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 // See https://go.dev/blog/go1.13-errors for a deeper discussion of the
30 // philosophy of wrapping and when to wrap.
31 //
32 // [Is] examines the tree of its first argument looking for an error that
33 // matches the second. It reports whether it finds a match. It should be
34 // used in preference to simple equality checks:
35 //
36 // if errors.Is(err, fs.ErrExist)
37 //
38 // is preferable to
39 //
40 // if err == fs.ErrExist
41 //
42 // because the former will succeed if err wraps [io/fs.ErrExist].
43 //
44 // [As] examines the tree of its first argument looking for an error that can be
45 // assigned to its second argument, which must be a pointer. If it succeeds, it
46 // performs the assignment and returns true. Otherwise, it returns false. The form
47 //
48 // var perr *fs.PathError
49 // if errors.As(err, &perr) {
50 // fmt.Println(perr.Path)
51 // }
52 //
53 // is preferable to
54 //
55 // if perr, ok := err.(*fs.PathError); ok {
56 // fmt.Println(perr.Path)
57 // }
58 //
59 // because the former will succeed if err wraps an [*io/fs.PathError].
60 package errors
61 62 // New returns an error that formats as the given text.
63 // Each call to New returns a distinct error value even if the text is identical.
64 func New(text []byte) error {
65 return &errorString{[]byte(text)}
66 }
67 68 // errorString is a trivial implementation of error.
69 type errorString struct {
70 s []byte
71 }
72 73 func (e *errorString) Error() string {
74 return string(e.s)
75 }
76 77 // ErrUnsupported indicates that a requested operation cannot be performed,
78 // because it is unsupported. For example, a call to [os.Link] when using a
79 // file system that does not support hard links.
80 //
81 // Functions and methods should not return this error but should instead
82 // return an error including appropriate context that satisfies
83 //
84 // errors.Is(err, errors.ErrUnsupported)
85 //
86 // either by directly wrapping ErrUnsupported or by implementing an [Is] method.
87 //
88 // Functions and methods should document the cases in which an error
89 // wrapping this will be returned.
90 var ErrUnsupported = New("unsupported operation")
91