just try to compile it with ghc -fallow-overlapping-instances -Wall --make Main.hs
or inseart sth. like this at the first line: {-# OPTIONS -fglasgow-exts -fffi -fallow-undecidable-instances -fallow-overlapping-instances #-} - marc Am Montag, 29. August 2005 05:25 schrieb Juan Carlos Arevalo Baeza: > David Menendez wrote: > > >Juan Carlos Arevalo Baeza writes: > > > > > > > >The way Haskell type classes > >work, the overlap is determined without looking at the context, so "Show > >a" will overlap with every possible instance for Show, including Show > >Int, which is predefined. > > > > > > Ah. :-P Bummer. > > >I'm not sure what the official justification for that is, but reason is > >to avoid situations like this: > > > > class A t where a :: t > > class B t where b :: t > > class C t where c :: t > > > > instance A t => C t where c = a > > instance B t => C t where c = b > > > > instance A Char where a = 'a' > > instance B Char where b = 'b' > > > >What should c :: Char evaluate to? > > > > > > Right. Ambiguity. But as long as there's no ambiguity, there's no > reason to be this restrictive. Oh, well. > > JCAB > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users