Tour of Go 2.0

Errors

Go handles errors with values, not exceptions. The built-in error interface looks like this:

type error interface {
	Error() string
}

Any type with an Error() string method satisfies it — the same implicit satisfaction you’ve seen throughout this chapter.

Functions typically return an error as their last value. Callers check it:

i, err := strconv.Atoi("42")
if err != nil {
	fmt.Printf("couldn't convert number: %v\n", err)
	return
}
fmt.Println("Converted integer:", i)

nil means success. Non-nil means something went wrong.

main.go ▶ Run in Playground 
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)
	}
}