- Haskell-Cafe - Haskell.org

[Redirect] polymorphism and existential types
by dons＠cse.unsw.edu.au 30 Nov '06

30 Nov '06

Redirected to haskell-cafe(a)haskell.org, the right list. -- Don ----- Forwarded message from Louis-Julien Guillemette <guillelj(a)iro.umontreal.ca> ----- Date: Mon, 27 Nov 2006 16:15:26 -0500 (EST) From: Louis-Julien Guillemette Subject: [Haskell] polymorphism and existential types Supposing a polymorphic value (of type, say, forall a . ExpT a t) is stored inside an existential package (of type, say, forall a . Exp a), I wonder how to recover a polymorphic value when eliminating the existential. The ``natural way'' to write this doesn't work: {-# OPTIONS -fglasgow-exts #-} data ExpT a t data Exp a = forall t . Exp (ExpT a t) f :: (forall a . ExpT a t) -> () f e = () g :: (forall a . ExpT a t) -> () g e = let e1 :: forall a . Exp a e1 = Exp e in case e1 of Exp e' -> f e' {- Test.hs:18:17: Inferred type is less polymorphic than expected Quantified type variable `a' is mentioned in the environment: e' :: ExpT a t (bound at Test.hs:18:11) In the first argument of `f', namely `e'' In the expression: f e' In a case alternative: Exp e' -> f e' Failed, modules loaded: none. -} In particular, the solution of pushing the forall inside doesn't work for me (it breaks other parts of my code...) : data Exp' = forall t . Exp' (forall a . ExpT a t) g' :: (forall a . ExpT a t) -> () g' e = let e1 :: Exp' e1 = Exp' e in case e1 of Exp' e' -> f e' Any help welcome! Louis-Julien _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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The Future of MissingH
by John Goerzen 30 Nov '06

30 Nov '06

Hi, Josef Svenningsson posted a comment on my blog today that got me to thinking. He suggested that people may be "intimidated by the size of MissingH, confused by the undescriptive name, and don't quite know what's in there." And I think he's right. I've been passively thinking about what MissingH should be for awhile now, and wonder if you all would have some suggestions. First, let me back up and describe a bit of what MissingH is. It gets its name because I think that it has useful functions that are "missing" from the standard Haskell libraries. But I can see how it's not very descriptive. The major features are: * A general-purpose modular logging infrastructure * A virtual filesystem component (similar to VFS in Gnome, but written in Haskell) * A configuration file parser, compatible with Python's and Perl's, which can also modify and generate config files and do variable interpolation * A bunch of tools for setting up pipes to/from processes * Tools to format numbers as quantities, with default configs for the SI system (1000-based) and binary system (1024-based) * Tools to track the progress of long-running actions and display a status bar on the terminal * Pure-Haskell un-gzip code, *DBM infrastructure, etc * Assorted list, string, regexp, bit, pointer, etc. utilities * MIME and email parsers. I wrote most of the code myself, but did pull some large chunks from others where licenses were compatible. The API reference is at http://gopher.quux.org:70/devel/missingh/html/index.html So, some of the questions are: Should this all be one library? I initially wrote it that way to make resolving dependencies easier for end users. Also, it still has utility because modules make use of each other's features. The list and string functions are used almost everywhere, for instance. HVFS support is also fairly pervasive. Should the module naming scheme be changed? Modules are currently named things like MissingH.List, MissingH.IO.HVFS, MissingH.ProgressMeter, etc. I guess these could go to System.ListExt, System.IO.HVFS, System.Console.ProgressMeter, etc. Could, and should, any of this be integrated into the Haskell libraries project? (That set of libraries formerly known as fptools) A few bits of code have already found their way into them. But I bet there could be a lot more. Maybe even the majority of it. How could greater community participation be encouraged, while still encouraging quality control? I have received some very good contributions to MissingH from people, and that's been great. I've also received some that just aren't that great -- they don't have Haddock docs, the code is opaque, they don't come with unit tests, etc. But by and large, I've been maintaining it mostly myself. Whenever I write some code, I think about whether this is generic code that could be useful in other projects. If so, I consider whether it would be appropriate for MissingH (and it usually is). It would be wonderful if others would be interested in contributing code that solves some need as well. There is a public darcs repo already, and I have received some darcs patches from people (thanks!) I'm not trying to rid myself of MissingH, but I think that it could be a nice resource for small tools from the entire community, if we can maintain a structure and QA footprint to it. What else should be done to make this a valuable resource for Haskell programmers? And a showcase for what is possible with Haskell? The floor's open... Thanks, -- John

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Lazy data from pipe using MissingH module
by Dougal Stanton 30 Nov '06

30 Nov '06

Newbie here working on a little program for fetching podcasts. I've been using the MissingH.Cmd module in concert with curl to download the RSS feeds like this: > fetchFeed :: Subscription -> IO (Either Error [Episode]) > fetchFeed sub = do > (pid, feed) <- pipeFrom "curl" (curlOpts ++ > ["--url", (slocation sub)]) > let eps = parseEpisodes (stitle sub) feed > forceSuccess pid > return eps According to the API docs I have to forceSuccess pid *after* I use the data. Will this construct do that, or does the compiler have free reign to move the line beginning 'let ...' wherever it feels? It seems to work occasionally for me but then just hangs, so I thought this might be the problem. Cheers, D. -- Dougal Stanton <ithika(a)gmail.com> <http://brokenhut.livejournal.com> Word attachments considered harmful.

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generating javascript
by jeff p 30 Nov '06

30 Nov '06

Hello, Are there any Haskell tools to generate javascript? Has anyone made a combinator library for javascript? thanks, Jeff

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SimonPJ and Tim Harris explain STM - video
by Bayley, Alistair 30 Nov '06

30 Nov '06

http://channel9.msdn.com/Showpost.aspx?postid=231495 The links to the video are a couple of yellow buttons at the bottom of the article: "Watch" or "Download". I haven't watched this yet (it's nearly an hour long, I think). Found via reddit (http://reddit.com) Haskeller's on TV (sort of...) woot woot! Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell-cafe] c2hs and cabal?
by Magnus Therning 29 Nov '06

29 Nov '06

On Tue, Nov 28, 2006 at 11:11:30 +0000, Duncan Coutts wrote: >On Tue, 2006-11-28 at 10:56 +0000, Magnus Therning wrote: >> On Tue, Nov 28, 2006 at 00:33:42 +0000, Duncan Coutts wrote: >> >On Mon, 2006-11-27 at 22:26 +0000, Magnus Therning wrote: >> >> Can I use cabal to build packages that incorporates C libraries and FFI >> >> Haskell created using c2hs? >> > >> >Yes you can. Cabal understands a bit about .chs files. I think all you >> >need to do is specify the module in the exposed-modules or >> >other-modules and Cabal will find the .chs file and try to build a .hs >> >file from it. > >> I'm still having some problems though :-( I thought I'd start with a >> minimal, rather silly, test to see if I can get Cabal to do what I want. >> I've stuck what I have at >> http://www.therning.org/magnus_files/cnh.tar.gz >> >> It fails to build for me with the following error: >> >> Preprocessing executables for cnh-0.1... >> Building cnh-0.1... >> [1 of 2] Compiling Foo ( src/Foo.hs, dist/build/cnh/cnh-tmp/Foo.o ) >> src/Foo.chs:6:8: parse error on input `import' >> >> I tried using '-v5' but it didn't really offer anymore insight for a >> novice like me :( > >Check the generated Foo.hs file to see if it looks obviously bogus. Can't really see anything obviously bogus about the following: % cat Foo.chs module Foo where #include "foo.h" fooOne i = {# call foo1 #} i % cat Foo.hs -- GENERATED by C->Haskell Compiler, version 0.14.5 Travelling Lightly, 12 Dec 2005 (Haskell) -- Edit the ORIGNAL .chs file instead! {-# LINE 1 "src/Foo.chs" #-}module Foo where fooOne i = foo1 i foreign import ccall safe "src/Foo.h foo1" foo1 :: (CInt -> (IO CInt)) Or am I missing something? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. "There are of course many problems connected with life, of which some of the most popular are `Why are people born?' `Why do they die?' `Why do they spend so much of the intervening time wearing digital watches?'" -- The Book.

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Compatibility between Data.ByteString.Base and Data.ByteString.Lazy
by Neil Bartlett 29 Nov '06

29 Nov '06

Hi, Firstly my apologies if this is an outrageously newbie question. I am trying to write a binary protocol parser using Data.ByteString. I have created a module "ByteParser" containing my parsing utilities, which imports ByteString as: import qualified Data.ByteString as B In my Main module, where all the IO happens, I want to use lazy ByteStrings so I ask for the following imports: import ByteParser import qualified Data.ByteString.Lazy as L The problem is that when I try to call a function in ByteParser with an L.ByteString, the compiler complains that: Couldn't match expected type `Data.ByteString.Base.ByteString' against inferred type `L.ByteString' Does this mean that the lazy version of ByteString is a completely separate, incompatible type from the base version? Obviously I can work around this problem by making ByteParser import Data.ByteString.Lazy instead of Data.ByteString, but then ByteParser would not be able to work with strict ByteStrings. Ideally I would like ByteParser to be agnostic about the use of lazy versus strict ByteStrings. Is this a sensible and/or possible thing to do? Many thanks, Neil

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haddock infix questions
by Nicolas Frisby 29 Nov '06

29 Nov '06

I found nothing in the Gmane archives, so I turn to you. I'm hoping to Haddock-a-chop my library but the ole infix type constructor (and there are many of them) is causing a parse error. It seems a common enough problem, so I was wondering... 1) What ended up happening with David Waern's SoC project? I tried the darcs repo at http://darcs.haskell.org/SoC/haddock.ghc/, but got some error messages that I think indicate I don't have an appropriate GHC As a Library package ("`HsDoc' not in scope"). I'm running GHC 6.6 on a Mac. I'll avoid a manual build of a snapshot GHC until it's my last option, so I haven't made strides to check if HsDoc is actually in the dev GHC repo. 2) Is there a good workaround out there for such a parse error? I'm thinking someone might have a handy pre-processor to help out Haddock? Thanks for your time, Nick

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RE: [Haskell] ANNOUNCE: Visual Haskell prerelease 0.2
by Bayley, Alistair 29 Nov '06

29 Nov '06

> From: haskell-bounces(a)haskell.org > [mailto:haskell-bounces@haskell.org] On Behalf Of Johannes Waldmann > > sounds great - but can I use this without buying Visual Studio first, > i. e. does it work with some free beta version or similar? > > and, does it then work under wine :-) > > seriously, how hard would it be to adapt VH to Eclipse? > the interfaces (for syntax highlighting, typechecking etc.) > should be similar - in theory. Johannes, Are you aware that there is a Haskell plugin for eclipse? http://eclipsefp.sourceforge.net/ It's probably not as sophisticated as VH (I haven't tried VH, so cannot comment with authority), but you get syntax highlighting/colouring, compile-on-save, and compile errors are highlighted in the source editor. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell] ANNOUNCE: Visual Haskell prerelease 0.2
by Pepe Iborra 28 Nov '06

28 Nov '06

On 28/11/2006, at 12:25, Krasimir Angelov wrote: > On 11/28/06, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de> wrote: >> seriously, how hard would it be to adapt VH to Eclipse? >> the interfaces (for syntax highlighting, typechecking etc.) >> should be similar - in theory. > Parsing and typechecking in VH is all done by GHC via the ghc-api. Thus strong interoperability support is needed. > Ah, and about Eclipse. Visual Studio and Eclipse are very different > platforms and you have to rewrite VSHaskell almost from scratch if you > want an Eclipse port. Perhaps the only common parts will be Cabal & > GHC. > Not only the interfaces are completely different, but an entirely new set of interoperability problems would need to be solved. The guys from EclipseFP have a document[1] about that topic. That said I would love being able to run an eclipsified VH in my Mac :) [1] - http://eclipsefp.sourceforge.net/haskell/ ExtendingEclipseInHaskell.pdf Cheers pepe

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