[Haskell-cafe] ghc overlapping instances

Steffen Mazanek haskell at steffen-mazanek.de
Tue Dec 4 09:36:20 EST 2007


I want to quickcheck a property on a datatype representing
programs (=[Stmt]) and need to define a specific instance

instance Arbitrary [Stmt]

(mainly to restrict the size of the list).

In quickcheck an instance Arbitrary of lists is already defined.
Which parameters do I have to give ghc such that it accepts
such an instance? In hugs -98 +o is enough. I have
tried -XOverlappingInstances, -XFlexibleInstances and also
-XIncoherentInstances, however I still got an overlapping
instances error for this declaration.

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