go - 期待 nil 但得到一个 nil 值的接口(interface)作为返回，这应该是 nil

Question

运行时间 https://play.golang.org/p/sl12vfS9vP

package main

import "fmt"

func main() {
    err := run()
    if err != nil {
        fmt.Printf("%#v", err)
    }
}

func run() (err error) {
    return check()
}

func check() *Result {
    return nil
}

type Result struct {
    message string
}

func (result *Result) Error() string {
    return result.message
}

Answer 1

这是在 FAQ 上讨论的和 Go Traps网站:

An interface value is nil only if the inner value and type are both unset, (nil, nil). In particular, a nil interface will always hold a nil type. If we store a pointer of type *int inside an interface value, the inner type will be *int regardless of the value of the pointer: (*int, nil). Such an interface value will therefore be non-nil even when the pointer inside is nil.

(...)

To return a proper nil error to the caller, the function must return an explicit nil:

func returnsError() error {
    if bad() {
        return ErrBad
    }
    return nil
}