January 2008 - Haskell-Cafe

Cabal and DB packages
by manu 08 Jan '08

08 Jan '08

Hello, I'm trying to do something that should be fairly simple, installing some DB package so I can use MySQL or SQLite. However I've had troubles building HSQL, HaskellDB and Takusen before giving up (I am using ghc 6.8.1 and Cabal-1.2.2.0). ------------------ with HSQL : Database/HSQL.hsc:66:7: Could not find module `System.Time': it is a member of package old-time-1.0.0.0, which is hidden so I added 'old-time' to the 'build-depends' line in the .cabal file but then it fails building with Building hsql-1.7... [1 of 2] Compiling Database.HSQL.Types ( Database/HSQL/Types.hs, dist/ build/Database/HSQL/Types.o ) Database/HSQL/Types.hs:134:18: Not in scope: type variable `forall' Database/HSQL/Types.hs:134:25: Not in scope: type variable `a' Database/HSQL/Types.hs:134:27: Illegal operator `.' in type `forall a . (Int -> FieldDef -> (FieldDef -> CString -> Int -> IO a) -> IO a)' (Use -XTypeOperators to allow operators in types) etc... At this point, it goes over my head... --------------- What the hell, I'll use HaskellDB and FlatDB, but then I get this message while building HaskellDB : src/Database/HaskellDB/HDBRec.hs:106:11: Illegal signature in pattern: l f a Use -XPatternSignatures to permit it when I add 'PatternSignatures' to the 'Extensions:' line in .cabal, it fails configuring with : Setup.hs: haskelldb.cabal:8: Parse of field 'extensions' failed: Jeeeesus, nevermind, I'll try Takusen --------------- with Takusen : $ runhaskell Setup.hs configure Setup.hs:26:7: Could not find module `Distribution.Program': Use -v to see a list of the files searched for. I give up... ---------------- Is there any simple way to do it though ? Is there any problems with Cabal that I need to work around ? Help !! Manu

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Newbie question: the need for parentheses
by Fernando Rodriguez 08 Jan '08

08 Jan '08

Hi, I have this function that sums up a list of numbers: suma [] = 0 suma (h:t) = h + suma t However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t Thanks! :-)

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Compiler backend question
by Peter Verswyvelen 08 Jan '08

08 Jan '08

If I understand it correctly, the GHC compiler either directly generates machinecode, or it uses C as an intermediate language. I also read somewhere that C is not the most efficient intermediate representation for functional languages, and that one gets better performance when generating native machine code. However, from the discussions in this list, I could conclude that the low level machine code optimizer of GHC is far from state-of-the-art compared to industry standard C/C++ compilers. I was wondering, why doesn't GHC use the GCC (or any other standard compiler) backend intermediate code? The backend of GCC generates highly optimized code no? Or is the intermediate code format of GCC (or other compilers) not suitable for Haskell? Another question regarding the backend: a cool feature of the Microsoft Visual C++ (MVC) compiler is its ability to perform "LTCG" (link-time-code-generation), performing whole program optimization. It something like this possible with Haskell / (or the GCC backend?). Would it be possible to feed all the generated C code of the GHC compiler into the MVC compiler, to generate one big MVC / LTCG generated executable? It would be interesting to see how much the whole program optimization approach (which can do analysis on the program as if it was a single module) would improve performance... Cheers, Peter

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Concurrency questions
by Andrew Coppin 08 Jan '08

08 Jan '08

Just a couple of things I was wondering about... 1. Is there some way to assign a "priority" to Haskell threads? (The behaviour I'd like is that high priority threads always run first, and low priority threads potentially never run at all unless there's an available processor which is completely idle.) 2. I have a situation where I have a thread generating some data and putting it into a mutable array, and another thread trying to read that data. Is there a way I can make the reader thread block if it tries to read a cell that hasn't been computed yet, but not introduce too much overhead for cells that have been filled in? 3. Would it be hard to make it so that the number of real threads (the RTS +N flag) could be adjusted at runtime?

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Implicit parameters and Arrows/Yampa?
by Peter Verswyvelen 08 Jan '08

08 Jan '08

If I understand it correctly, implicit parameters in Haskell allow you to pass values to functions with explicitly adding a parameter to each of the functions being "called" (I appologize for my imperative terminology here. How would I say this correctly? Being "evaluated"?) The arrows always use tuples to group the input and output parameters, like: foo :: SF (Int,Int,Int) (Int,Int) foo = proc (x,y,z) -> do p <- cat -< (x,y) q <- dog -< z returnA -< (p,q) where cat = proc (x,y) -> returnA -< (x+y) dog = proc z -> returnA -< 10*z Suppose I don't want to explicitly pass the x and y parameters to the cat (and deeper) arrows, but make them implicit. I guess that would be impossible? I mean, I can't use implicit parameters language extension to make the arrow input parameters implicit? Thanks, Peter

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Wikipedia on first-class object
by Cristian Baboi 07 Jan '08

07 Jan '08

http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions. ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

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ANN: Build fixed for regex-base,posix,compat,pcre
by ChrisK 07 Jan '08

07 Jan '08

ANNOUNCEMENT: Build fixed for regex-base, regex-posix, regex-compat, regex-pcre The changes are mainly to the Cabal build files to support ghc-6.8 and ghc-6.6 simultaneously. They definitely work with cabal version 1.2.3.0 (required for regex-pcre). The regex-base, regex-posix, and regex-compat packages also seem to work with cabal-1.2.2.0 (but not regex-pcre). The darcs repositories under http://darcs.haskell.org/packages/regex-unstable/ hold the new code. I have uploaded new versions of the most popular regex-* packages to hackage: Mon Jan 7 13:18:00 PST 2008 ChrisKuklewicz regex-pcre-0.94.1 Mon Jan 7 13:05:57 PST 2008 ChrisKuklewicz regex-posix-0.93.1 Mon Jan 7 13:04:49 PST 2008 ChrisKuklewicz regex-base-0.93.1 Mon Jan 7 06:50:54 PST 2008 ChrisKuklewicz regex-compat-0.91 I would like to thank everyone who sent me updated, and particularly Olivier Boudry who sent patches. These build fixes have only been tested with ghc-6.6.1 on Mac OS X 10.5.1 (powerpc) and with ghc-6.8.2 on Linux (x86). Happy New Year, Chris Kuklewicz

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GHC's dynamic linker and Windows
by Peter Verswyvelen 07 Jan '08

07 Jan '08

I recently tried the latest version of HS-PLUGINS, and it gave an error on Windows. After a bit of Googling it seemed Conal Elliot had the same problem. I reported this problem to the author. This is also (one of) the reason why I could not get YI running on Windows. Now, according to the documentation, HS-PLUGINS seems to be mainly a handy wrapper around GHC's dynamic linker and compiler. Suppose I would use GHC's dynamic linker, interpreter and compiler directly (aha "GHC as a library"); does this work on Windows, or are there known problems with these? Thanks, Peter

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Type Mismatch
by Cetin Sert 07 Jan '08

07 Jan '08

Hi, I'm new to Haskell programming and have the following problem. ----------------------------------------------------- (|>) f g = g f data Stream a where S :: (s -> Step s a) -> s -> Stream a data Step s a = Done | Yield a s | Skip a s toStream :: [a] -> Stream a toStream ax = S step ax where step [] = Done step (a:ax) = Yield a ax fromStream :: Stream a -> [a] fromStream (S step s) = loop s where loop s = case step s of Done -> [] Skip a s' -> loop s' Yield a s' -> a : loop s' filterStream :: (a -> Bool) -> Stream a -> Stream a filterStream p (S step s) = S filter s where filter s = case step s of Done -> Done Skip a s' -> Skip a s' Yield a s' -> if p a then Yield a s' else Skip a s' mapStream :: (a -> b) -> Stream a -> Stream b mapStream f (S step s) = S map s where map s = case step s of Done -> Done Skip a s' -> Skip (f a) s' Yield a s' -> Yield (f a) s' class Streamable a where to :: a -> Stream a instance Streamable [a] where to = toStream -- ERROR: see below s f x = x |> toStream |> f |> fromStream smap x = s (mapStream x) sfilter x = s (filterStream x) (%) a b = mod a b main = do print ([0..20] |> sfilter (\x -> x % 2 == 0)) --------------------------------------------------------- Error 1 Couldn't match expected type `a' (a rigid variable) against inferred type `[a]' `a' is bound by the instance declaration at C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src/Main.hs:63:0 Expected type: [a] -> Stream [a] Inferred type: [[a]] -> Stream [a] In the expression: toStream In the definition of `to': to = toStream C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src\Main.hs 64 8 How can I make the types match so that I can declare lists streamable? Is there something like in-place type annotations as in ML/OCaml/F#? Best Regards, Cetin Sert corsis.de/blog

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haskell package installation problem
by Brian Park 07 Jan '08

07 Jan '08

Hi, I was installing various haskell packages from hackage. When I was installing HaXml, I think it was complaining about Text.PrettyPrint.HughesPJ not installed or something. (can't remember the specific message and I can't reproduce now...) So I installed "pretty-1.0.0.0" package as well. Ever since then, when I try to install other haskell packages, I get the following error message: "brian@bp-desktop:~/Download/mtl-1.1.0.0$ runghc Setup.hs configure <interactive>: /usr/local/lib/ghc-6.8.2/lib/Cabal-1.2.3.0/HSCabal-1.2.3.0.o: unknown symbol `prettyzm1zi0zi0zi0_TextziPrettyPrintziHughesPJ_lvl18_closure' ghc-6.8.2: unable to load package `Cabal-1.2.3.0'" Does anyone know what the problem is? Currently installed packages are: ========= /usr/local/lib/ghc-6.8.2/package.conf: Cabal-1.2.3.0, HTTP-3001.0.4, HUnit-1.2.0.0, X11-1.4.1, array-0.1.0.0, base-3.0.1.0, bytestring-0.9.0.1, containers-0.1.0.1, directory-1.0.0.0, filepath-1.1.0.0, (ghc-6.8.2), haskell98-1.0.1.0, hpc-0.5.0.0, hxt-7.4, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parsec-2.1.0.0, polyparse-1.1, pretty-1.0.0.0, process-1.0.0.0, random-1.0.0.0, readline-1.0.1.0, rts-1.0, template-haskell-2.2.0.0, unix-2.3.0.0, xmonad-0.5, xmonad-contrib-0.5 ========= Thank you, - Brian

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