src/errors/example_test.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package errors_test

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"time"
)

// MyError is an error implementation that includes a time and message.
type MyError struct {
	When time.Time
	What string
}

func (e MyError) Error() string {
	return fmt.Sprintf("%v: %v", e.When, e.What)
}

func oops() error {
	return MyError{
		time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
		"the file system has gone away",
	}
}

func Example() {
	if err := oops(); err != nil {
		fmt.Println(err)
	}
	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
}

func ExampleNew() {
	err := errors.New("emit macho dwarf: elf header corrupted")
	if err != nil {
		fmt.Print(err)
	}
	// Output: emit macho dwarf: elf header corrupted
}

// The fmt package's Errorf function lets us use the package's formatting
// features to create descriptive error messages.
func ExampleNew_errorf() {
	const name, id = "bimmler", 17
	err := fmt.Errorf("user %q (id %d) not found", name, id)
	if err != nil {
		fmt.Print(err)
	}
	// Output: user "bimmler" (id 17) not found
}

func ExampleJoin() {
	err1 := errors.New("err1")
	err2 := errors.New("err2")
	err := errors.Join(err1, err2)
	fmt.Println(err)
	if errors.Is(err, err1) {
		fmt.Println("err is err1")
	}
	if errors.Is(err, err2) {
		fmt.Println("err is err2")
	}
	// Output:
	// err1
	// err2
	// err is err1
	// err is err2
}

func ExampleIs() {
	if _, err := os.Open("non-existing"); err != nil {
		if errors.Is(err, fs.ErrNotExist) {
			fmt.Println("file does not exist")
		} else {
			fmt.Println(err)
		}
	}

	// Output:
	// file does not exist
}

func ExampleAs() {
	if _, err := os.Open("non-existing"); err != nil {
		var pathError *fs.PathError
		if errors.As(err, &pathError) {
			fmt.Println("Failed at path:", pathError.Path)
		} else {
			fmt.Println(err)
		}
	}

	// Output:
	// Failed at path: non-existing
}

func ExampleUnwrap() {
	err1 := errors.New("error1")
	err2 := fmt.Errorf("error2: [%w]", err1)
	fmt.Println(err2)
	fmt.Println(errors.Unwrap(err2))
	// Output
	// error2: [error1]
	// error1
}