Simon Peyton-Jones
No I didn't intend to put more in the header, perhaps less. I've added more clarification.
Simon
Thanks Simon, I agree with keeping it terse; I agree with your "yuk" rating for `of'. At risk of bikeshedding over surafce syntax (for a feture that's still only a gleam in the eye) ... I think we're going to see two idioms for overlapping instances: Idiom 1: total instance (this would apply to all the HList examples). We only need one instance group for the whole; then it's the type family decl that seems superfluous. Perhaps we could allow: type family Equal a b :: Bool where Equal a a = True Equal a b = False type family HasMember a (b :: '[]) :: Bool where HasMember a '[] = False -- (not overlapping) HasMember a ( a ': bs ) = True HasMember a ( b ': bs ) = HasMember a bs Idiom 2: an instance group discriminated by the outermost type constructor, or by one of the arguments (this might apply for Monad Transformers). Then although the instance header is superfluous, it might be useful documentation: module SomeLibrary where type family F a b :: ... module MyModule where data MyType = ... type instance F MyType b where -- total function for a ~ MyType F MyType Int = ... F MyType (Int, b) = ... F MyType b = ... AntC