A Tour of Go Errors

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)
    }
}