An error is anything that can describe itself as an error string. The idea is captured by the predefined, built-in interface type, error, with its single method, Error, returning a string:

type error interface {    Error() string}

The fmt package‘s various print routines automatically know to call the method when asked to print an error.

package main  import (    "fmt"    "time")type MyError struct {    When time.Time     What string}func (e *MyError) Error() string {    return fmt.Sprintf("at %v, %s",        e.When, e.What)}func run() error {    return &MyError{        time.Now(),        "it didn‘t work",    }}func main() {    if err := run(); err != nil {        fmt.Println(err)    }}

A Tour of Go Errors