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Package errors

import "errors"
Overview
Index
Examples

Overview ▾

Package errors implements functions to manipulate errors.

The New function creates errors whose only content is a text message.

An error e wraps another error if e's type has one of the methods 

Unwrap() error
Unwrap() []error

If e.Unwrap() returns a non-nil error w or a slice containing w, then we say that e wraps w. A nil error returned from e.Unwrap() indicates that e does not wrap any error. It is invalid for an Unwrap method to return an []error containing a nil error value.

An easy way to create wrapped errors is to call fmt.Errorf and apply the %w verb to the error argument: 

wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)

Successive unwrapping of an error creates a tree. The Is and As functions inspect an error's tree by examining first the error itself followed by the tree of each of its children in turn (pre-order, depth-first traversal).

Is examines the tree of its first argument looking for an error that matches the second. It reports whether it finds a match. It should be used in preference to simple equality checks: 

if errors.Is(err, fs.ErrExist)

is preferable to 

if err == fs.ErrExist

because the former will succeed if err wraps io/fs.ErrExist.

As examines the tree of its first argument looking for an error that can be assigned to its second argument, which must be a pointer. If it succeeds, it performs the assignment and returns true. Otherwise, it returns false. The form 

var perr *fs.PathError
if errors.As(err, &perr) {
	fmt.Println(perr.Path)
}

is preferable to 

if perr, ok := err.(*fs.PathError); ok {
	fmt.Println(perr.Path)
}

because the former will succeed if err wraps an *io/fs.PathError.

▾ Example

Code:

if err := oops(); err != nil {
    fmt.Println(err)
}

Output:

1989-03-15 22:30:00 +0000 UTC: the file system has gone away

Variables

ErrUnsupported indicates that a requested operation cannot be performed, because it is unsupported. For example, a call to os.Link when using a file system that does not support hard links.

Functions and methods should not return this error but should instead return an error including appropriate context that satisfies 

errors.Is(err, errors.ErrUnsupported)

either by directly wrapping ErrUnsupported or by implementing an Is method.

Functions and methods should document the cases in which an error wrapping this will be returned. 

var ErrUnsupported = New("unsupported operation")

func As 1.13 

func As(err error, target any) bool

As finds the first error in err's tree that matches target, and if one is found, sets target to that error value and returns true. Otherwise, it returns false.

The tree consists of err itself, followed by the errors obtained by repeatedly calling its Unwrap() error or Unwrap() []error method. When err wraps multiple errors, As examines err followed by a depth-first traversal of its children.

An error matches target if the error's concrete value is assignable to the value pointed to by target, or if the error has a method As(any) bool such that As(target) returns true. In the latter case, the As method is responsible for setting target.

An error type might provide an As method so it can be treated as if it were a different error type.

As panics if target is not a non-nil pointer to either a type that implements error, or to any interface type.

▾ Example

Code:

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 Is 1.13 

func Is(err, target error) bool

Is reports whether any error in err's tree matches target.

The tree consists of err itself, followed by the errors obtained by repeatedly calling its Unwrap() error or Unwrap() []error method. When err wraps multiple errors, Is examines err followed by a depth-first traversal of its children.

An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.

An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines 

func (m MyError) Is(target error) bool { return target == fs.ErrExist }

then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library. An Is method should only shallowly compare err and the target and not call Unwrap on either.

▾ Example

Code:

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 Join 1.20 

func Join(errs ...error) error

Join returns an error that wraps the given errors. Any nil error values are discarded. Join returns nil if every value in errs is nil. The error formats as the concatenation of the strings obtained by calling the Error method of each element of errs, with a newline between each string.

A non-nil error returned by Join implements the Unwrap() []error method.

▾ Example

Code:

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 New 

func New(text string) error

New returns an error that formats as the given text. Each call to New returns a distinct error value even if the text is identical.

▾ Example

Code:

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

Output:

emit macho dwarf: elf header corrupted

▾ Example (Errorf)

The fmt package's Errorf function lets us use the package's formatting features to create descriptive error messages.

Code:

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 Unwrap 1.13 

func Unwrap(err error) error

Unwrap returns the result of calling the Unwrap method on err, if err's type contains an Unwrap method returning error. Otherwise, Unwrap returns nil.

Unwrap only calls a method of the form "Unwrap() error". In particular Unwrap does not unwrap errors returned by Join.

▾ Example

Code:

err1 := errors.New("error1")
err2 := fmt.Errorf("error2: [%w]", err1)
fmt.Println(err2)
fmt.Println(errors.Unwrap(err2))

Output:

error2: [error1]
error1