On Tue, Dec 04, 2007 at 03:36:20PM +0100, Steffen Mazanek wrote:
Hello,
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.
You shouldn't use lists if you need to have special instance behavior - lists are for perfectly ordinary sequences of things. If a program is just a bunch of unrelated statements, then use [], otherwise use a custom (new)type. Stefan