October 2008 - Haskell-Cafe

the Haskell notion of class vis-a-vis universal algebra?
by Galchin, Vasili 23 Oct '08

23 Oct '08

Hello, What is the relationship of a Haskell class to universal algebra? (a refresher ... http://en.wikipedia.org/wiki/Universal_algebra) ... it seems that all types that belong to a class are models? E.g. all "monads" have to satisfy the "monad laws" stated in the Monad class, i.e. equational axioms! Vasili

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Help converting Perl to Haskell
by Jason Dagit 23 Oct '08

23 Oct '08

Hello, I'm not very perl literate, but I want to convert a perl script to Haskell. This bit of perl is part of darcs' test suite. I was hoping to make it "more portable" by writing it in Haskell. By more portable I mean, works in windows without cygwin/mingw/msys and avoids the need for perl also. Depending on a Haskell compiler seems reasonable since darcs is written in Haskell :) Correct me if I'm wrong, but if I want to make this work on windows I can't use System.Posix, right? If so, what is the portable way to set environment variables? I see[1] that getEnv exists in System.Environment, but setEnv is in System.Posix.Env. I looked in System.Win32 and I didn't see anything that looked like it would manipulate environment variables. Will I have to use System.Process.runProcess on Win32? I'd like this to work on ghc 6.6 and System.Process looks new? Another thing I noticed is that System.Directory doesn't give me a way to exactly emulate this bit of perl: mkdir 'test_output', 0750; If I use System.Directory.setPermissions, then I think the closest I can approximate those permissions is: setPermissions "test_output" (Permissions {readable = True, writable = True, executable = True, searchable = True}) Which just makes sure it has the 7 for user, but doesn't seem to change the other permissions. Maybe that won't matter in practice, but I wanted to make it as close as possible to minimize unforeseen problems. I guess I could write a function that uses setPermissions on win32 and uses the appropriate thing from System.Posix when not on windows. Below is the full perl script if you want to see what I'm starting from. I think most of it should be easy to replace. \begin{perl} #!/usr/bin/perl -w # Run each of the shell tests, capturing the output, and reporting passed # or failed. use Cwd; use File::Find; # Place to put test output to avoid cluttering test/ mkdir 'test_output', 0750; # Override the users default .darcs settings $ENV{HOME} = cwd(); mkdir '.darcs'; system 'echo ALL --ignore-times >> .darcs/defaults'; # Used for finding darcs, but may not be defined by the shell $ENV{PWD} = cwd(); # Put the right darcs first in PATH my $darcspath="$ENV{HOME}/.."; if ($ENV{DARCS}) { # User has asked for a particular darcs... my $actualdarcs=`which $ENV{DARCS}`; my $darcspath=`dirname "$actualdarcs"`; } $ENV{PATH} = "$darcspath:$ENV{PATH}"; # Some environment variables can act as defaults that we don't want $ENV{EMAIL} = $ENV{DARCS_EMAIL} = 'tester'; # These two environment variables will turn off darcs' "Christmas mode". # It will make the tests run a tad faster, and make darcs' output # independent of the testing systems locale and environment. $ENV{DARCS_DONT_COLOR} = 1; $ENV{DARCS_DONT_ESCAPE_ANYTHING} = 1; my $OK = 1; my @Failures; my @Passes; `which bash`; if( $? != 0 ) { die "You need bash to run the shell tests!" } for my $test (@ARGV) { my $test_out = "test_output/$test.out"; printf "Running %-40s", "$test ..."; my $output = `bash $test 2>&1`; if( $? == 0 ) { push @Passes, $test; print " passed.\n"; } else { $OK = 0; push @Failures, $test; print " FAILED!\n"; print "Output from failed $test:\n$output"; } # give ourselves write permissions to every file in a tmp dir # (in case a script sets permissions and fails to clean up # after itself) my @tmpdirs = glob("tmp* temp*"); if (@tmpdirs > 0) { find (sub { chmod 0755, $_; }, @tmpdirs); } } my $CWD = cwd(); if ($CWD =~ /bugs/ && $#Passes >= 0) { print "Some tests passed:\n"; print "\t$_\n" for @Passes; } if ($OK) { print "All tests successful!\n"; } else { print "TESTS FAILED!\n"; print "\t$_\n" for @Failures; } # Exit with non-zero if anything failed. exit !$OK; \end{perl} Thanks! Jason [1] http://www.haskell.org/ghc/docs/latest/html/libraries/

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Cabal warning
by Henk-Jan van Tuyl 22 Oct '08

22 Oct '08

L.S., I keep getting the following warning for each "cabal install" command: Warning: Error parsing config file C:\Documents and Settings\[User]\Application Data\cabal\config: On line 1: GHC Warning: Using default configuration. The first line of this file is: compiler: GHC What can I do about this? -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Spine-lazy "multiqueue"
by Luke Palmer 22 Oct '08

22 Oct '08

Hi, I need a rather strange data structure, and I can't find any existing implementations or think of a way to implement it. It's a "multiqueue", basically a map of queues. The trick is that it should be lazy in its spine and still support efficient access. For example, the following should hold: dequeue 1 (foldr (enqueue 1) empty [42..]) = Just (42, ...) (where dequeue :: k -> QMap k v -> Maybe (v, QMap k v), similarly for enqueue) I also need a unionWith :: ([v] -> [v] -> [v]) -> QMap k v -> QMap k v -> QMap k v. I realize that these might be some pretty tough requirements to meet. Any pointers or original ideas? Thanks, Luke

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Template Haskell
by Andrew Coppin 22 Oct '08

22 Oct '08

If I'm understanding this correctly, Template Haskell is a way to auto-generate repetative Haskell source code. The thing that worries me is... if you need to write repetative Haskell source code, doesn't that mean that Haskell itself is broken in the first place?

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ghc error: requested module name differs from name found in interface file
by Larry Evans 22 Oct '08

22 Oct '08

With a file containing: > module Main where > > import Array > import Control.Functor.Fix I get: > make > ghc -i/root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 -c catamorphism.example.hs > > catamorphism.example.hs:19:0: > Bad interface file: /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2/Control/Functor/Fix.hi > Something is amiss; requested module main:Control.Functor.Fix differs from name found in the interface file category-extras-0.53.5:Control.Functor.Fix > make: *** [all] Error 1 I used cabal to install category-extras: > /home/evansl/download/haskell/libs # cabal install category-extras > Resolving dependencies... > Downloading category-extras-0.53.5... > Configuring category-extras-0.53.5... > Preprocessing library category-extras-0.53.5... ... > /usr/bin/ar: creating dist/build/libHScategory-extras-0.53.5.a > Installing library in /root/.cabal/lib/category-extras-0.53.5/ghc-6.8.2 > Registering category-extras-0.53.5... > Reading package info from "dist/installed-pkg-config" ... done. > Saving old package config file... done. > Writing new package config file... done. > /home/evansl/download/haskell/libs # What should I do to import the Functor.Fix as shown here: http://comonad.com/reader/2008/recursion-schemes/ TIA. -Larry

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Verifying a list of properties using QuickCheck
by Thomas van Noort 22 Oct '08

22 Oct '08

Hi, I would like to verify a list of properties using QuickCheck. Of course, I can test a single property using: quickCheck :: Testable prop => prop -> IO () Then, I can check a list of properties my mapping this function over a list: quickCheckL :: Testable prop => [prop] -> IO () quickCheckL = mapM_ quickCheck This gives me a result for each property: Prelude Test.QuickCheck> quickCheckL [1==1,2==2] OK, passed 100 tests. OK, passed 100 tests. However, I would like a single result for the complete list of properties instead of a result for each property. I realize that this restricts the properties to be of the same type, but that isn't a problem for my application. Did I miss a library function that provides me this functionality? Regards, Thomas

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ANNOUNCE: rewriting-0.1
by Thomas van Noort 22 Oct '08

22 Oct '08

Generic rewriting library for regular datatypes =============================================== This package provides rewriting functionality for regular datatypes. Regular datatypes are recursive datatypes such as lists, binary trees, etc. This library cannot be used with mutually recursive datatypes or with nested datatypes. This library has been described in the paper: A Lightweight Approach to Datatype-Generic Rewriting. Thomas van Noort, Alexey Rodriguez, Stefan Holdermans, Johan Jeuring, Bastiaan Heeren. ACM SIGPLAN Workshop on Generic Programming 2008. More information about this library can be found at: http://www.cs.uu.nl/wiki/GenericProgramming/Rewriting Features -------- * Generic rewriting machinery * Generic traversals (top-down, bottom-up, etc.) * Rewrite rules are defined concisely as values instead of functions, which allows for better observability * Rewrite rules are defined in the original domain and do not require a manual extension for metavariables Requirements ------------ * GHC 6.10.1 (tested with 6.10.0.20081007) * Cabal 1.2.1 (or higher) Download & Source ----------------- Use cabal-install: cabal install rewriting Get the package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rewriting Get the source: svn checkout https://svn.cs.uu.nl:12443/repos/dgp-haskell/rewriting Bugs & Support -------------- Report issues, request features, or just discuss the library with the authors, maintainers, and other interested persons at: http://www.haskell.org/mailman/listinfo/generics

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Fwd: enhancing type classes with properties
by Alberto G. Corona 22 Oct '08

22 Oct '08

I´m just thinking aloud, but, because incorporating deeper mathematics concepts has proven to be the best solution for better and more flexible programming languages with fewer errors, I wonder if raising the type classes incorporating axioms can solve additional problems. At first sight it does: class Abelian a where (+) :: a -> a -> a property ((+))= a+b == b+a this permits: 1- safer polimorphism: I can safely reuse the operator + if the type and the property is obeyed. The lack of ability to redefine operators is a problem for DSLs that must use wreid symbols combinations with unknow meanings. To use common operators with fixed properties is very good. the same aplies for method names. 2- the compiler can use the axions as rewrite rules. 3- in debugging mode, it is possible to verify the axiom for each value a generated during execution. Thus, a generator is not needed like in quickcheck. The logic to quickcheck can be incorporated in the debugging executable. 3 guaranties that 1 and 2 are safe. a type class can express a relation between types, but it is not possible to define relation between relations.

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Build without install, using cabal
by Mauricio 22 Oct '08

22 Oct '08

Hi, I would like to include a few source files as 'executable' sections in a .cabal package description. However, although I do want to use main=mainDefault features, I do not want those packages to be installed when I run 'Setup.hs install'. Is it possible to do that? I believe there's no flag that indicates install step, since (I guess) flags are checked only at configure time. Thanks for your help, Maurício

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