组成两个比较函数? [英] composing two comparison functions?

问题描述

我想先按一个属性排序，然后再按另一个属性排序(如果第一个属性相同).

I'd like to sort by one property and then by another (if the first property is the same.)

Haskell组合两个比较函数(即与sortBy一起使用的函数)的惯用方式是什么?

What's the idiomatic way in Haskell of composing two comparison functions, i.e. a function used with sortBy?

给予

f :: Ord a => a -> a -> Ordering
g :: Ord a => a -> a -> Ordering

组成f和g将产生:

h x y = case v of
          EQ -> g x y
          otherwise -> v
        where v = f x y

推荐答案

vitus指出了Monoid对于Ordering的非常酷的实例.如果将其与实例instance Monoid b => Monoid (a -> b)结合使用，则表明您的合成功能只是(准备好):

vitus points out the very cool instance of Monoid for Ordering. If you combine it with the instance instance Monoid b => Monoid (a -> b) it turns out your composition function is just (get ready):

mappend

签出:

Prelude Data.Monoid> let f a b = EQ
Prelude Data.Monoid> let g a b = LT
Prelude Data.Monoid> :t f `mappend` g
f `mappend` g :: t -> t1 -> Ordering
Prelude Data.Monoid> (f `mappend` g) undefined undefined 
LT
Prelude Data.Monoid> let f a b = GT
Prelude Data.Monoid> (f `mappend` g) undefined undefined 
GT

+1获得强大而简单的抽象

+1 for powerful and simple abstractions

这篇关于组成两个比较函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！