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
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
