在普通函数上返回像 Golang 中的“ok"这样的地图 [英] Return map like 'ok' in Golang on normal functions

问题描述

在 Go 中，如下工作(注意使用 map 有一个返回，另一个有两个返回)

In Go, the following works (note one use of the map has one return, the other has two returns)

package main

import "fmt"

var someMap = map[string]string { "some key": "hello" }

func main() {
    if value, ok := someMap["some key"]; ok {
        fmt.Println(value)
    }

    value := someMap["some key"]
    fmt.Println(value)
}

然而，我不知道如何用我自己的函数来做同样的事情.是否有可能通过像 map 这样的可选返回来实现类似的行为?

However, I have no idea how to do this same thing with my own function. Is it possible to have similar behavior with an optional return like map?

例如:

package main

import "fmt"

func Hello() (string, bool) {
    return "hello", true
}

func main() {
    if value, ok := Hello(); ok {
        fmt.Println(value)
    }

    value := Hello()
    fmt.Println(value)
}

无法编译(由于错误multiple-value Hello() in single-value context)......有没有办法使这个语法适用于函数Hello()?

Wont compile (due to the error multiple-value Hello() in single-value context) ... is there a way to make this syntax work for the function Hello()?

推荐答案

map 是不同的，因为它是一个内置的 type 而不是一个函数.访问 map 元素的 2 种形式由 Go 语言规范指定: 索引表达式并由编译器支持.

map is different because it is a built-in type and not a function. The 2 forms of accessing an element of a map is specified by the Go Language Specification: Index Expressions and backed by the compiler.

对于函数，您无法做到这一点.如果一个函数有 2 个返回值，你必须期望"两者都有或根本没有.

With functions you can't do this. If a function has 2 return values, you have to "expect" both of them or none at all.

但是，您可以将任何返回值分配给空白标识符:

However you are allowed to assign any of the return values to the Blank identifier:

s, b := Hello()    // Storing both of the return values

s2, _ := Hello()   // Storing only the first

_, b3 := Hello()   // Storing only the second

您也可以选择不存储任何返回值:

You can also choose not to store any of the return values:

Hello()            // Just executing it, but storing none of the return values

注意:您也可以将两个返回值都分配给空白标识符，尽管它没有任何用处(除了验证它恰好有 2 个返回值):

Note: you could also assign both of the return values to the blank identifier, although it has no use (other than validating that it has exactly 2 return values):

_, _ = Hello()     // Storing none of the return values; note the = instead of :=

您也可以在 Go Playground 上尝试这些.

You can also try these on the Go Playground.

辅助函数

如果您多次使用它并且不想使用空白标识符，请创建一个丢弃第二个返回值的辅助函数:

If you use it many times and you don't want to use the blank identifier, create a helper function which discards the 2nd return value:

func Hello2() string {
    s, _ := Hello()
    return s
}

现在你可以:

value := Hello2()
fmt.Println(value)

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