Brian Hulley writes:
Also, as a second point, could functional dependencies in type classes be written using a similar syntax eg instead of
class Insert t c a | c a -> t where insert :: t -> c a -> c a
we could write:
class Insert (h (c a)) c a where insert :: h (c a) -> c a -> c a
or class Insert t@(h (c a)) c a where -- re-using as-pattern syntax insert :: t -> c a -> c a
to avoid having to have a special syntax just for functional dependencies and/or to be able to write more complicated fundeps more succinctly?
You might be interested in associated types and associated type
synonyms[1]. These are alternatives to functional dependencies which are
less powerful but possibly easier to use. (Of course, no Haskell
compiler supports them.)
For example:
class Collection c where
type Elem c
insert :: Elem c -> c -> c
...
instance Collection [a] where
type Elem [a] = a
insert = (:)
...
Your example, in the syntax of associated type synonyms, would look
something like this:
class Insert c a where
type T c a
insert :: T c a -> c a -> c a
[1]: http://research.microsoft.com/Users/simonpj/papers/assoc-types/
--
David Menendez