mniip, you might want to take a look at a paper Richard Eisenberg and I wrote together in 2014: Eisenberg, R. A., and Stolarek, J.: Promoting Functions to Type Families in Haskell, ACM SIGPLAN Notices 49(12):95-106 (originally published at: Haskell Symposium 2014, Göteborg, Sweden) Janek Dnia środa, 26 kwietnia 2017, mniip napisał:
Oops, got cut off :S
We could then write stuff like
type family Foldr (f :: k ~> l ~> l) (z :: l) (xs :: [k]) :: l where Foldr f z '[] = z Foldr f z (x ': xs) = f x (Foldr f z xs)
type family Flip (f :: k ~> l ~> m) (x :: l) (y :: k) :: m where Flip f x y = f y x
-- Lift a type constructor into a type family type family Apply (con :: k -> l) (x :: k) :: l where Apply con x = con x
type family (.) (f :: l ~> m) (g :: k ~> l) (x :: k) :: l where (.) f g x = f (g x)
stuff :: Foldr (Flip (Apply . Apply (,))) () '[Int, String, Char] stuff = ((((), 'x'), "moo"), 3)
This idea is very fresh, and I certainly haven't explored all the aspects, so I would welcome constructive (and intuitionistic) criticism regarding both usefulness and mathematical soundness of this.
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