null而不是== [英] null instead of ==

问题描述

我刚刚开始学习Haskell的兴趣。我按照 learnyouahaskell.com 。

I have just started to learn Haskell out of interest. I follow learnyouahaskell.com.

我发现这里：

null 检查列表是否为空。如果
是，它返回 True ，否则
返回 False 。使用此函数
而不是 xs == [] （如果你有一个
列表 xs ）

null checks if a list is empty. If it is, it returns True, otherwise it returns False. Use this function instead of xs == [] (if you have a list called xs)

为什么？当两者产生相同的结果时，为什么我们应该使用 null 而不是 == ？

Why is that? Why should we use null instead of == when both produce the same result?

感谢。

推荐答案

将列表与 == 需要元素是可比的（表示为 Eq a ）。

Comparing the lists with == requires elements to be comparable (denoted as Eq a).

Prelude> :t (==[])
(==[]) :: (Eq a) => [a] -> Bool

例如， [sin] == [] 将不工作，因为你不能比较函数。它可能看起来很蠢，但是类型系统必须判断一个表达式的类型而不用看它的值。

For example, [sin] == [] won't work, since you can't compare functions. It might seem stupid, but the type system must judge type of an expression without looking at its value.

一个替代的检查是 length xs = = 0 ，这不需要相等，但如果你的列表是无限的，不会停止（try length [1 ..] == 0 ）。这是为什么有一个专门的函数。

An alternate check would be length xs == 0, this doesn't require equality but won't stop if your list is infinite (try length [1..] == 0). That's why there's a dedicated function.

null [] = True
null _ = False

Prelude> :t null
null :: [a] -> Bool     -- Notice lack of (Eq a).

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