Matthew Brecknell wrote:
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.
It wasn't my code Simon was replying to.
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?
As far as my limited understanding goes this should work, because we are using an associated *data* type here. This means we can't have instances saying 'type B a = Int', we have to use either a newtype or a data declaration for that. As a side effect, the compiler can determine 'a' from 'B a'. This wouldn't be possible for associated type synonyms, but those aren't completely implemented yet anyway (again, as far as I know). Bertram