Lua，if 语句惯用法未能返回正确的 bool 值

Question

local a = (true==true) and false or nil -- returns nil
local a = (true==true) and true or nil -- returns true
local a = (true==true) and not false or nil -- returns true
local a = (true==true) and not true or nil -- returns nil

当值为 true 时返回正确的 boolean，但当 false 时返回失败。为什么？

Answer 1

bool 习惯用法通过使用快捷求值来工作(仅在必要时求值第二个操作数)。

如果您以明确的优先级重写表达式，您就会明白为什么会得到nil:

(true and false) or nil     =>  false or nil  => nil
(true and true) or nil      =>  true or nil   => true
(true and not false) or nil =>  true or nil   => true
(true and not true) or nil  =>  false or nil  => nil

Logical Operators Programming in Lua 部分解释这个成语:

Another useful idiom is (a and b) or c (or simply a and b or c, because and has a higher precedence than or), which is equivalent to the C expression

a ? b : c

provided that b is not false. For instance, we can select the maximum of two numbers x and y with a statement like

max = (x > y) and x or y

为什么b不能为false？因为计算总是返回 false。

1 > 0 and false  --> false
1 < 0 and false  --> false