(I'm using GHC 4.04 patchlevel 1...)
Suppose I have a type involving higher kinds such as
data H f a = H (f a)
and now suppose I want to define equality on this type.
I *cannot* say
instance Eq (f a) => Eq (H f a) where
H x == H y = x == y
because I get an error message
Illegal constaint `Eq (f a)' in instance context
(Instance contexts must constrain only type variables)
(BTW, typo in "constaint"...)
However, I *can* say
data H f a = H (f a) deriving (Eq)
which presumably generates code internally that is *identical*
to the instance declaration I tried to write.
Is this inconsistency between what can be written manually
and what can be derived deliberate?
-Chris
