该方法是否应该与接口的签名显式地收缩？

Question

I am new with the golang, I am not quite understand why the below demo program could be executed successfully,

type fake interface {
    getAge(valueInt int,  valStr string) (age int, name string, err error)
}

type Foo struct {
    name string
}

func (b *Foo) getAge(valueInt int, valStr string) (age int, retErr error) {
    age = valueInt
    return age, nil
}
func main() {
    inst := &Foo{name:"foo"}
    value, _ := inst.getAge(2, "foo")
    fmt.Println(value)
}

The interface wants to return three value, but the method getAge only return two, but it still works. How to understand this behavior in golang?

Thanks!

Answer 1

Foo doesn't implement fake. This is apparent if you extend your code sample a bit (try it on the Go playground):

package main

import "fmt"

type fake interface {
    getAge(valueInt int, valStr string) (age int, name string, err error)
}

type Foo struct {
    name string
}

func (b *Foo) getAge(valueInt int, valStr string) (age int, retErr error) {
    age = valueInt
    return age, nil
}


func bar(f fake) {
  _, name, _ := f.getAge(10, "")
}

func main() {
    inst := &Foo{name: "foo"}
    value, _ := inst.getAge(2, "foo")
    fmt.Println(value)

    bar(inst)
}

This produces a compile error that's pretty descriptive:

prog.go:28:5: cannot use inst (type *Foo) as type fake in argument to bar:
    *Foo does not implement fake (wrong type for getAge method)
        have getAge(int, string) (int, error)
        want getAge(int, string) (int, string, error)