{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleInstances #-}

– uncurryG converts a function of the form a -> b -> c -> d to the uniform
– representation (a,(b,(c,()))) -> d
– curryG converts the uniform representation (a,(b,(c,()))) -> d to a function
– of the form a -> b -> c -> d

class Fn a c where
type Fnq a c :: *
uncurryG :: Fnq a c -> a -> c
curryG :: (a -> c) -> Fnq a c

instance Fn () c where
type Fnq () c = c
uncurryG f () = f
curryG f = f ()

instance (Fn b c) => Fn (a,b) c where
type Fnq (a,b) c = a -> Fnq b c
uncurryG f (a,b) = uncurryG (f a) b
curryG f = a -> curryG (b -> f (a,b))