December 2006 - Haskell-Cafe

I'd like start with Haskell, but...
by Waldemar Biernacki 18 Dec '06

18 Dec '06

Hello! I'd like to start programming in Haskell. But as an "industry programmer" I have a hope to use Haskell in my every-day work. Big part of my every-day work are GUI applications (in MS-Windows) working with SQL databases (PostgreSQL on Linux servers). My question: Is there a TRUE possibility to use Haskell for such applications? Is anybody there who have ANY experience in the field? If answer would be positive what GUI+database libraries could be used in such a case? I've seen some GUI libraries web pages, but they seem to be NOT maintenanced any more. cheers Waldemar

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type hackery question
by jeff p 18 Dec '06

18 Dec '06

Hello, The HList paper gives a type cast: class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x and a type equality check: class TypeEq x y b | x y -> b instance TypeEq x x HTrue instance TypeCast HFalse b => TypeEq x y b The type cast succeeds causing its two argument types to be unified, or simply fails if the arguments are not unifiable. The type equality always succeeds "returning" HTrue if the arguments are syntactically equal, and HFalse if the arguments are not syntactically equal; thus the type equality will return false for two arguments which could be unified. If I understand correctly, these two classes do not provide a way to check for unifiability. Is it possible to write a class which checks to see if two given type arguments are unifiable? thanks, Jeff

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greencard and cabal, how to do it right?
by Marc Weber 17 Dec '06

17 Dec '06

I want to have a look at fruit which is using GCJNI, greencard, ... That's why I want to learn how to use greencard with cabal. If this is only a RTFM problem, can you point me to the docs? When using the cabal builtin preprocessor I get the error that --libdir has to be specified. So I tried adding this manually using a custom proprocessor beeing copied mostly from the already existing one: -- setup.hs ----------------------------------------------------------- import Distribution.PackageDescription import Distribution.Simple.Utils (rawSystemVerbose, dieWithLocation) main= do defaultMainWithHooks $ defaultUserHooks { hookedPreProcessors = [ myGreencardPP ] } where myGreencardPP :: (String, BuildInfo -> LocalBuildInfo -> PreProcessor) myGreencardPP = ("gc", ppGreenCard' ["--libdir ../c_lib"] []) -- << --libdir is added here -- copied from Distribution.PreProcess ppGreenCard' :: [String] -> [String] -> BuildInfo -> LocalBuildInfo -> PreProcessor ppGreenCard' args inputArgs bi lbi = maybe (ppNone "greencard") pp (withGreencard lbi) where pp greencard inFile outFile verbose = rawSystemVerbose verbose greencard (args ++ ["-tffi", "-o" ++ outFile, inFile] ++ inputArgs) ppNone :: String -> PreProcessor ppNone name inFile _ _ = dieWithLocation inFile Nothing $ "no " ++ name ++ " preprocessor available" ---------------------------------------------------------------------------- I don't understand why the command does work but not when beeing executed by cabal: The commandline shows the directory content (no Greencard.hs). (1>) Then it executes ./setup -v build which shows the commandline failing (2>) Then it executes this commandline from shell (using bash eval and getting the greencard line by grepping the output from ./setup -v build) The final ls (3>) shows that Greencard.hs is there. I've marked these parts using > and * * I've also replaced the greencard executable by a shell script printing pwd which is the same.. Whats going wrong here? How to do it right? > $ ls; ./setup -v build || echo -e "\n\n ===> build failed \n\n"; eval `./setup -v build 2>&1 | grep /usr/local/bin/greencard` && echo -e "\n\n =====> this works why?" ; ls 1> Greencard.cabal Greencard.gc Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Preprocessing executables for greencard-0.0... 2> */usr/local/bin/greencard --libdir ../c_lib -tffi -oGreencard.hs Greencard.gc* unrecognized option `--libdir ../c_lib' Usage: greencard [OPTION]... SOURCE Run Green Card, a foreign function interface preprocessor for Haskell, over SOURCE Green Card home page: http://www.haskell.org/greencard -?, -h --help print out this help message and exit -V --version output version information and exit -v --verbose print more information -d --debug output extra debug information --print-libdir output default library location --libdir=DIR use base library location -i DIRS --include-dir=DIRS Add DIRS (colon separated) to the include search path -s SUFS --suffix=SUFS colon separated list of source file suffixes -t TARGET --target=TARGET generate Haskell code for a particular system (ghc, ffi) -S --safe-code call C `safely' (GHC only) -n --no-inline put C code in a separate file (GHC only) -4 --haskell1.4 generate code compatible with a Haskell 1.4 system -m SCHEME --name-mangling-scheme=SCHEME controls mapping of external names into Haskell ones (std, classic) -D --stub-debug include debugging code in generated code -f DLL --dllname=DLL generate 'foreign imports' to dynamic library DLL (FFI backend only) -C CCONV --callconv=CCONV use specific calling convention (stdcall, ccall) -o FILE --output=FILE write Green Card Haskell output to FILE -c FILE --output-c=FILE write Green Card C output to FILE -p PRE --output-prefix=PRE write Green Card Haskell output to PRE.{hs,c} Building greencard-0.0... /usr/bin/ghc -Idist/build -o dist/build/greencard/greencard --make -hide-all-packages -i -idist/build/autogen -i. -odir dist/build/greencard/greencard-tmp -hidir dist/build/greencard/greencard-tmp -package haskell98-1.0 -package base-2.0 -package QuickCheck-1.0 Greencard.gc -lc_lib -L../c_lib/ ghc-6.5.20060917: no input files Usage: For basic information, try the `--help' option. ===> build failed =====> this works why? 3> Greencard.cabal Greencard.gc *Greencard.hs* Greencard_stub_ffi.c Greencard_stub_ffi.h dist greencard_stub_ffi.c greencard_stub_ffi.h setup setup.hi setup.hs Marc

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Re: A suggestion for the next high profile Haskell project
by Paul Johnson 17 Dec '06

17 Dec '06

Magnus Therning <magnus(a)therning.org> wrote: > When reading this[1] I couldn't help thinking that rewriting GPG is an > excellent opportunity for using Haskell to have an impact on the world. The Fedora project is planning to rewrite the foundations of RPM. I've noticed that the higher level RPM tools are written in Python, and are (1) very slow, (2) single threaded, so window redraws don't happen promptly. This would also be a cool place for Haskell to shine. Paul.

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Re: [Haskell-cafe] Aim Of Haskell
by Neil Bartlett 17 Dec '06

17 Dec '06

I think this hits the nail on the head. To be blunt, the presence of so many academics and scientists in the Haskell community is intimidating to those of us that work "in industry". Our brains are, after all, not as highly trained as yours, and we care about different things than you do. Now I don't mean to say that the academics and scientists should go away! Far from it. Just that it would be great to hear more about the mundane aspects of programming occasionally. Like, how exactly do I read from a relational database with Haskell? Or process an XML file? Or build an event-driven GUI? And crucially, why does Haskell do those things better than Java, or C#, or Ruby? If somebody could write some articles on those subjects, and get them up on popular websites like Digg or Reddit, this would be far more helpful than yet another monad tutorial. The Haskell web server that Simon Peyton-Jones et al described in their paper would be a great example. But where's the download? How do I get a copy to play with? In the "real world", things don't stop with the publication of a paper ;-) I think Haskell has huge potential to improve mainstream programming, if it could only catch on a bit. I don't know how to make that happen, unfortunately (if I did, I would do it, and hopefully get rich in the process). But whatever Haskell needs, it's not getting at the moment. Neil > Hello Kaveh, > > Sunday, December 10, 2006, 6:15:23 PM, you wrote: > >> chosen one. But Haskell seems to be buzz-full research platform. Now again to the top : what is the aim of Haskell project? If it is going to be used in real world applications it needs more attention to real world application developers and their needs. > > you are right - just now Haskell is a huge technology with non-obvious path to learn. there is some work to make Haskell more pragmatic, but it's an chicken-and-egg problem - we have a small number of pragmatic programmers > that use Haskell and therefore it's hard to change Haskell to suit their needs, on the other hand this means that pragmatic programmers can't grok > Haskell > > on the way to make Haskell more pragmatic i especially mention renewal of > Haskell standard to include modern language extensions, modern programming > environments such as WinHugs or BusinessObjects, development of web/db/gui > libraries, and definition of core (standard) libraries set > > one particular thing that we still lack is something like book "Haskell in > real world" > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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what are the points in pointsfree?
by Steve Downey 16 Dec '06

16 Dec '06

i'm not naive enough to think they are the composition function, and i've gathered it has something to do with free terms, but beyond that i'm not sure. unless it also has something to do with fix points?

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Composing monads (sort of)
by Mark Wassell 16 Dec '06

16 Dec '06

Hi, I have a set of functions: f1 :: DBRecord -> Maybe Int f2 :: Int -> IO Maybe DBRecord f3 :: DBRecord -> Maybe Int The odd numbered functions are field accessors, accessing a field that might hold an identifier for another record. The even numbered functions are record fetch functions that get records from the database. I want to compose these so that I can navigate structures of joined records in the database. How can I concisely compose these functions without having to write a cascade of case statements such as: case f1 rec1 of Nothing -> return Nothing Just id1 -> do rec2 <- f2 id2 return $ case rec2 of Nothing -> return Nothing Just rec2' -> case f3 rec2' of .... I understand that if I was just dealing with Maybe I could use the fact that Maybe is a monad. I am also not sure if composing the IO and the Maybe will get me what I want (some of the functions only return Maybe Int). Cheers Mark PS Heard this on the 'West Wing' and thought it was appropriate in a way: A coach goes up to a player and asks "Are you ignorant or apathetic?". The player replies "I don't know and I don't care".

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Fwd: Build failure on FC6, ghc66
by Conrad Parker 16 Dec '06

16 Dec '06

Hi Haskellers, I received the following bug report from someone trying to build HOgg on Fedora Core 6 (FC6) with its ghc66 package. The build error is: Could not find module `Data.Map': it is a member of package base, which is hidden I've built HOgg on Debian unstable with its ghc6.6 package without problems. I know that non-base libraries have been separated out in the GHC 6.6 release. Is this problem FC6 specific or a bug in HOgg's cabal setup? any clues? cheers, Conrad. ---------- Forwarded message ---------- From: Ian Malone <ibmalone(a)gmail.com> Date: 11-Dec-2006 06:36 Subject: Re: [annodex-announce] HOgg 0.2.0 Released To: Conrad Parker <conrad(a)metadecks.org> Hi, On 06/12/06, Conrad Parker <conrad(a)metadecks.org> wrote: > HOgg 0.2.0 Released > ------------------- > > The HOgg package provides a commandline tool for manipulating Ogg files, > and a corresponding Haskell library. > > http://snapper.kfish.org/~conrad/software/hogg/ > Can't get it to build on FC6, using their ghc66 package, tried installing hugs98 and haddock as well (although they don't seem to be related to the problem). "Could not find module `Data.Map'" [ian@atlas hogg-0.2.0]$ ./Setup.hs configure Configuring hogg-0.2.0... configure: /usr/bin/ghc-pkg configure: Using install prefix: /usr/local configure: Binaries installed in: /usr/local/bin configure: Libraries installed in: /usr/local/lib/hogg-0.2.0/ghc-6.6 configure: Private binaries installed in: /usr/local/libexec configure: Data files installed in: /usr/local/share/hogg-0.2.0 configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.6 configure: Using package tool: /usr/bin/ghc-pkg configure: Using ar found on system at: /usr/bin/ar configure: Using haddock found on system at: /usr/bin/haddock configure: No pfesetup found configure: Using ranlib found on system at: /usr/bin/ranlib configure: Using runghc found on system at: /usr/bin/runghc configure: Using runhugs found on system at: /usr/bin/runhugs configure: No happy found configure: No alex found configure: Using hsc2hs: /usr/bin/hsc2hs configure: No c2hs found configure: No cpphs found configure: No greencard found [ian@atlas hogg-0.2.0]$ ./Setup.hs build Preprocessing library hogg-0.2.0... Preprocessing executables for hogg-0.2.0... Building hogg-0.2.0... Codec/Container/Ogg/MessageHeaders.hs:17:7: Could not find module `Data.Map': it is a member of package base, which is hidden -- imalone

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Stack, Heap and GHC
by Felix Breuer 16 Dec '06

16 Dec '06

Hello everyone, I have been trying to run a Haskell program of mine that does an extensive computation with very large amounts of data. I compiled the program with ghc --make. When I run it it terminates after some time with the message: Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. The program isn't that well written so the overflow did not surprise me, I expected that it might run out of memory. What did surprise me was the *stack* overflow. I do not use recursion in my program except for a couple of fold operations over very large lists. So I have a number of questions: 1) Which Haskell operations cost space on the stack, which cost space on the heap? I guess this is implementation dependent, so I looked into the GHC manual but did not find an answer. Where can I look these things up? 2) What could be possible sources of a stack overflow? (Apart from a recursive but not tail-recursive function.) 3) I tried using +RTS -K<size> as suggested, but these options do not seem to be passed through if I use --make. How can I use both, these compilation flags and --make? Thanks, Felix

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Re: [Haskel-cafe] Re: what are the points in pointsfree?
by Steve Downey 16 Dec '06

16 Dec '06

I spent time working with cubic mappings. With two critical points, there are Julia sets that consist of infinitely many disconnected components that are still locally connected, corresponding two one critical point escaping and one falling into one of the basins of attraction. Of course that was about 15 years ago, and I haven't really kept up. Too much time keeping up with software. On 12/16/06, Jacques Carette <carette(a)mcmaster.ca> wrote: > > Steve Downey wrote: > > No fair. Although I've a B.S. in Mathematics, I spent most of my time > > in complex analytic dynamical systems, rather than hanging with the > > algebraists. Except for a bit where I did some graph theory. > > Never thought I'd run into a fellow dynamicist on haskell-cafe! I did > my PhD on the links between the geometry and dynamics of Julia sets (co > supervised by the (late) Adrien Douady and John Hubbard). What > 'flavour' of this stuff are you interested in? > > Jacques > > PS: now I have converted to being a computer scientist, which is why I > hang out on haskell-cafe! >

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