On Sun, 22 Dec 2013, Dr. ERDI Gergo wrote:
If it's only A and B, perhaps abominations like these could be considered:
-- implicit foralls pattern Show t => P t :: (Num t, Eq b) => b -> T t
-- explicit foralls pattern forall t. Show t => P t :: forall b. (Num t, Eq b) => b -> T t
I'm not 100% sure what that 't' in 'P t' is supposed to be in your example. 'P' is not like a type constructor at all; it's a lot more like a data constructor.
Thinking further about it, I think this could work, using a syntax similar to data constructor definitions instead of sticking to the function type syntax: pattern (Num a, Eq b) => P a b :: (Show a) => T a or with explicit foralls (using the fact that we can deduce which tyvars are universial vs existential simply by seeing if they occur in 'T a'): pattern forall a b. (Num a, Eq b) => P a b :: (Show a) => T a my only concern with this one is that the direction of the first double arrow doesn't "feel right". Other examples with this syntax: -- Number literal patterns pattern Z :: (Num a, Eq a) => a pattern Z = 0 -- Monomorphic patterns pattern TrueAnd Bool :: [Bool] pattern TrueAnd b = [True, b] -- Infix notation pattern a :< Seq a :: Seq a pattern x :< xs <- (Seq.viewl -> x Seq.:< xs) I'm liking this so far. Bye, Gergo -- .--= ULLA! =-----------------. \ http://gergo.erdi.hu \ `---= gergo@erdi.hu =-------' RICE: Race Inspired Cosmetic Enhancement