Question

我正在尝试编写一个可变函数组合函数。这基本上是(.)，除了第二个参数函数是可变参数。这应该允许表达式：

map even . zipWith (+)

或只是

map even . zipWith

目前，如果我添加IncoherentInstances并且需要第一个参数函数的非多态实例，那么我已经达到的效果。

{-# 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

有什么想法吗？它甚至可能吗？

Answer 1

可以使用多态函数对其进行打字：

{-# 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

但我怀疑你可以避免IncoherentInstances

Variadic撰写功能？

1 个答案: