Hello,
I was thinking that it could maybe be useful to allow duplicate instance
declarations, if the class has no methods. From my naive point of view, i
can not see problems, scince there are no conflicting methods to choose
from. But the following program fails to be compiled by ghc-6.6:
> {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
> class GT a b
> class Succ a b
> data Zero
> data One
> data Two
> data Three
> instance Succ One Zero
> instance Succ Two One
> instance Succ Three Two
> instance Succ a b => GT a b
> instance (Succ a b, GT b c) => GT a c
The former two lines lead to:
test5.lhs:22:1:
Duplicate instance declarations:
instance (Succ a b) => GT a b -- Defined at test5.lhs:22:1
instance (Succ a b, GT b c) => GT a c -- Defined at test5.lhs:23:1
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