November 2006 - Haskell-Cafe

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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c2hs and cabal?
by Magnus Therning 28 Nov '06

28 Nov '06

Can I use cabal to build packages that incorporates C libraries and FFI Haskell created using c2hs? Any pointers on how to do that? /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. If voting could really change things it would be illegal.

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GHCi and HXT
by Alexis Hazell 28 Nov '06

28 Nov '06

Hi all, i'm having a bit of difficulty using GHCi to try out HXT. i brought the Text.XML.HXT.Arrow module into scope using :m. Then i entered: let contact = mkelem "stream:stream" [ sattr "xmlns:stream" "http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr "to" "livejournal.com" ] [] and got the error message: <interactive>:1:14: Ambiguous type variable `a' in the constraint: `ArrowXml a' arising from use of `mkelem' at <interactive>:1:14-19 Probable fix: add a type signature that fixes these type variable(s) The type signature of mkelem is mkelem :: (ArrowXml a) => String -> [a n XmlTree] -> [a n XmlTree] -> a n XmlTree So then i modified it to: let contact = mkelem "stream:stream" [ sattr "xmlns:stream" "http://etherx.jabber.org/streams", sattr "xmlns" "jabber:client", sattr "to" "livejournal.com" ] [] :: ArrowXml a => a XmlTree XmlTree And got: <interactive>:1:14: Ambiguous type variable `a' in the constraint: `ArrowXml a' arising from instantiating a type signature at <interactive>:1:14-194 Probable fix: add a type signature that fixes these type variable(s) i don't understand why i need to specify type signatures when they should already be in scope? And if i /do/ need to specify the type signatures for some reason, how do i do so in a manner that works? Any help much appreciated! Alexis.

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