Re: default class method types... (unification)

6 Sep 2002


      fre 2002-09-06 klockan 15.01 skrev MR K P SCHUPKE:
...
If I have a class:
class A m n where
    f :: m a -> n a
I can declare instances
instance A Maybe Maybe where
    f = id
instance A [] [] where
    f = id
but if I try and put a default method:
    f = id
I get a unification error (less polymorphic than expected).This seems
a little odd - is there a work around for this?
Because that default method only works when m=n, which it's not in the
general case.

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Martin Norbäck          d95mback@dtek.chalmers.se              
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