December 2008 - Haskell-Cafe

Comparing on multiple criteria
by Martijn van Steenbergen 24 Dec '08

24 Dec '08

Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) To reverse the order of a criterion or a set of criteria, flip can be used: compareTuple' = mconcat [comparing fst, flip $ comparing snd] I think it would be really neat if these two use cases were also described in comparing's documentation. Who is the right person to ask this of? Thanks in advance, Martijn.

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The Applicative Functor Monad
by Jeremy Shaw 24 Dec '08

24 Dec '08

Hello, I want to make a Monad which is almost exactly like the Writer monad, except instead of using mappend to glue Monoids together, it uses <*> to glue applicative functors together. Here is the code: import Control.Applicative import Data.Monoid -- * Sample Implementation of the Writer Monad data Writer w a = Writer { runWriter :: (w, a) } instance (Monoid w) => Monad (Writer w) where return a = Writer (mempty, a) (>>=) = bindWriter bindWriter :: (Monoid w) => Writer w a -> (a -> Writer w b) -> Writer w b bindWriter (Writer (w,a)) f = let (Writer (w', b)) = f a in Writer (w `mappend` w', b) -- * Sample Implementation of the Applicative Functor Monad data AF af a = AF { runAF :: (af, a) } bindAF :: (Applicative f) => AF (f (a -> b)) x -> (x -> AF (f a) y) -> AF (f b) y bindAF (AF (f, x)) g = let (AF (a, y)) = g x in AF (f <*> a, y) -- instance (Applicative f) => Monad (AF (f ... As you can see, the similarity is striking. Alas, AF and bindAF do not quite have the right type signatures to be used for an instance of the Monad class. Is there some clever work-around I missing? (Aside from, -fno-implicit-prelude). Thanks! - jeremy

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shortest paths, prioritiy queues
by Johannes Waldmann 24 Dec '08

24 Dec '08

Hello. I am looking for a single-source shortest-path implementation (Dijkstra's algorithm, cf. Chapter 25 in Cormen,Leiserson,Rivest). An efficient implementation would use Binomial or Fibonacci heaps (Chap.s 20 + 21). The problem seems to be "(fib-)heap-decrease-key" which assumes that we have, with the key, a pointer to its position in the heap, because that is where there will be a destructive update. How is this dealt with in Okasaki's book + library? - Thanks, J.W.

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Renaming a package on Hackage
by Jeff Heard 24 Dec '08

24 Dec '08

How does one go about renaming a package on Hackage? I finally figured out a name for Thingie, and I'd like to change the package name to reflect that, so that future versions look to the new package and not the old one.

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Time for a new logo?
by Don Stewart 24 Dec '08

24 Dec '08

I noticed a new haskell logo idea on a tshirt today, http://image.spreadshirt.net/image-server/image/configuration/13215127/prod… Simple, clean and *pure*. Instead of the "we got lots going on" of the current logo. Any graphic designers want to try some variations on this theme of purity? A new year, and a new mature logo... -- Don

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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 23 Dec '08

23 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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