August 2010 - Haskell-Cafe

hsb2hs preprocessor looking for a maintainer
by Joachim Breitner 05 Aug '10

05 Aug '10

Hi, out of a discussion in haskell-devscripts, John MacFarlane wrote a very nice tool, called hsb2hs, that allows you to include any binary (or text) file as a constant in your program. This can be useful in various instances, e.g. when creating programs that should not depend on data files installed in the right location, or larger, multi-line strings that you’d like to edit on their own, not as a long string constant in the Haskell file (I once wished I had this when including some CSS in a CGI script written in Haskell). The current code is on http://github.com/jgm/hsb2hs and you can read more about it in the subthread starting at http://lists.debian.org/debian-haskell/2010/08/msg00013.html Unfortunately, John says that he does not have time to develop it further, and I probably don’t either. Is there anyone here interested in picking up the projects? Possible TODOs involve the optimization I suggested in http://lists.debian.org/debian-haskell/2010/08/msg00015.html and making Cabal support .hsb files directly (just as with .y etc. files). Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Why is toRational a method of Real?
by Omari Norman 05 Aug '10

05 Aug '10

Why is toRational a method of Real? I thought that real numbers need not be rational, such as the square root of two. Wouldn't it make more sense to have some sort of Rational typeclass with this method? Thanks. --Omari

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Problem with System.Timeout in Windows
by Adolfo Builes 05 Aug '10

05 Aug '10

Hi All: I have been trying to use System.timeout in windows, but for some reason it doesn't work, a concrete example is: import Network.HTTP import System.Timeout main = do rsp <- timeout 10000000 $ simpleHTTP $ getRequest "http://10.1.2.3" case rsp of Just rsp -> print $ show rsp Nothing -> error "Nothing" In this case I would expect to receive "Nothing" after 10 seconds, but it ignores the timeout function. I'm compiling with -threaded, but it does not work either, I'm using ghc 6.12.2. Does someone know what could be going wrong ? Thanks !

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hGetContents: resource exhausted
by Lally Singh 05 Aug '10

05 Aug '10

Hey all, This is on OpenSolaris. Simple attempts to build cabal packages give me this error, and I don't know what it means. Here's an example: [07/25 18:51::lally@sol type-level]$ runghc Setup.hs configure Configuring type-level-0.2.4... Setup.hs: fd:8: hGetContents: resource exhausted (Resource temporarily unavailable) Setup.hs: fd:8: resource exhausted Does anyone know what this means? I'm trying to just get the llvm bindings installed (requiring mtl & type-level). Cheers, -Lally

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can Haskell do everyting as we want?
by Qi Qi 05 Aug '10

05 Aug '10

As more I learn haskell, I am more interested in this function programming language. I am intended to more focus on haskell than other languages like python, Java, or C++. But I am still wonder whether haskell can do everyting as other languages do, such as python, perl, Java and C++. Is there anyone happen to come into any tasks that haskell is not able to achieve? Thanks. -- Qi Qi

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Playing with ATs again
by Andrew Coppin 05 Aug '10

05 Aug '10

OK, so if you do something like class Container c where type Element c :: * then we now have a clean and concise way to discover what type of element any given container holds. (Regardless of whether it's element type is parametric, hard-coded, class-constrained or anything else.) I really like this! Now suppose that instead of a container type with an associated element type, what we want is several pairs of types having a one-to-one relationship, and we want to be able to traverse that relationship in either direction. What's the best way to do that? (I tried just having two mirror image classes like the one above. But that doesn't work; there's no guarantee that going one way and then going back the other way will definitely take you back to where you started. I tried to make that a constraint, and got an error message that basically says "this feature is not implemented". Great. Still, there's probably a better way to do this.)

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ANN: blaze-builder 0.1
by Jasper Van der Jeugt 05 Aug '10

05 Aug '10

Dear all, I have just released a package called blaze-builder on Hackage. It is basically a builder monoid supporting fast string concatenation. It was originally written for the blaze-html package, but because we thought it could be useful elsewhere as well, we are releasing it as a separate package. Here is a blogpost with more information: http://jaspervdj.be/posts/2010-08-05-blaze-builder.html And here is the Hackage link: http://hackage.haskell.org/package/blaze-builder All feedback is welcome, Kind regards, Jasper Van der Jeugt

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subversion with haskell
by Andrew U. Frank 05 Aug '10

05 Aug '10

hackage contains a package Hs2SVN HsSVN seems to do what i need, but unfortunately, i cannot install it - i have the error : checking for stdint.h... yes checking for unistd.h... yes checking svn_error.h usability... no checking svn_error.h presence... no checking for svn_error.h... no configure: error: SVN headers are required. Hint: EXTRA_CPPFLAGS cabal: Error: some packages failed to install: HsSVN-0.4.3 failed during the configure step. The exception was: ExitFailure 1 i installed all development packages i could see related to svn - which one is really needed? can you clarify and point me in the right direction? (and put the information in the package cabal file so it is copied to hackage) thank you andrew

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packages on hackage: how to download files which are not included in the darcs inventory
by Andrew U. Frank 05 Aug '10

05 Aug '10

i try to use jsmw. i see that there are multiple example files in a directory. this directory is not included in darcs and does not download automatically. is there an easy way to download directories (i can get the files individually with curl - but this is slow and errorprone)? help is appreciated! andrew

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ANNOUNCE: uu-parsinglib version 2.5.3; extended with merging combinators
by S. Doaitse Swierstra 05 Aug '10

05 Aug '10

I have uploaded a new version of the uu-parsinglib. It contains, besides the extension of the abstract interpretation part and the fixing of some very subtle corner cases in that part, some nice new functionality: The call of the parser: -- run ((,,,) `pMerge` (pSome pa <||> pMany pb <||> pOne pc <||> pNatural `pOpt` 5)) "babc45a" results in: -- > Result: (["a", "a"],["b","b"],"c",45) and -- run ((,,,) `pMerge` (pSome pa <||> pMany pb <||> pOne pc <||> pNatural `pOpt` 5)) "bbc" results in -- > Result: (["a"],["b","b"],"c",5) -- > Correcting steps: -- > Inserted 'a' at position (0,3) expecting one of ['0'..'9', 'b', 'a'] -- pSome means that the number of elements occurring in the input should be >= 1 pMany >= 0 pOne = 1 pOpt <= 1 The implementation can be found in the Derived module, and is remarkably simple. Doaitse

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