Bertram Felgenhauer:
How does
class F a where data B a :: * data E a :: * wrap :: B a -> E a unwrap :: E a -> B a
sound? 'B a' would represent the 'b' in your previous attempt,
class F a b | a -> b where ...
I'm with Simon in thinking that this code is suspicious. For any given call to "wrap" or "unwrap", how is the compiler supposed to determine which instance to use, given that "a" cannot be uniquely determined from the type of the function? The same question also applies to Matthew's original formulation using functional dependencies:
class G a b | a -> b where data E a :: * wrap :: b -> E a unwrap :: E a -> b
Simon's reformulation doesn't have this problem:
class G a where data E a :: * wrap :: a -> E a unwrap :: E a -> a