by Jerry, JiJie 09 Mar '02

09 Mar '02

Good day everyone, I was fiddling around with this tiny echo client/server haskell program from 'The Great Language Shootout' site (http://www.bagley.org/~doug/shootout/) and got stuck. The code (attached) has been reformatted with minimal API tweak (mkPortNumber, writeSocket, readSocket) to please my ghc-5.02.2, and all what I get is something stuck forever after the first iteration: $ ./echo 3 Client wrote: Hello there sailor Server recv: Hello there sailor Server read: Hello there sailor Server wrote: Hello there sailor After adding all these print statement, I still don't have a clue what's jammed there. Hope someone here can shred some light. BTW, I'd also like to take this chance to ask how to debug a haskell program in general? Regards, Jerry

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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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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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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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Re: [xml-dev] ANNOUNCE: HXML 0.2, XML parser for Haskell
by postmaster＠bol.net.in 06 Mar '02

06 Mar '02

Mail not delivered to darpeshp(a)bol.net.in. Quota exceeded. Try sending later.

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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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Re: Haskell-Cafe digest, Vol 1 #342 - 1 msg
by Zhe Fu 06 Mar '02

06 Mar '02

I want to read a file's content into a string. content :: FilePath -> String content f = readFile f But when there is a type error: Type error in explicitly typed binding *** Term : content *** Type : FilePath -> IO String *** Does not match : FilePath -> String How can I solve it? Thanks. Zhe

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Re: how to read a file's content to a string?
by Ashley Yakeley 06 Mar '02

06 Mar '02

At 2002-03-05 14:43, Zhe Fu wrote: >content :: FilePath -> String >content f = readFile f You have the wrong type signature for 'content'. This should solve it: content :: FilePath -> IO String content f = readFile f Of course, you'll need to change whatever uses 'content'. -- Ashley Yakeley, Seattle WA

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Lazy Evaluation
by Nguyen Phan Dung 05 Mar '02

05 Mar '02

Hello, Sorry for asking such a silly question: Haskell is using lazy evaluation. So, it means we should be able to declare and use a list of billions elements without any trouble. So I declare my list as follow: mylist :: [Integer] mylist = [1..1000000000] In Hugs, I type mylist to print out all the elements inside. However, after printing about 22000 elements, the system crashs & outputs: "Garbage collection fails to reclaim sufficient memory" Would you please help me with this problem? Thank you very much, Phan Dung.

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