英文:
Return map like 'ok' in Golang on normal functions
问题
在Go语言中，以下代码是有效的（注意map的用法，一个返回一个值，另一个返回两个值）：
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，我不知道如何在自己的函数中实现类似的行为。是否可以像map一样具有可选的返回值？
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)
}
这段代码无法编译（由于错误multiple-value Hello() in single-value context）... 是否有办法使函数Hello()的语法能够工作？
英文:
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)
}
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?
For example:
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)
}
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()?
答案1
得分: 47
map不同于函数，因为它是一种内置的类型而不是函数。访问map元素的两种形式由Go语言规范：索引表达式指定，并由编译器支持。
对于函数，你不能这样做。如果一个函数有两个返回值，你必须“期望”它们都存在或者都不存在。
然而，你可以将任何一个返回值赋给空白标识符：
s, b := Hello() // 存储两个返回值
s2, _ := Hello() // 只存储第一个返回值
_, b3 := Hello() // 只存储第二个返回值
你也可以选择不存储任何返回值：
Hello() // 只执行它，不存储任何返回值
注意：你也可以将两个返回值都赋给空白标识符，尽管它没有用处（除了验证它确实有两个返回值）：
_, _ = Hello() // 不存储任何返回值；注意使用 = 而不是 :=
你也可以在Go Playground上尝试这些代码。
辅助函数
如果你多次使用它，并且不想使用空白标识符，可以创建一个辅助函数来丢弃第二个返回值：
func Hello2() string {
s, _ := Hello()
return s
}
现在你可以这样做：
value := Hello2()
fmt.Println(value)
Go 1.18泛型更新： Go 1.18添加了泛型支持，现在可以编写一个通用的First()函数来丢弃第二个（或任何后续的）返回值：
func First[T any](first T, _ ...any) T {
return first
}
这在github.com/icza/gog中可用，作为gog.First()（声明：我是作者）。
使用它：
value := First(Hello())
fmt.Println(value)
英文:
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.
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.
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
You can also choose not to store any of the return values:
Hello() // Just executing it, but storing none of the return values
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 :=
You can also try these on the Go Playground.
Helper function
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
}
And now you can do:
value := Hello2()
fmt.Println(value)
Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic First() function which discards the second (or any further) return values:
func First[T any](first T, _ ...any) T {
return first
}
This is available in github.com/icza/gog, as gog.First() (disclosure: I'm the author).
Using it:
value := First(Hello())
fmt.Println(value)
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