1// Copyright 2011 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package errors implements functions to manipulate errors.
6//
7// The [New] function creates errors whose only content is a text message.
8//
9// An error e wraps another error if e's type has one of the methods
10//
11//	Unwrap() error
12//	Unwrap() []error
13//
14// If e.Unwrap() returns a non-nil error w or a slice containing w,
15// then we say that e wraps w. A nil error returned from e.Unwrap()
16// indicates that e does not wrap any error. It is invalid for an
17// Unwrap method to return an []error containing a nil error value.
18//
19// An easy way to create wrapped errors is to call [fmt.Errorf] and apply
20// the %w verb to the error argument:
21//
22//	wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)
23//
24// Successive unwrapping of an error creates a tree. The [Is] and [As]
25// functions inspect an error's tree by examining first the error
26// itself followed by the tree of each of its children in turn
27// (pre-order, depth-first traversal).
28//
29// [Is] examines the tree of its first argument looking for an error that
30// matches the second. It reports whether it finds a match. It should be
31// used in preference to simple equality checks:
32//
33//	if errors.Is(err, fs.ErrExist)
34//
35// is preferable to
36//
37//	if err == fs.ErrExist
38//
39// because the former will succeed if err wraps [io/fs.ErrExist].
40//
41// [As] examines the tree of its first argument looking for an error that can be
42// assigned to its second argument, which must be a pointer. If it succeeds, it
43// performs the assignment and returns true. Otherwise, it returns false. The form
44//
45//	var perr *fs.PathError
46//	if errors.As(err, &perr) {
47//		fmt.Println(perr.Path)
48//	}
49//
50// is preferable to
51//
52//	if perr, ok := err.(*fs.PathError); ok {
53//		fmt.Println(perr.Path)
54//	}
55//
56// because the former will succeed if err wraps an [*io/fs.PathError].
57package errors
58
59// New returns an error that formats as the given text.
60// Each call to New returns a distinct error value even if the text is identical.
61func New(text string) error {
62	return &errorString{text}
63}
64
65// errorString is a trivial implementation of error.
66type errorString struct {
67	s string
68}
69
70func (e *errorString) Error() string {
71	return e.s
72}
73
74// ErrUnsupported indicates that a requested operation cannot be performed,
75// because it is unsupported. For example, a call to [os.Link] when using a
76// file system that does not support hard links.
77//
78// Functions and methods should not return this error but should instead
79// return an error including appropriate context that satisfies
80//
81//	errors.Is(err, errors.ErrUnsupported)
82//
83// either by directly wrapping ErrUnsupported or by implementing an [Is] method.
84//
85// Functions and methods should document the cases in which an error
86// wrapping this will be returned.
87var ErrUnsupported = New("unsupported operation")
88