error struct proposal about slim HOT 3 CLOSED

package main

import (
    "errors"
    "fmt"
)

type MyError struct {
    error
    ExtraInfo string
}

func (err *MyError) Error() string {
    return fmt.Sprintf("msg: %s, extra: %s", err.error, err.ExtraInfo)
}

//func ToError(err MyError) error {
//}

func New(err error, extra string) *MyError {
    stdErr := err
    switch err.(type) {
    case *MyError:
        fmt.Println("arg err is instance of MyError")
        stdErr = err.(*MyError).error
    default:
        fmt.Println("arg err is instance of standard error")
    }

    return &MyError{
        error:     stdErr,
        ExtraInfo: extra,
    }
}

func main() {
    err := errors.New("this is standard error")

    myErr := New(err, "extra error info")

    // fmt.Printf("lsl-debug: type: %T\n", err)
    // fmt.Printf("lsl-debug: type: %T\n", myErr)

    // fmt.Println(err)
    // fmt.Println(myErr)

    // MyError as standard error
    myErr2 := New(myErr, "extra error again")
    fmt.Println(myErr2)
}

 ☛  go run test.go
arg err is instance of standard error
arg err is instance of MyError
msg: this is standard error, extra: extra error again