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SYB with class: Bug in Derive.hs module
by Roman Cheplyaka 05 Sep '12

05 Sep '12

There's a bug in syb-with-class reported by Alexey Rodriguez Yakushev in 2008 [1]. I can confirm that the bug is still there (syb-with-class-0.6.1.3, ghc 7.4.1). [1]: http://www.haskell.org/pipermail/haskell-cafe/2008-March/041179.html Here's an even simpler test case: {-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, TemplateHaskell, OverlappingInstances, DeriveDataTypeable #-} import Data.Generics.SYB.WithClass.Basics import Data.Generics.SYB.WithClass.Derive data Foo = Foo Foo | Bar deriving (Typeable, Show) deriveData [''Foo] f :: (Data NoCtx ast, Typeable ast) => ast -> TypeRep f = typeOf main = print $ f $ Foo Bar The cause of this bug is a self-referencing instance created by deriveData: instance (Data ctx Foo, Sat (ctx Foo)) => Data ctx Foo where ... What's the proper way to fix it? -- Roman I. Cheplyaka :: http://ro-che.info/

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How to waitForProcess?
by Magicloud Magiclouds 04 Sep '12

04 Sep '12

Hi, I have code like this and it leaves lots of zombies of flow-export. Then I add waitForProcess.... Well I do not know where to put it. Before or after 'hGetContents' both make the program hung. exportCSV :: FilePath -> IO [String] exportCSV file = do csv_ <- withBinaryFile file ReadMode $ \i -> do (_, Just o, _, h) <- createProcess $ CreateProcess (RawCommand "/usr/bin/flow-export" ["-f2"]) Nothing Nothing (UseHandle i) CreatePipe Inherit True False hGetContents o return $ tail $ lines csv_ -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Over general types are too easy to make.
by timothyhobbs＠seznam.cz 04 Sep '12

04 Sep '12

I'd have to say that there is one(and only one) issue in Haskell that bugs me to the point where I start to think it's a design flaw: It's much easier to type things over generally than it is to type things correctly. Say we have a >data BadFoo = > BadBar{ > badFoo::Int} | > BadFrog{ > badFrog::String, > badChicken::Int} This is fine, until we want to write a function that acts on Frogs but not on Bars. The best we can do is throw a runtime error when passed a Bar and not a Foo: >deBadFrog :: BadFoo -> String >deBadFrog (BadFrog s _) = s >deBadFrog BadBar{} = error "Error: This is not a frog." We cannot type our function such that it only takes Frogs and not Bars. This makes what should be a trivial compile time error into a nasty runtime one :( The only solution I have found to this is a rather ugly one: >data Foo = Bar BarT | Frog FrogT If I then create new types for each data constructor. >data FrogT = FrogT{ > frog::String, > chicken::Int} >data BarT = BarT{ > foo :: Int} Then I can type deFrog correctly. >deFrog :: FrogT -> String >deFrog (FrogT s _) = s But it costs us much more code to do it correctly. I've never seen it done correctly. It's just too ugly to do it right :/ and for no good reason. It seems to me, that it was a design mistake to make data constructors and types as different entities and this is not something I know how to fix easily with the number of lines of Haskell code in existence today :/ >main = do > frog <- return (Frog (FrogT "Frog" 42)) > print $ > case frog of > (Frog myFrog) -> deFrog myFrog > badFrog <- return (BadBar 4) > print $ > case badFrog of > (notAFrog@BadBar{}) -> deBadFrog notAFrog The ease with which we make bad design choices and write bad code(in this particular case) is tragically astounding. Any suggestions on how the right way could be written more cleanly are very welcome! Timothy Hobbs

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Re: [Haskell-cafe] From monads to monoids in a small category
by Alberto G. Corona 04 Sep '12

04 Sep '12

Not to mention the ugly formatting ;) 2012/9/5 Richard O'Keefe <ok(a)cs.otago.ac.nz>: > > On 4/09/2012, at 10:39 PM, Alberto G. Corona wrote: > >> "Monads are monoids in the category of endofunctors" >> >> This Monoid instance for the endofunctors of the set of all elements >> of (m a) typematch in Haskell with FlexibleInstances: >> >> instance Monad m => Monoid (a -> m a) where >> mappend = (>=>) -- kleisly operator >> mempty = return >> >> The article can be found here: >> >> http://haskell-web.blogspot.com.es/2012/07/from-monads-to-monoids-in-small.… >> >> I would appreciate some comments. > > s/kleisly/Kleisli/ > In the article, > s/Lets/Let's/ > /Here 'm b' as/ s/as/is/ > s/this_are/this are/ > s/first, is/first is/ > s/haskell/Haskell/ > s/polimorphic/polymorphic/ > s/x=/x =/ > s/let's/Let's/ > s/condition, associativity/condition, associativity,/ > /if not where that way, .* guess/ > I *think* you mean to say something like > (If it were not so, it would be impossible to > define the denotational semantics of imperative > languages in terms of monads, I guess.) > Generally, it's "according TO", not "according WITH", > and "associated WITH", not "associated TO". > > instance Functor a > doesn't seem to be legal Haskell. > > At this point I stopped reading. > >

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Asking for advice on programming style and also performance
by Harald Bögeholz 04 Sep '12

04 Sep '12

Dear Haskell Cafe, I am working on a solver for Slitherlink puzzles in Haskell. Today I am completely puzzled that a small change I made had a huge performance impact. I thought the change to be merely cosmetic to make a function a little more elegant and the code more readable. But this change slowed down my program by a factor of 100! I would greatly appreciate advice from experienced Haskellers why this has happened. The program is hosted on github; this is the direct link to the change with unexpected outcome: https://github.com/ctbo/slitherlink/commit/b6f58699258ef68ddee21a1346bd1844… The program itself isn't commented at all, but I have written a sketch of how it works in the wiki: https://github.com/ctbo/slitherlink/wiki/How-it-works While I am at it: I would also appreciate any advice you might have on the programming style in general. I am relatively new to Haskell and this is my first nontrivial program. So any suggestions for improvements are welcome! Thanks for your time, -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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Cabal install fails due to recent HUnit
by Erik Hesselink 03 Sep '12

03 Sep '12

Hi all, All cabal installs using cabal-install-0.10.2 are currently failing for us. This is due to the cabal file for HUnit-1.2.5.0, which was recently uploaded to hackage. The ouput I'm getting from cabal is just: Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file "HUnit/1.2.5.0/HUnit.cabal" If I unpack HUnit-1.2.5.0 and call 'cabal configure', I get: cabal: HUnit.cabal:57: The 'type' field is required for test suites. The available test types are: exitcode-stdio-1.0 The relevant lines from the cabal file are: Test-Suite hunit-tests-optimize-0 Type: exitcode-stdio-1.0 These look fine to me. Does anyone have any idea how to go about fixing this (on hackage at least)? Could this package temporarily be removed, to avoid breaking everyone's cabal? Erik

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hstats median algorithm
by David Feuer 03 Sep '12

03 Sep '12

The median function in the hstats package uses a naive O(n log n) algorithm. Is there another package providing an O(n) option? If not, what would it take to get the package upgraded?

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hellno - a somewhat different approach to tackling cabal hell
by Danny B 03 Sep '12

03 Sep '12

Like many of us, I've suffered from cabal dependency hell and sought relief. I wasn't exactly happy with sandboxes - because using per-project ones meant package duplication and shared sandboxes suffer from the same issues as the GHC package database itself, i.e. reinstalls can break other projects, etc. So I wrote hellno, which is so named because that's the expression one makes when seeing cabal hell suddenly manifest itself and that's also the expression one makes upon encountering yet ANOTHER cabal wrapper. To quote the README: > Generally, with hellno you'll get the same result as for blowing away your user > package database and doing a nice clean install but without having to recompile > everything and with ability to easily revert back and change between projects. Hellno works by keeping all the compiled packages to itself in a database, so that when you ask it to bring in the dependencies of a project, it will use the precompiled packages if available or install the deps and save them for later reuse. Hellno puts symlinks in the user package database pointing to packages in its storage to make them visible. Mutating the user db is hardly elegant, although shouldn't result in much trouble. It's been working fine for me, so I figured it may be useful to others. You can get hellno from GitHub: https://github.com/db81/hellno I don't mean to say that sandboxing is inherently bad or should not be used, but I guess it's better to consider more than one way.

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Installation issues with Centos
by Manish Trivedi 03 Sep '12

03 Sep '12

# uname -a Linux <<hosename>> 2.6.32-220.2.1.el6.centos.plus.x86_64 #1 SMP Thu Dec 22 23:32:31 GMT 2011 x86_64 x86_64 x86_64 GNU/Linux # cat /proc/version Linux version 2.6.32-220.2.1.el6.centos.plus.x86_64 ( mockbuild(a)c6b18n3.bsys.dev.centos.org) (gcc version 4.4.6 20110731 (Red Hat 4.4.6-3) (GCC) ) #1 SMP Thu Dec 22 23:32:31 GMT 2011 I am trying to install Haskell framework on this server. - Installation for GHC went ok. no error - Installed ghc-7.4.1 from "ghc-7.4.1-x86_64-unknown-linux.tar" then while trying to install (in configure step) Haskell platform version 2012.2.0.0. I get following error: [root@qa-engine1 haskell-platform-2012.2.0.0]# ./configure ************************************************** * * Haskell Platform Source Installer * * For the Haskell Platform 2012.2.0.0 and GHC 7.4.1 * ************************************************** checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu checking for ghc... /usr/local/lib/ghc-7.4.1/ghc checking for ghc-pkg... /usr/local/lib/ghc-7.4.1/ghc-pkg checking for hsc2hs... /usr/local/lib/ghc-7.4.1/hsc2hs checking version of ghc... 7.4.1 checking version of ghc-pkg matches... yes checking ghc actually works... no configure: error: Your installation of ghc does not appear to work. It cannot compile a simple program (see config.log for the details). If you installed ghc from a generic binary tarball then it is worth checking that you have the 'gmp' C library and header files installed. (On Debian-based systems this package is called libgmp3-dev.) ==================================== I have made sure that libc and gmp are indeed installed. while trying use yum to make sure I have gmp installed. ==================================== [root@qa-engine1 haskell-platform-2012.2.0.0]# yum install gmp Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: mirror.stanford.edu * extras: centos.mirror.facebook.net * updates: mirror.stanford.edu Setting up Install Process Package gmp-4.3.1-7.el6_2.2.x86_64 already installed and latest version Nothing to do ======================================== Any help is appreciated. Btw, I could make haskell latest version work on my mac osx. -Manish

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Compiling a haskell project as a shared library and loading it in C/C++
by Farid Neshat 02 Sep '12

02 Sep '12

So I'm trying to follow [this blog][1]. So I tried the code, couldn't compile the haskell part, probably due to the fact the guide uses version 6, but I have version 7 of ghc. By changing the paramaters I could compile it with the following: ghc --make -dynamic -shared -fPIC -no-hs-main -optl '-shared' -optc '-DMODULE=Test' -o Test.so Test.hs module_init.c But the when trying to load it, on dlopen, I get the following error: /usr/lib/ghc/ghc-prim-0.2.0.0/libHSghc-prim-0.2.0.0-ghc7.4.1.so: undefined symbol: stg_forkOnzh I suspect it's because the haskell runtime is not linked to the shared library but adding -L/usr/lib/ghc/ didn't really help. I searched a lot, but couldn't find anymore example that somebody have done this. [Here's also some docs][2] for compiling shared libraries: [1]: http://weblog.haskell.cz/pivnik/building-a-shared-library-in-haskell [2]: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/using-shared-libs.ht…

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