December 2002 - Haskell

help
by Suja 10 Dec '02

10 Dec '02

Hi, I want to know how to execute a haskell program that does not contain a main module. I would also like to know the difference between .lhs and .hs extensions.

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Re: How to force recursion
by Cai john 09 Dec '02

09 Dec '02

Thanks for the help everyone,problem solved. Very stupid of me not to see that tree instead of empty would solve the problem. As for the 'other' errors, I wrote too fast and can't find the edit button. My first post contains the better implementation. John __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com

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How to force recursion
by Cai john 09 Dec '02

09 Dec '02

How do I make this work: insertTree tree (x:xs) n = insertTree (insert tree a n) xs n insert -> common insertion into a tree which returns a tree __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com

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ANNOUNCE: GHC version 5.04.2 released
by Simon Marlow 08 Dec '02

08 Dec '02

============================================================== The (Interactive) Glasgow Haskell Compiler -- version 5.04.2 ============================================================== We are pleased to announce a new patchlevel release of the Glasgow Haskell Compiler (GHC), version 5.04.2. This is a bugfix-only release. For all the changes since 5.02.3, see the release notes: http://www.haskell.org/ghc/docs/latest/users_guide/release-5-04.html How to get it ~~~~~~~~~~~~~ The easy way is to go to the WWW page, which should be self-explanatory: http://www.haskell.org/ghc/ We supply binary builds in the native package format for various flavours of Linux and BSD, and in InstallShield form for Windows folks. Binary builds for other platforms are available as a .tar.gz which can be installed wherever you want. The source distribution is also available from the same place. Once you have the distribution, please follow the pointers in the README file to find all of the documentation about this release. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language; the current language version is Haskell 98, agreed in December 1998. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page http://www.haskell.org/ghc/ Haskell home page http://www.haskell.org/ comp.lang.functional FAQ http://www.cs.nott.ac.uk/~gmh/faq.html System requirements ~~~~~~~~~~~~~~~~~~~ To compile programs with GHC, you need a machine with 64+MB memory, GCC and perl. This release is known to work on the following platforms: * i386-unknown-{linux,*bsd,mingw32} * sparc-sun-solaris2 * alpha-dec-osf3 * powerpc-apple-darwin (MacOS/X) Ports to the following platforms should be relatively easy (for a wunderhacker), but haven't been tested due to lack of time/hardware: * hppa1.1-hp-hpux{9,10} * i386-unknown-solaris2 * mips-sgi-irix{5,6} * {rs6000,powerpc}-ibm-aix The builder's guide on the web site gives a complete run-down of what ports work; it can be found at http://www.haskell.org/ghc/docs/latest/building/building-guide.html Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at http://www.haskell.org/mailman/listinfo/glasgow-haskell-users http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see http://www.haskell.org/mailman/listinfo/ Please report bugs using our SourceForge page at http://sourceforge.net/projects/ghc/ or send them to glasgow-haskell-bugs(a)haskell.org. GHC users hang out on glasgow-haskell-users(a)haskell.org. Bleeding edge CVS users party on cvs-ghc(a)haskell.org.

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Re: arrows
by Ashley Yakeley 06 Dec '02

06 Dec '02

At 2002-06-29 14:43, Wolfgang Jeltsch wrote: >To simplify things a bit, let's take a simpler Parser type which doesn't >use monad state transformers but simple state transformers instead. This >makes the type ParserState simpler. You can think of a parser state as >an infinite list of substate-token pairs now. The tokens denote the >input stream. The substates may be used to encapsulate parser states of >a subordinate parsing process. When a token is read, the corresponding >token-substate pair is discarded. Sounds very complicated. Wouldn't a simple stream transformer work? newtype MyParser baseMonad token output = MkMyParser (baseMonad token -> baseMonad output); > (>>=): sequencing of parsers > return: returning values > fail: generation of always failing parsers > mzero: always failing parser > mplus: parsing using alternative parsers > fmap: applying a function to the result of a parser > (>>>): providing high-level tokens and parsing using these tokens > identity: reading a single token > pure: reading a single token and applying a function to it I think I can do all of those for MyParser. There's a one-token look-ahead variant: newtype MyLookaheadParser m t a = MkMyLookaheadParser (m t -> Maybe t -> m (Maybe t,a)); >(I don't allow >finite lists here to make some things a bit easier an more elegant. You >can easily produce an infinite list from a finite one by applying >something like (++ repeat Nothing) . map Just to the finite list.) Wouldn't it be easier to allow finite token lists and have reading past the last token be the same as mzero? -- Ashley Yakeley, Seattle WA

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./configure
by Christian Maeder 06 Dec '02

06 Dec '02

Hi, when installing ghc and hugs under SuSE linux 8.1 I got "configure: error: can not guess host type; you must specify one" It worked after I added "--host=i386-linux" Christian

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Re: Completeness of pattern matching
by ketil＠ii.uib.no 06 Dec '02

06 Dec '02

ketil(a)ii.uib.no (Ketil Z. Malde) writes: > Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> writes: > > > Ingo Wechsung <iw(a)contexo.de> writes: > >> I wonder if the compiler could check, if all possible combinations have > >> been checked in a pattern match. > > > In ghc, use the compile-time option -fwarn-incomplete-patterns > > Is there a warning to warn if a block is delimited (or not) with a mix > of tabs and spaces? Or somehow otherwise warn against dangerously > looking layout? Talking about warnings, it is probably patently stupid to put {-# OPTIONS -fglasgow-exts -package lang #-} at the top of my Main.lhs. But why does GHC (5.04-1) fail to find any of my imports as a consequence? Bug? -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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RE: stupid strictness question
by Simon Marlow 05 Dec '02

05 Dec '02

> Now, we define: > > > data SMaybe a = SNothing | SJust !a deriving Show > > Now, we run: > > *Strict> Just (undefined::Int) > Just *** Exception: Prelude.undefined > *Strict> Just $! (undefined::Int) > *** Exception: Prelude.undefined > *Strict> SJust $! (undefined::Int) > *** Exception: Prelude.undefined > *Strict> SJust (undefined::Int) > SJust *** Exception: Prelude.undefined > > I can't figure out why this last one is different from the > one before it, or the one before that. This one is a GHCi (not GHC) bug. You may have seen this message while loading the source containing the strict constructor definition: WARNING: ignoring polymorphic case in interpreted mode. Possibly due to strict polymorphic/functional constructor args. Your program may leak space unexpectedly. which means that GHCi essentially ignored the strictness flag on the polymorphic field of the SJust constructor. To work around the bug, you can compile that module with GHC. The good news is that this bug will be fixed in the next major release. Cheers, Simon

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stupid strictness question
by Hal Daume III 04 Dec '02

04 Dec '02

i know i know, this has been asked a million times, but i was searching through the archives with no success, so i figured i'd bug you guys. according to the report: A declaration of the form data cx => T u1 ... uk = ... | K s1 ... sn | ... where each si is either of the form ! ti or ti, replaces every occurance of K in an expression by (\x1 ... xn -> ( ((K op1 x1) op2 x2) ... ) opn xn) where opi is the lazy apply function $ if si is of the form ti, and opi is the strict apply function $! (see Section 6.2) if si is of the form !ti. Pattern matching on K is not affected by strictness flags. so. we define: > data L = L Int deriving Show > data S = S !Int deriving Show and, as expected, we get: *Strict> L undefined L *** Exception: Prelude.undefined *Strict> L $! undefined *** Exception: Prelude.undefined *Strict> S undefined *** Exception: Prelude.undefined Now, we define: > data SMaybe a = SNothing | SJust !a deriving Show Now, we run: *Strict> Just (undefined::Int) Just *** Exception: Prelude.undefined *Strict> Just $! (undefined::Int) *** Exception: Prelude.undefined *Strict> SJust $! (undefined::Int) *** Exception: Prelude.undefined *Strict> SJust (undefined::Int) SJust *** Exception: Prelude.undefined I can't figure out why this last one is different from the one before it, or the one before that. Interestingly, Hugs disagrees (the previous was with ghc 5.04.1): Strict> Just (undefined::Int) Just Program error: {undefined} Strict> Just $! (undefined::Int) Program error: {undefined} Strict> SJust $! (undefined::Int) Program error: {undefined} Strict> SJust (undefined::Int) Program error: {undefined} Which is what I expected. Can someone clarify here? - Hal

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Re: Design patterns in Haskell
by Chris.Angus＠Lawson.com 04 Dec '02

04 Dec '02

I spent an awful lot of time doing a brain-dump into these pages and am a bit dissapointed that they seemed to have dissappeared without trace. Were these archived anywhere Keith Wansbrough <Keith.Wansbrough(a)cl.cam.ac.uk> Sent by: haskell-admin(a)haskell.org 03/12/2002 18:02 To: matt hellige <matt(a)immute.net> cc: haskell(a)haskell.org, (bcc: Chris Angus/Lawson) Subject: Re: Design patterns in Haskell > size. while there's really no substitute for experience, i really > believe we could benefit from some patterns. There was a list of design patterns for Haskell on the Wiki (back in the days when it worked): http://haskell.org/wiki/wiki?CommonHaskellIdioms --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory. _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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