- Haskell-Cafe - Haskell.org

Associated data types
by Eyal Lotem 10 Dec '08

10 Dec '08

If we have associated type synonyms, is there still reason to have associated data types? For example, we can replace: class C b where data D b ... instance C Int where data D Int = D | E with: class C b where type D b ... data DInt = D | E instance C Int where type D Int = DInt Or perhaps allow, for convenience, this form (which would desugar to the above): class C b where type D b ... instance C Int where data D Int = D | E

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Translating RWH into other languages
by Bulat Ziganshin 10 Dec '08

10 Dec '08

Hello Don, There is a team of people which want to translate RWH book into Russian. is it ok? can you help us by establishing "subversion" of RWH book on your site. if it will contain copy of the English book for the beginning and allow team members to further edit it - it will be great! -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Does ghc use Language.Haskell.*?
by Maurício 10 Dec '08

10 Dec '08

Hi, I've just coded a small utility to prettyprint my code, based on Language.Haskell.Parser and friends. Much to my surprise it wasn't able to parse itself, complaining about a non-ASC character (í). Why can ghc read the program, but not the standard library parser? Does ghc use something else? Is it possible to use whatever ghc uses to build a prettyprinter? Thanks, Maurício

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a haskell_proposals subreddit
by Jason Dusek 10 Dec '08

10 Dec '08

There is a subreddit for people to propose libraries: http://www.reddit.com/r/haskell_proposals/ The idea being, that Web 2.0 will help us to allocate our collective talents more efficiently when it comes to extensions (and perhaps clue us in when our pet project is something people really want). -- _jsn

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File name encodings
by Günther Schmidt 10 Dec '08

10 Dec '08

Hi, how can I convert a file name with for example umlauts (ü, ä ... ) into UTF8, or some other format that I can actually use it? Günther -- On a German Win XP

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Building/installing haskelldb and friends with ghc 6.10.1
by Laurent Giroud 10 Dec '08

10 Dec '08

Hi everyone, I have been trying to install haskelldb in the last few days and encountered a number of hurdles which raised a few questions and for which I'd appreciate some insight and advice. Note that I am posting here because they are not specifically haskelldb related. I am using Cabal to install haskelldb, haskelldb-hsql, haskelldb-hsql- mysql (and others) and discovered that the packages available on hackage do not build properly with ghc 6.10.1. The typical error being of the following kind: Could not find module `System.Time': it is a member of package old-time-1.0.0.1, which is hidden These ones are relatively easy to fix by finding the locally cached .tar.gz and modifying the .cabal file to include the corresponding packages (there were several of them which were discovered in successive attempts). But I finally stumbled on hsql-mysql due to it apparently not being actively maintained anymore (this seemingly simple conclusion actually required quite a lot of research). I read on haskell-cafe that Frederik Eaton had been apparently fixing these problems but could find nowhere a version exhibiting the will to compile gracefully, a darcs repository at http://code.haskell.org/ HSQL/ apparently containing patches from him also proved reluctant to build, although for different reasons. I wasn't courageous enough to pursue and try to fix them at that point of the adventure. Other searches in haskell cafe lead me to people advising to use HDBC instead, after which I contemplated suicide for a few seconds before coming back to my senses and cabal-installing haskelldb-hdbc which quickly led me to error messages of the exact same kind mentioned above. Strangely enough, hdbc 1.1.5 itself installs quite fine using Cabal, however, haskell-hdbc insists on depending on version 1.0.1. I haven't been able so far to find what configuration file to modify in order for it to try to build with the latest Cabalized version (I admit I didn't search much). I then discovered this page http://www.haskell.org/haskellwiki/Upgrading_packages which hints that some of the existing packages in Hackage need "a bit" of work to be usable with ghc 6.10.1 as well as ghc 6.8.x apparently. HSQL and haskelldb-HDBC seem indeed to be in the list of packages which would require some effort in order to be usable with the latest ghc versions (can't remember where I read that though). Pursuing any further would seem to require to delve deeper in the intricacies of the Cabal system and fix either HSQL or HaskellDB-HDBC. Since I really want to use HaskellDB I'll probably try these avenues, but I would appreciate comments at this point from persons knowledgeable in the evocated areas: Are the databases interface for haskelldb still officially maintained and where is this information available ? Is the information that this and that package are compatible (or not) with ghc 6.8.x or 6.10.1 available somewhere without compiling first hand the packages ? It would be immensely valuable to the haskell newcomer to have the incompatible packages signaled on http://hackage.haskell.org/packages/archive/pkg-list.html for example. What would you recommend at this point : fix hsql, or haskelldb-hdbc so it uses 1.1.5 instead of 1.1, try an alternative to haskelldb (are there any ?) ? Thanks in advance for your comments and advices. Best regards, Laurent -- Laurent Giroud ml(a)niaow.com

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scanr producer?
by Louis Wasserman 09 Dec '08

09 Dec '08

Is there a change in strictness with the following transformation? It seems like scanr should be a good consumer and a good producer, intuitively. scanrFB :: (a -> b -> b) -> b -> [a] -> (b -> c -> c) -> c -> c scanrFB f x ls c n = snd (foldr (\ x (b, bs) -> let b' = f x b in (b', b' `c` bs)) (x, n) ls) {-# RULES "scanr" [~1] forall f x l . scanr f x l = build (scanrFB f x l); -# RULES} -- Louis Wasserman wasserman.louis(a)gmail.com

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FFI, TH or GHCI, foreign bindings and the linker(s)
by Andrea Rossato 09 Dec '08

09 Dec '08

Hello, I know that this is somehow a recurring question, still the archives have not been helpful for finding a working solution. I'm writing the bindings[1] to bibutils[2], a set of utilities for converting from and to different bibliographic databases (MODS, bibtex, etc.). bibutilis uses a static library (which is not installed) to build a set of different binaries which are the installed utilities. Suppose I build the library in /tmp/bibutils. If I build the haskell bindings (hs-bibutils) with: runhaskell Setup.lhs configure --extra-include-dirs=/tmp/bibutils/lib --extra-lib-dirs=/tmp/bibutils/lib where libbibutils.a and the header files are located, then I can build a simple test[3] program with ghc --make test.hs and it works perfectly. When I tried to link citeproc-hs to hs-bibutils everything worked fine, too. But when I tried building pandoc, which uses the Template Haskell extension to generate some code at compilation time, I got a linker error when TH started its process: [...] [ 6 of 29] Compiling Text.Pandoc.ODT ( Text/Pandoc/ODT.hs, dist/build/Text/Pandoc/ODT.o ) Loading package ghc-prim ... linking ... done. [...] Loading package template-haskell ... linking ... done. ghc: /tmp/bibutils_3.43/lib/bibutils.o: unknown symbol `fields_add' Loading package hs-bibutils-0.1 ... linking ... ghc: unable to load package `hs-bibutils-0.1' So I went back to the test file with the original bindings. If I try to load it and run it on ghci I get the very same error: Prelude Main> main Loading package syb ... linking ... done. Loading package hs-bibutils-0.1 ... linking ... <interactive>: /tmp/bibutils/lib/bibutils.o: unknown symbol `fields_add' ghc: unable to load package `hs-bibutils-0.1' After searching the we I found this: http://article.gmane.org/gmane.comp.lang.haskell.cafe/23635 which states that GHCi cannot load static libraries (the same can be argued by reading the GHCi docs, indeed). After reading this thread: http://article.gmane.org/gmane.comp.lang.haskell.cafe/40412 I came to know that GHCi and GHC are not using the same linker, and, as far as I understand, TH uses the first one too. So I tried to build a dynamic library. ar -t /tmp/bibutils/lib/libbibutils.a to get the list of objects to link with: gcc -shared -o libbibutils.so *.o after compiling them with the -fPIC flag. But this library is not being loaded because of some undefined symbols: lib/libbibutils.so: undefined reference to `corps' lib/libbibutils.so: undefined reference to `asis' lib/libbibutils.so: undefined reference to `progname' These symbols refer to some variables defined as external entities, for instance: extern char progname[]; these entities are initialized by a stub.c file in the Haskell bindings: http://code.haskell.org/~arossato/hs-bibutils/cbits/stub.c Unfortunately my knowledge of such low level stuff is very very limited and I'm not seeing any way out. Is there one? Thanks, Andrea [1] http://code.haskell.org/~arossato/hs-bibutils/ [2] http://www.scripps.edu/~cdputnam/software/bibutils/ [3] the test.hs file import Text.Bibutils main :: IO () main = do init_globals "mods2bibtex" bibl <- bibl_init param <- bibl_initparams mods_in bibtex_out setFormatOpts param [bibout_brackets, bibout_uppercase] setBOM param setVerbose param bibl_read param bibl "/tmp/prova.biblio" mods_in bibl_write param bibl "-" bibtex_out return ()

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Timber mailing list
by ryah dahl 09 Dec '08

09 Dec '08

Is there a user mailing list for the Timber language? (http://www.timber-lang.org/)

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Exact parsing of decimal numbers
by Emil Axelsson 09 Dec '08

09 Dec '08

Hello, I don't know enough about the RealFrac class to answer this myself: Can I be sure that Numeric.readFloat :: ReadS Rational never exhibits any rounding errors? Thanks, / Emil

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