Custom Errors #

// It's possible to use custom types as `error`s by
// implementing the `Error()` method on them. Here's a
// variant on the example above that uses a custom type
// to explicitly represent an argument error.

package main

import (
	"errors"
	"fmt"
)

// A custom error type usually has the suffix "Error".
type argError struct {
	arg     int
	message string
}

// Adding this `Error` method makes `argError` implement
// the `error` interface.
func (e *argError) Error() string {
	return fmt.Sprintf("%d - %s", e.arg, e.message)
}

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

		// Return our custom error.
		return -1, &argError{arg, "can't work with it"}
	}
	return arg + 3, nil
}

func main() {

	// `errors.As` is a more advanced version of `errors.Is`.
	// It checks that a given error (or any error in its chain)
	// matches a specific error type and converts to a value
	// of that type, returning `true`. If there's no match, it
	// returns `false`.
	_, err := f(42)
	var ae *argError
	if errors.As(err, &ae) {
		fmt.Println(ae.arg)
		fmt.Println(ae.message)
	} else {
		fmt.Println("err doesn't match argError")
	}
}

$ go run custom-errors.go
42
can't work with it