- Haskell-Cafe - Haskell.org

Defining a containing function on polymorphic list
by Raeck Zhao 24 Dec '08

24 Dec '08

I am trying to define a containing function to see if a value is one of the elements within a list which is polymorphic, but failed with the following codes: > contain :: a -> [a] -> Bool > contain x [] = False > contain x (y:ys) = if x == y then True else contain x ys it seems that the problem is the 'operator' == does not support a polymorphic check? Any way can solve the problem? or any alternative solution to achieve the purpose? Thanks! Raeck _________________________________________________________________ It’s the same Hotmail®. If by “same” you mean up to 70% faster. http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_…

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Type classes vr.s functions
by Brian Hurt 24 Dec '08

24 Dec '08

So, style question for people, if I can. I have a certain problem- basically, I have a bunch of functions which need a special function, of type a -> Foo say. And a bunch of other functions which can define that function on some type of interest, and then what to call the first batch of functions. I can do this either by defining a type class, something like: class Fooable a where toFoo :: a -> Foo or I can simply have all the functions which need a toFoo take an extra agrument. Performance really isn't that important here, so it's really a matter of style- which approach would people prefer in this case? Brian

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understanding enumerator/iteratee
by Jason Dusek 24 Dec '08

24 Dec '08

So, it looks Iteratee takes a "step" on the resource -- whatever it is -- and Enumerator manages the resource and sequences the steps of the Iteratee. The Enumerator, then, defines our way of managing a particular resource -- how to take a step, how to close it, &c. -- while the Iteratee describes a computation that computes an output and also tells us when to free the resource. Is that a correct interpretation? I find the material on Enumerator and Iterator to be vague (or at least not very concrete). I have seen code for a random access Iteratee/Enumerator pair, as well. In that case, the role of the Iteratee is to provide the "next step" to take -- one of forward <n>, backward <n>, close? -- _jsn

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Can I build and install GHC 6.10.1 without previous installed ghc
by Wang, Chunye (NSN - CN/Beijing) 24 Dec '08

24 Dec '08

Hi All, I have our own Linux distribution installed in my server. it is based on RedHat 9.0. Because it is the server, I can not update it to any other linux distribution Here is my situation, I have no machine with any version of ghc installed. This is what I did % wget http://haskell.org/ghc/dist/6.10.1/ghc-6.10.1-src.tar.bz2 % wget http://haskell.org/ghc/dist/6.10.1/ghc-6.10.1-src-extralibs.tar.bz2 % tar -jxf ghc-6.10.1-src-extralibs.tar.bz2 %tar -jxvf ghc-6.10.1-src.tar.bz2 % cd ghc-6.10.1 % sh boot grep: packages: No such file or directory Booting . Booting libraries/base Booting libraries/directory /usr/share/aclocal/libgcrypt.m4:23: warning: underquoted definition of AM_PATH_LIBGCRYPT run info '(automake)Extending aclocal' or see http://sources.redhat.com/automake/automake.html#Extending-aclocal /usr/share/aclocal/ao.m4:9: warning: underquoted definition of XIPH_PATH_AO Booting libraries/editline Booting libraries/network Booting libraries/old-time Booting libraries/process Booting libraries/regex-posix Booting libraries/time Booting libraries/unix % ./configure 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 Canonicalised to: x86_64-unknown-linux checking for ghc... no checking for ghc-pkg matching ... no checking for ghc-pkg... no checking whether ghc has editline package... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/local/bin/ld configure: error: GHC is required unless bootstrapping from .hc files. I know http://hackage.haskell.org/trac/ghc/wiki/Building/Prerequisites said that GHC 6.6 is required to build and install the GHC 6.10.1 So I try to follow the instructions http://hackage.haskell.org/trac/ghc/wiki/Building/Porting. Because I don't have any ghc installed in any machine. I have to use the "Porting GHC to a new platform " %./configure --enable-hc-boot --enable-hc-boot-unregisterised 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 Canonicalised to: x86_64-unknown-linux checking for ghc... no checking for ghc-pkg matching ... no checking for ghc-pkg... no checking whether ghc has editline package... no checking for nhc... no checking for nhc98... no checking for hbc... no checking for ld... /usr/local/bin/ld checking for path to top of build tree... ./configure: line 2733: -v0: command not found ./configure: line 2737: utils/pwd/pwd: No such file or directory configure: error: cannot determine current directory I think it is because the utils/pwd/pwd.hs is not compiled and linked into the ``pwd''. So I create a bash version of pwd #!/bin/bash pwd And m make it executeble, chmod +x utils/pwd/pwd Then I try to run the ./configure --enable-hc-boot --enable-hc-boot-unregisterised again %./configure --enable-hc-boot --enable-hc-boot-unregisterised again %cd includes %make %cd .. Finnally, I found I still need ghc is intalled on the host machine. %perl --version This is perl, v5.8.5 built for x86_64-linux-thread-multi Copyright 1987-2004, Larry Wall Perl may be copied only under the terms of either the Artistic License or the GNU General Public License, which may be found in the Perl 5 source kit. Complete documentation for Perl, including FAQ lists, should be found on this system using `man perl' or `perldoc perl'. If you have access to the Internet, point your browser at http://www.perl.com/, the Perl Home Page. %gcc --version gcc (GCC) 4.1.2 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %make --version GNU Make 3.80 Copyright (C) 2002 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %autoconf --version autoconf (GNU Autoconf) 2.59 Written by David J. MacKenzie and Akim Demaille. Copyright (C) 2003 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %automake --version automake (GNU automake) 1.9.2 Written by Tom Tromey <tromey(a)redhat.com>. Copyright 2004 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. %sed --version GNU sed version 4.1.2 Copyright (C) 2003 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE, to the extent permitted by law. %diff --version diff (GNU diffutils) 2.8.1 Copyright (C) 2002 Free Software Foundation, Inc. This program comes with NO WARRANTY, to the extent permitted by law. You may redistribute copies of this program under the terms of the GNU General Public License. For more information about these matters, see the file named COPYING. Written by Paul Eggert, Mike Haertel, David Hayes, Richard Stallman, and Len Tower. %python -V Python 2.3.4 Best Regards Chunye Wang <chunye.wang(a)nsn.com>

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monad constraint + record update
by Peter Padawitz 24 Dec '08

24 Dec '08

I'd like to define a monad Set for types in the class Eq. But how can the arguments of Set be constrained when Set is defined as an instance of Monad? instance Eq a => Monad Set where ... obviously cannot work. Is there a standard update function for fields in data types, something that OO programmers do with assignments like obj.attr := value ? Peter

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intercalate and (byte)strings
by wman 24 Dec '08

24 Dec '08

I encountered the following code : -- B == Data.ByteString ; L == Data.ByteString.Lazy contents' = B.intercalate B.empty $ L.toChunks contents with a previously unencountered function intercalate. A quick google query later i knew that it's just intersperse & concat nicely bundled and started wondering why anybody would do this, as simple contents' = B.concat $ L.toChunks contents would do (probably nearly) the same. The only thing I am able to come up with is that it somehow helps streamline the memory usage (if it has some meaning). Is there some reason to use intercalate <empty> <list> instead of concat <list> (probably when dealing with non-lazy bytestrings) ? Thx, wman.

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Detecting system endianness
by Mauricio 24 Dec '08

24 Dec '08

Hi, Is there some way I can check the endianness of the machine my haskell code is running in? Thanks, Maurício

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Is this related to monomorphism restriction?
by Maurício 24 Dec '08

24 Dec '08

Hi, Why isn't the last line of this code allowed? f :: (TestClass a) => a -> Integer f = const 1 a = (f,f) g = fst a The only thing I can think about is monomorphism restriction, but it's allowed (or even the third line would not be accepted). Is there something I could read to understand that? Thanks, Maurício

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Parsec question
by Erik de Castro Lopo 24 Dec '08

24 Dec '08

Hi all, I'm rather new to Haskell and I'm diving right into the deep end writing a parser using Parsec. In particular I'm using Text.ParserCombinators.Parsec.Language to do some of the heavy lifting and have this: import qualified Text.ParserCombinators.Parsec.Language as L import qualified Text.ParserCombinators.Parsec.Token as T lexer = T.makeTokenParser L.emptyDef { L.identStart = letter <|> char '_', L.identLetter = alphaNum <|> char '_', .... identifier :: CharParser st String identifier = T.identifier lexer and now I need to parse things "this.that.the.other". I'd like to have a function with the following signature: qualifiedIdentifier :: CharParser st [ String ] which should return [ "this", "that", "the", "other" ] and write it in terms of identifier and thats where I'm stuck. Anyone care to whack me with the cluestick? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "If you don't have freedom as a principle, you can never see a reason not to make an exception." -- Richard Stallman.

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Re: [Haskell-cafe] Re: [Haskell] ANN: HLint 1.0
by Claus Reinke 23 Dec '08

23 Dec '08

>>> Well, sort of. Ok, we can parse that. Let's assume a variable x holds >>> the output of :show modules as a String. We call lines on it, then map >>> words on it, do a !! 2 on it, and we get ["Util.hs,", "Recorder.hs,", >>> "Game.hs,", "Monadius.hs,", "Demo.hs,"]. Chuck in a map (filter (\= >>> ',')), and we get a good list. We can turn the list into a string >>> suitable for hlint with a quick unwords. >>> >>> So our long sought after command becomes ':def hoogle (\_ -> return $ >>> ":! " ++ (unwords $ map (filter (\= ',')) $ (map words $ lines x) !! >>> 2))'. But wait, how do we get 'x'? How do we call :show modules inside >>> a Haskell expression? >> The first url includes a link to a .ghci mini-tutorial (section 4) that, >> among other things, implements >> :redir <var> <cmd> -- execute <cmd>, redirecting stdout to <var> > Perhaps my cold has fogged my head too much, but I'm not sure how > :redir would help. I could do :redir var "hlint .", but that's as > unsatisfactory as :! "hlint ." You were asking about getting the output of ':show modules' into a variable 'x', so that you can process it further. ':redir x :show modules' should do just that. There is another example command for implementing ':edit' this way (by now a native ghci command). Claus

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