24-errors.go

// In Go it’s idiomatic to communicate errors via an explicit, separate return value. This contrasts with the exceptions used in languages like Java and Ruby and the overloaded single result / error value sometimes used in C. Go’s approach makes it easy to see which functions return errors and to handle them using the same language constructs employed for any other, non-error tasks.

package main

import (
	"errors"
	"fmt"
)

func f1(arg int) (int, error) {
	if arg == 42 {

		return -1, errors.New("can't work with 42")

	}

	return arg + 3, nil
}

type argError struct {
	arg  int
	prob string
}

func (e *argError) Error() string {
	return fmt.Sprintf("%d - %s", e.arg, e.prob)
}

func f2(arg int) (int, error) {
	if arg == 42 {

		return -1, &argError{arg, "can't work with it"}
	}
	return arg + 3, nil
}

func main() {

	for _, i := range []int{7, 42} {
		if r, e := f1(i); e != nil {
			fmt.Println("f1 failed:", e)
		} else {
			fmt.Println("f1 worked:", r)
		}
	}
	for _, i := range []int{7, 42} {
		if r, e := f2(i); e != nil {
			fmt.Println("f2 failed:", e)
		} else {
			fmt.Println("f2 worked:", r)
		}
	}

	_, e := f2(42)
	if ae, ok := e.(*argError); ok {
		fmt.Println(ae.arg)
		fmt.Println(ae.prob)
	}
}