This is correct behavior. The former has a CUSK, as all open type families have CUSKs with un-annotated kinds defaulting to Type. The latter does not have a CUSK, because the result kind is unknown. You therefore cannot specialize the k variable in the definition of the latter. There is a ticket (#10141) about improving the error message here to educate the user about CUSKs, but there's no progress on it. Richard

On Jan 19, 2017, at 5:02 AM, Simon Peyton Jones wrote:

Richard

This works

type family F (a :: k) type instance F Maybe = Char But this does not. Surely it should?

type family F (a :: k) where -- = r | r -> a where F Maybe = Char The latter is rejected with

Foo.hs:6:5: error: * Expecting one more argument to `Maybe' Expected kind `k', but `Maybe' has kind `* -> *' * In the first argument of `F', namely `Maybe' In the type family declaration for `F' If you agree I’ll open a ticket.

Simon