go - 困惑 : implement multiple interfaces in Go

Question

在下面的代码中，类型ErrNegativeSqrt 实现了Stringer 和error 接口(interface)。因为在 Sqrt 方法中返回类型是 fmt.Stringer，所以我认为执行结果是:

0 nil

0 Impl Stringer type

但实际结果是下面的，为什么？

0 nil

0 Impl error type

package main

import (
    "fmt"
)

type ErrNegativeSqrt float64

func Sqrt(x ErrNegativeSqrt) (float64, fmt.Stringer) {
    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    }
    return 0, nil
}

func (e ErrNegativeSqrt) String() string {
    return "Impl Stringer type"
}

func (e ErrNegativeSqrt) Error() string {
    return "Impl error type"
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

Answer 1

documentation for Package fmt指出:

... special formatting considerations apply for operands that implement certain interfaces. In order of application:

...

3. If the %v verb is used with the # flag (%#v) and the operand implements the GoStringer interface, that will be invoked.

If the format (which is implicitly %v for Println etc.) is valid for a string (%s %q %v %x %X), the following two rules apply:

4. If an operand implements the error interface, the Error method will be invoked ...

5. If an operand implements method String() string, that method will be invoked ...

由于您使用的是 fmt.Println，规则 4 和 5 开始发挥作用，它们更喜欢调用 Error() 而不是 String()