在Golang中向结构添加方法时，为什么必须声明变量名？

Question

Let's say I have a struct

type Rectangle struct {
    length, width int
}

and I want to add a method to it:

func (r Rectangle) Area() int {
    return r.length * r.width
}

Why must I give it a variable name here r?

Answer 1

Because there is no implicit identifier denoting the actual receiver value (like this in Java), and if you want to refer to the fields or methods of the receiver value (Rectangle value), you need an identifier that you can use.

Note that the spec does not require you to name the receiver value, e.g. the following using the blank identifier is a valid syntax:

func (_ Rectangle) Foo() string {
    return "foo"
}

Or even this: omitting the receiver name (the parameter name):

func (Rectangle) Foo() string {
    return "foo"
}

Relevant section from the spec: Method declarations:

MethodDecl   = "func" Receiver MethodName ( Function | Signature ) .
Receiver     = Parameters .

Where Parameters is:

Parameters     = "(" [ ParameterList [ "," ] ] ")" .
ParameterList  = ParameterDecl { "," ParameterDecl } .
ParameterDecl  = [ IdentifierList ] [ "..." ] Type .

As you can see in the last line, the IdentifierList is optional (but Type is required).