Variadic compose function?

问题

I'm trying to write a variadic function composition function. Which is basically the (.) except that the second argument function is variadic. This should allow expressions like:

map even . zipWith (+)

or just

map even . zipWith

Currently what I've reached works if I add IncoherentInstances and requires a non-polymorphic instance for the first argument function.

{-# LANGUAGE FlexibleInstances, OverlappingInstances, MultiParamTypeClasses, 
FunctionalDependencies, UndecidableInstances, KindSignatures #-}

class Comp a b c d | c -> d where
    comp :: (a -> b) -> c -> d

instance Comp a b (a :: *) (b :: *) where
    comp f g = f g

instance Comp c d b e => Comp c d (a -> b) (a -> e) where
    comp f g = comp f . g

Any ideas? Is it even possible?

回答1:

It is possible to type-hack it into working with polymorphic functions:

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses,
  IncoherentInstances, UndecidableInstances,
  FunctionalDependencies, TypeFamilies,
  NoMonomorphismRestriction #-}


class Comp a b c | a b -> c where
    (...) :: a -> b -> c

instance (a ~ c, r ~ b) => Comp (a -> b) c r where
    f ... g = f g

instance (Comp (a -> b) d r1, r ~ (c -> r1)) => Comp (a -> b) (c -> d) r where
    f ... g = \c -> f ... g c

t1 = map even ... zipWith (+)
t2 = map even ... zipWith
t3 = (+1) ... foldr

But I doubt you can avoid IncoherentInstances

来源：https://stackoverflow.com/questions/9656797/variadic-compose-function