Why does method signature have to perfectly match interface method

Question

I began to learn Go and have trouble understanding the following:

package main

import "fmt"

type I interface {
    foo(interface {})
}

type S struct {}

func (s S) foo(i int) {
    fmt.Println(i)
}

func main() {
    var i I = S{}
    i.foo(2)
}

This fails with:

cannot use S literal (type S) as type I in assignment:
        S does not implement I (wrong type for foo method)
                have foo(int)
                want foo(interface {})

I don't understand why Go doesn't accept the foo(int) signature given the fact that int implements interface {}. Can anyone help with an explanation?

Answer 1

I think your understanding of interface isn't sound. Interface{} itself is a type. It consists of two things : underlying type and underlying value.

Golang doesn't have overloading. Golang type system matches by name and requires consistency in the types

So, when you are defining a function taking a interface type as a parameter:

foo(interface {})

This is a different function from a function taking int type:

foo(int)

So you should change the following line to

func (s S) foo(i interface{}) {
    fmt.Println(i)
}

Or better yet to this:

type I interface {
    foo()
}

type S struct {
    I int
}

func (s S) foo() {
    fmt.Println(s.I)
}

func main() {
    var i I = S{2}
    i.foo()
}