从科特林的lambda隐式返回 [英] Implicit return from lambda in Kotlin

问题描述

即使您省略了return语句，lambda的最后一行似乎总是返回该值.这样对吗?它在任何地方都有记录吗?

It seems that the last line of a lambda always returns that value even if you omit the return statement. Is this correct? Is it documented anywhere?

fun main(args: Array<String>) {
    val nums = arrayOf(1, 2, 3)
    val numsPlusOne = nums.map { it -> 
        val r = it + 1
        r
    }
    // numsPlusOne = [2, 3, 4]
}

推荐答案

是的，这是正确的，如果lambda的最后一条语句是表达式，则将其视为返回值.

Yes, this is correct, if the last statement of a lambda is an expression, it is considered its return value.

以下是参考引用的内容(感谢 @KirillRakhman ):

我们可以使用限定的return 语法.否则，将隐式返回最后一个表达式的值.因此，以下两个片段是等效的:

We can explicitly return a value from the lambda using the qualified return syntax. Otherwise, the value of the last expression is implictly returned. Therefore, the two following snippets are equivalent:

ints.filter {
    val shouldFilter = it > 0 
    shouldFilter
}

ints.filter {
    val shouldFilter = it > 0 
    return@filter shouldFilter
}

最后一个语句的语义对于 if也是如此没有三元运算符)，when和 try-catch 块，这些语句本身就是表达式:

The last statement semantics is also true for if (that's why there's no ternary operator), when and try-catch blocks, and these statements are expressions themselves:

val foo = if (bar) { 
    doSomething()
    baz 
} else { 
    doSomethingElse()
    qux 
}

^{另请参见: when和try- .}

^{See also: examples for when and try-catch.}

因此，lambda在这方面与语言结构保持一致.

So, lambdas are consistent with the language constructs in this respect.

如果您要在lambda中做出明确的return语句，请使用带有示例的另一个答案 ).相反，未标记的return与最接近的fun(忽略lambda)一起使用，因此只能在

If you want to make an explicit return statement in a lambda, use the return@label syntax (also, another answer with examples). Non-labeled return, on contrary, works with the nearest fun (ignoring lambdas) and thus can only occur in those lambdas which are inlined.

有一个语言建议为发射添加特殊语法 em>来自代码块的值，但被拒绝.

There was a language proposal to add special syntax for emitting a value from a code block, but it was rejected.

这篇关于从科特林的lambda隐式返回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！