Overriding errors.Is() not working with custom errors

Question

I am trying to create a custom error in Go such that I can create new instances of that error with a custom message, and then handle that specific type of error. However, when I try to do this, then my overridden errors.Is() is not being executed. Here is my code:

package main

import (
        "fmt"
        "errors"
)

type Error struct {
        Message string
        Code int
}

func (e *Error) Error() string {
        return e.Message
}

func (e Error) Is(target Error) bool {
        // This is never printed - this method never excutes for some reason
        fmt.Println("compared!")
        return e.Code == target.Code
}

var NotFoundError *Error = &Error{Code: 404, Message: "The page was not found"}

func NewError(errorType *Error, message string) error {
        rc := *errorType
        rc.Message = message

        return &rc
}

func FetchImage() error {
        return NewError(NotFoundError, "That image is gone")
}

func main() {
        err := FetchImage()

        // Returns false
        fmt.Println(errors.Is(err, NotFoundError))
}

In this case, the call to errors.Is() returns false. However, even though I have supplied my own Is() function, that function is not being called at all. That is, the string "compared!" is never printed.

Why isn't my Is() function working as desired?

Answer 1

type Error struct {
        Message string
        Code int
}

func (e *Error) Error() string {
        return e.Message
}

func (e *Error) Is(tgt error) bool {
        // This is never printed - this method never excutes for some reason
        fmt.Println("compared!")
        target, ok := tgt.(*Error)
        if !ok{
          return false
        }
        return e.Code == target.Code
}

您的 Error 结构未Is正确实现方法，参数不应error为Error。

请参阅实际操作：

https://play.golang.org/p/vRQndE9ZRuH