可变组合函数? | 可变组合函数

本文介绍了可变组合函数?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

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

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 (+)

或者只是

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，我所达到的效果.

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

有什么想法吗?甚至有可能吗?

Any ideas? Is it even possible?

推荐答案

可以通过 type-hack 它来处理多态函数:

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

但我怀疑你能否避免 IncoherentInstances

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