界面参考显示了奇怪的输出

Question

I am trying to understand Go's interface concept and create following code:

package main

import "fmt"

type Failer interface {
    Error() string
}

type Succer interface {
    Success() string
}

type Result interface {
    Failer
    Succer
}

type Fail struct{}

func (*Fail) Error() string {
    return "Error"
}

type Succ struct{}

func (*Succ) Success() string {
    return "Success"
}

type Combi struct{}

func (*Combi) Error() string {
    return "Error"
}

func (*Combi) Success() string {
    return "Success"
}

func main() {

    var r Result
    var s Succer

    c := &Combi{}
    r = c
    s = c
    fmt.Println(r.Error())
    fmt.Println(s)
}

As output I've got

Error 
Error

Why? I've expect as output error and success, because s it is the interface of type Succer, there is no error return as string. And when I change the main function like this:

func main() {

    var r Result
    var s Succer

    c := &Combi{}
    r = c
    s = c

}

the compiler complain

# command-line-arguments
.\sample1.go:42: r declared and not used
.\sample1.go:43: s declared and not used

Why? I assign variables r and s a reference.

Answer 1

fmt.Println(s) prints "Error" because error is special-cased in the fmt package

        switch v := p.arg.(type) {
        case error:
            handled = true
            defer p.catchPanic(p.arg, verb)
            p.printArg(v.Error(), verb, depth)
            return

        case Stringer:
            handled = true
            defer p.catchPanic(p.arg, verb)
            p.printArg(v.String(), verb, depth)
            return
        }
    }

The fmt package first checks if an object is a Formatter, GoStringer, error, or Stringer , in that order, to obtain a value to print.

As for your last question, you have to use a variable, not just assign it. Printing them removes the error.