module Data.Function.Poly where
import Data.Constraint
import Data.HList
type family TypeListToArity (xs :: [*]) (r :: *) :: * where
TypeListToArity '[] r = r
TypeListToArity (x ': xs) r = x -> TypeListToArity xs r
type family ExpectArity (xs :: [*]) (f :: *) :: Constraint where
ExpectArity '[] f = ()
ExpectArity (x ': xs) (x -> remainder) = ExpectArity xs remainder
type family Head (xs :: [k]) :: k where
Head (x ': xs) = x
type family Tail (xs :: [k]) :: [k] where
Tail (x ': xs) = xs
class ExpectArity xs f => ConsumeArity (xs :: [*]) (f :: *) result | xs f -> result where
appN :: f -> HList xs -> result
instance ConsumeArity '[] r r where
appN r _ = r
instance ( ConsumeArity xs f r
, ExpectArity (x ': xs) (x -> f) )=> ConsumeArity (x ': xs) (x -> f) r where
appN f (HCons x xs) = appN (f x) xs