December 2008 - Haskell-Cafe

Building/installing haskelldb and friends with ghc 6.10.1
by Laurent Giroud 09 Dec '08

09 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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Cabal: defaultMainNoRead and source file location
by Mauricio 09 Dec '08

09 Dec '08

Hi, I've just seen this from Distribution.ModuleName (ghc 6.10): toFilePath $ ( simple "A.B.C" ) to which ghci answers: "A.B.C". Shouldn't it say "A/B/C"? The reason why I'm asking is that I've just created a Setup.hs with 'defaultMainNoRead', and 'Setup build' complains it can't find A.B.C. When I copy (move doesn't work) C.hs from <src_dir>/A/B to <src_dir>/A.B.C.hs, it doesn't complain, although it fails to generate my library saying: [1 of 1] Compiling A.B.C ( src/A/B/C.hs, dist/build/A/B/C.o) /usr/bin/ar: creating dist/build/libHSabc-0.a /usr/bin/ar: dist/build/A.B.C.o: No such file or directory Thanks for your tips, Maurício

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How to define Show [MyType] ?
by Dmitri O.Kondratiev 09 Dec '08

09 Dec '08

I am trying to define instance Show[MyType] so show (x:xs :: MyType) would return a single string where substrings corresponding to list elements will be separated by "\n". This would allow pretty printing of MyType list in several lines instead of one, as default Show does for lists. For example: data ShipInfo = Ship { name :: String, kind :: String, canons :: Int } deriving Show s1 = Ship {name ="HMS Fly", kind = "sloop", canons=16} s2 = Ship {name ="HMS Surprise", kind = "frigate", canons=42} -- Yet when I try to define: instance (Show ShipInfo) => Show [ShipInfo] where show (x:xs) = "<" ++ show x ++ ">" ++ show xs -- I get this error: Illegal instance declaration for `Show [ShipInfo]' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Show [ShipInfo]' Failed, modules loaded: none. -- On the other hand this definition: instance (Show a) => Show [a] where show (x:xs) = "<" ++ show x ++ ">" ++ show xs -- Gives a different error: Duplicate instance declarations: instance (Show a) => Show [a] -- Defined at C:/wks/haskell-wks/ShowMatrix.hs:37:0 instance (Show a) => Show [a] -- Defined in GHC.Show Failed, modules loaded: none. -- Note the last error implicitly tells us that defining Show [a] is possible in principle! -- In fact it already defined in GHC.Show !!! -- How to define Show [MyType] ? Thanks! Dmitri

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Data.Map add only if not exist and return true if added
by John Ky 09 Dec '08

09 Dec '08

Hi, I'm looking for a function for Data.Map that will insert a new key and value into the map if the key doesn't already exist in the map. When the key already exists, I don't want the value updated in the map. Additionally, I want to know whether the key/value was inserted or not so that I can return that fact as a boolean from my function. I can't work out which function in Data.Map is suitable for this task and I wanted to avoid two lookups. Thanks, -John

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Why does "instance Ord Pat" causes <<loop>>
by Martin Hofmann 09 Dec '08

09 Dec '08

I am storing the TH data types 'Exp' and 'Pat' in Maps and Sets. As a first attempt to quickly get rid of typechecker's complaints I defined some naive instances of Ord for Exp and Pat. Now it took me about a week to realise, that 'instance Ord Pat' causes ghc to loop. Apparently, there is a reason, why Pat and Exp are not instances of Ord. What is so bad about it? If Pat and Exp should not be instances of Ord, maybe a note in the source code or haddock would be helpful. On the other hand, what would argue against a lexicographic ordering (apart from the inefficient implementation of the following one)? Following some literate code to reproduce the <<loop>> (or stack overflow in GHCi), by commenting and uncommenting the appropriate lines: > {-# OPTIONS_GHC -fglasgow-exts -fth #-} > module Test where > import Language.Haskell.TH > import Data.Set ------------------- naive Ord > instance Ord Exp > instance Ord Pat ------------------- lexicographic Ord instance Ord Exp where compare l r = compare (show l) (show r) instance Ord Pat where compare l r = compare (show l) (show r) ------------------- > mkVP s = VarP $ mkName s > mkVE s = VarE $ mkName s > rule1 = (,) [mkVP "x_14"] (mkVE "y_14") > rule2 = (,) [InfixP (mkVP "x1_15") '(:) (mkVP "x_16")] (InfixE (Just (mkVE "y1_15")) (ConE '(:)) (Just (mkVE "ys_16"))) > stack_overflow = fromList [rule1,rule2] Thanks, Martin

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Reading showables
by John Ky 08 Dec '08

08 Dec '08

Hi, Is there a way to read Showables? main = do putStrLn $ show $ read Thanks -John

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