On Thu, 31 May 2007, Stefan Holdermans wrote: > Mingli, > > > > class Lattice e where > > > join :: e -> e -> e > > > meet :: e -> e -> e > > > > > > -- associative law > > > join x (join y z) = join (join x y) z > > > join (join x y) z = join x (join y z) > > If you are not to sell your soul to advanced and perhaps obscure type > hacking, you cannot express laws like this *in* Haskell. > > More concretely, one usually does not provide such laws as default > implementations of a class' methods. Instead, they are stated in, for > instance, comments and the documentation that goes with your library. > These then form an informal obligation for programmers that provide > instances of your class to let these instances obey the laws.
Like here: http://darcs.haskell.org/numericprelude/src/Algebra/Lattice.hs > If you provide an instance of the class you could use testing > framework, e.g., QuickCheck [1], to assert that the required > properties hold. I assume they can in some way also be used for GHC's optimizer. http://www.haskell.org/haskellwiki/Playing_by_the_rules _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
