golangのエラー型

main.go

package main

import (
	"fmt"
)

type MyErr struct {}

func (e MyErr) Error() string {
	return fmt.Sprint("error")
}

func main() {
	{
		var err interface{}
		if err == nil {
			fmt.Println("interface\t is nil")
		}
	}
	{
		var err error
		if err == nil {
			fmt.Println("error type\t is nil")
		}
	}
	{
		var err *MyErr
		if err == nil {
			fmt.Println("MyErr pointer\t is nil")
		}
		
	}
	{
		var err MyErr
		_ = err
		//cannot convert nil to type MyErr
		//if err == nil {
		//	fmt.Println("MyErr is nil")
		//}
		
	}
	{
		var my *MyErr
		var err error
		err = my
		if err != nil {
			fmt.Println("error\t is not nil")
			switch err := err.(type) {
			case *MyErr:
				_ = err
				fmt.Println("  *MyErr")

			}

		}

	}
	{
		var my MyErr
		var err error
		err = my
		if err != nil {
			fmt.Println("error\t is not nil")
			switch err := err.(type) {
			case MyErr:
				_ = err
				fmt.Println("  MyErr")
			}

		}

	}
	
}

interface	 is nil
error type	 is nil
MyErr pointer	 is nil
error	 is not nil
  *MyErr
error	 is not nil
  MyErr

error interfaceに何か代入した時点で nilではない。

 var my *MyErr
 fmt.Println(my == nil) //true
 var err error
 err = my
 fmt.Println(err == nil) //false