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.

The Unwrap, Is and As functions work on errors that may wrap other errors. An error wraps another error if its type has the method 

Unwrap() error

If e.Unwrap() returns a non-nil error w, then we say that e wraps w.

Unwrap unpacks wrapped errors. If its argument's type has an Unwrap method, it calls the method once. Otherwise, it returns nil.

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

errors.Unwrap(fmt.Errorf("... %w ...", ..., err, ...))

returns err.

Is unwraps its first argument sequentially 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 fs.ErrExist.

As unwraps its first argument sequentially 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 *fs.PathError.

▾ Example

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

func As 1.13 

func As(err error, target any) bool

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

The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

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(interface{}) 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

Failed at path: non-existing

func Is 1.13 

func Is(err, target error) bool

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

The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.

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

file does not exist

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

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. 

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.

▾ Example