Haskell(:)和(++)的区别 [英] Haskell (:) and (++) differences

问题描述

对于这样的问题，我感到抱歉.我不太清楚Haskell中:和++运算符的区别.

I'm sorry for a question like this. I'm not too sure about the difference of the : and ++ operator in Haskell.

x:y:[] = [x,y]  

还

[x] ++ [y] = [x,y]

至于反向功能对我来说是一个问题，

as for the reverse function which arose this question for me,

reverse ::[a]->[a]
reverse [] = []
reverse (x:xs) = reverse(xs)++[x]

为什么以下各项不起作用?

Why doesn't the following work?

reversex ::[Int]->[Int]
reversex [] = []
reversex (x:xs) = reversex(xs):x:[]

出现类型错误.

推荐答案

:运算符称为"cons"运算符，用于将head元素添加到列表的前面.因此，[]是一个列表，而x:[]在x之前是空白列表，从而形成了列表[x].如果然后使用y:[x]弊端，则会得到与y:x:[]相同的列表[y, x].

The : operator is known as the "cons" operator and is used to prepend a head element to a list. So [] is a list and x:[] is prepending x to the empty list making a the list [x]. If you then cons y:[x] you end up with the list [y, x] which is the same as y:x:[].

++运算符是列表串联运算符，它将两个列表作为操作数并将它们组合"为一个列表.因此，如果您有列表[x]和列表[y]，则可以将它们连接起来:[x]++[y]以获得[x, y].

The ++ operator is the list concatenation operator which takes two lists as operands and "combine" them into a single list. So if you have the list [x] and the list [y] then you can concatenate them like this: [x]++[y] to get [x, y].

请注意，:包含一个元素和一个列表，而++包含两个列表.

Notice that : takes an element and a list while ++ takes two lists.

对于无效的代码.

reversex ::[Int]->[Int]
reversex [] = []
reversex (x:xs) = reversex(xs):x:[]

reverse函数将得出一个列表.由于:运算符不将列表作为其第一个参数，因此reverse(xs):x无效.但是reverse(xs)++[x]是有效的.

The reverse function evaluates to a list. Since the : operator does not take a list as its first argument then reverse(xs):x is invalid. But reverse(xs)++[x] is valid.

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