March 2002 - Haskell-Cafe

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/

1 0

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

1 0

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

1 0

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.

1 0

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

1 0

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

2 1

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

1 0

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.

3 2

RE: Ground Up
by Simon Peyton-Jones 03 Mar '02

03 Mar '02

| Further, Haskell, though _very_ nice, is probably not "the | best choice" for a "pragmatic programming". It is too alive. | Different compilers are not 100% compatible each to other, | and even different versions of the same compilers may be | incompatible. You should be ready either to stick yourself | and your co-wokers to a certain version of the compiler or to | spend some significal time to keep you programs working with | any compiler. It's true that GHC and Hugs and nhc don't accept exactly the same language but a) Haskell 98 is a very stable language that all three accept, and are committed to continuing to do so. b) If you stick to one compiler of the three, you can be pretty sure that programs that used to work will continue to work. (GHC, for example, always has a "stable branch", currently 5.02.2, that we maintain in parallel with introducing new features and bugs into the head. Bleeding-edge folk can and do compile the head from source, but you don't have to do that!) It's true that we occasionally make incompatible changes, but not often. | Maybe you will find that it's better to use Ocaml or | Scheme. Thay have own neats, and seem to be more stable. c) I have a very high opinion of Caml and Scheme, and their impls, but I think you are being a bit unfair on Haskell here. Caml has only one impl, and Scheme has many incompatible variants (eg PLT Scheme), so in practice you have to stick to one impl unless you use a the R3RS subset. I don't think any of them are more stable than any particular one of the Haskell impls. Simon

3 2

Ground Up
by Jerry, JiJie 01 Mar '02

01 Mar '02

Good day everyone, I'm a haskell newbie trying to seeking advice to advance myself. My backgrounds are: * As a non-CS student, I have absolutely no knowledge of lambda calculus * And unfortunately I have to make painful decision on spending $30 on a book or three days' meal While my goals are: * Become a pragmatic haskell programmer in the shortest time * At the minimal expense What I have done for the past few weeks were: * I read almost all the free educational sources at www.haskell.org, * Subscribed to this mailing list and try to digest every mail * Read most of the "The Haskell School of Expression" (by Paul Hudak) and the non-theoretical chapters of the "An Introduction To Functional Programming Systems Using Haskell" (by AJT Davie), which are the only two introductory level haskell related books in our library However, my problems are: * I still don't understand most of the codes I found, like the various haskell libraries * I still have no clue of most (ok, almost all) of what is being discussed in this mailing list So I'm eager to know if there are cost-effective ways to achieve my goals or at least leverage myself from the beginner's level. Your advice would be greatly appreciated! Regards, Jerry

4 3