I've also been experiencing this a lot in class instances, such as:
class Foo f where foo :: a -> f a
data Bar f a = Foo f => Bar {bar :: f a}
instance Foo (Bar f) where foo a = Bar (foo a)
Is there any way to avoid repeating the Foo f constraint in the Bar f instance?
Dear Harry, et al, The problem here is that GADT-constructors simply "carry" the appropriate dictionaries. When you *produce* an element of a GADT where the constructor requires a dictionary, you must provide it there. In this case, for a variable f, you don't have a dictionary, so you could read the constraint and instance head as "if you give me a Foo-dictionary for f, then I will wrap it in a Bar." Another way of looking at it is that the type "Bar f" only 'exists' for 'f's that are 'Foo's. If you don't know whether a particular 'f' is a 'Foo', you don't know whether Bar f exists. In short, I think that there is no such way. Regards, Philip