为什么使用null函数而不是== []检查Haskell中的空列表? [英] Why use null function instead of == [] to check for empty list in Haskell?

问题描述

我正在阅读 学习Haskell的伟大成就的"Starting Out"一章！ .它说:

I am reading through the "Starting Out" chapter of Learn You a Haskell for Great Good!. It says:

null检查列表是否为空.如果是，则返回 True ，否则返回 False .使用此功能代替 xs == [] (如果您有一个名为 xs 的列表)

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)

我在ghci中尝试过

xs = []      -- and then,

xs == []
null xs

他们都是True.

我想知道有什么区别.

I wonder what's the difference.

我应该使用null函数而不是== []吗?

Should I use the null function instead of == [] and why?

推荐答案

您应使用null.在大多数情况下，这并不重要，但是无论如何这都是一个好习惯，因为有时候您可能想检查一系列不可比的事物是否为空.这是一个简短而清晰的示例，显示了这种差异:

You should use null. In most cases it doesn't matter, but it is a good habit to get into anyway, because occasionally you may want to check if a list of non-comparable things is empty. Here is a short, crisp example showing this difference:

> null [id]
False
> [id] == []
<interactive>:1:1: error:
    • No instance for (Eq (a0 -> a0)) arising from a use of ‘==’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: [id] == []
      In an equation for ‘it’: it = [id] == []

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