On Wed, Oct 8, 2008 at 2:59 PM, Tobias Bexelius
Or do I have too much faith in the -fallow-incoherent-instances flag now? :/
Overlapping instances are an "instance definition"-time feature; incoherent instances only become applicable at the call site for polymorphic functions in the presence of overlapping instances. As an example, consider the following overlapping instances: {-# LANGUAGE OverlappingInstances #-} class Foo a where foo :: a -> Bool instance Foo a where foo x = False instance Foo Integer where foo x = (x == 3) test1 :: String -> Bool test1 x = foo x -- uses "Foo a" instance test2 :: Integer -> Bool test2 x = foo x -- uses "Foo Integer" instance test3 :: a -> Bool test3 x = foo x -- incoherent! Consider (test3 (3 :: Integer)). This should definitely use the implementation for instance Foo Integer. But it can't, because test3 doesn't know what type its argument is! Incoherent instances still allows test3 to compile, but it may choose the "wrong" instance. In this case, test3 will use the "Foo a" instance no matter what type of argument you pass it. But the incoherency only arises at a call-site for the class method; if you take test3 out of this program, the instances are still overlapping but there is no compile problem (assuming -foverlapping-instances). If you are getting an error because of an instance definition, as opposed to a call site, incoherent instances won't help you. -- ryan