英文:
Assign return value to struct member with shorthand assignment/declaration
问题
为什么在给struct成员赋值时不能使用:=？
package main
import "fmt"
type Foo struct { Bar int64 }
func Baz() (int64, int64) { return 0, 0 }
func main() {
foo := Foo{}
var x int64
x, foo.Bar = Baz() // 正常
y, foo.Bar := Baz() // 错误
fmt.Printf("%#v\n", foo)
}
编译错误信息为：
non-name foo.Bar on left side of :=
英文:
Why can't I use := when a struct member is being assigned one of the return values?
package main
import "fmt"
type Foo struct { Bar int64 }
func Baz() (int64, int64) { return 0, 0 }
func main() {
foo := Foo{}
var x int64
x, foo.Bar = Baz() // ok
y, foo.Bar := Baz() // error
fmt.Printf("%#v\n", foo)
}
The compilation error is:
non-name foo.Bar on left side of :=
答案1
得分: 4
因为规范是这样规定的。不过，真的是这样的：
-
短变量声明只在标识符列表上定义：
ShortVarDecl = IdentifierList ":=" ExpressionList . -
IdentifierList = identifier { "," identifier } .
因此，在使用短变量声明语法时，不允许分配选择器。
有关详细信息，请参阅此相关问题。在那里，您可以找到这种行为背后的原因：
> := 符号是常见情况的简写。它并不意味着涵盖每种可能的声明。我更愿意保持原样，但在其他人发表意见之前，我不会关闭此问题。
英文:
Because the spec says so. No, really:
-
Short variable declarations are only defined on identifier lists:
ShortVarDecl = IdentifierList ":=" ExpressionList . -
Identifiers lists do not include Selectors:
IdentifierList = identifier { "," identifier } .
Therefore, you are not allowed to assign a selector when using the short variable declaration syntax.
See this related issue for details. There you can find the reasoning behind this behaviour:
> The := notation is a shorthand for common cases. It's not meant to cover every possible declaration one may write. I'd prefer to leave as is, but won't close this until others have weighed in.
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