func As(errerror, targetany)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 repeatedlycalling its Unwrap() error or Unwrap() []error method. When err wraps multipleerrors, 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 valuepointed to by target, or if the error has a method As(any) bool such thatAs(target) returns true. In the latter case, the As method is responsible forsetting target.

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

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

Deprecated: As of Go 1.13, this function simply callserrors.As.

package mainimport ("fmt""os""golang.org/x/xerrors")func main() {_, err := os.Open("non-existing")if err != nil {var pathError *os.PathErrorif xerrors.As(err, &pathError) {fmt.Println("Failed at path:", pathError.Path)}}}

Output:Failed at path: non-existing

package mainimport ("fmt""golang.org/x/xerrors")type MyError2 struct {Message stringframe   xerrors.Frame}func (m *MyError2) Error() string {return m.Message}func (m *MyError2) Format(f fmt.State, c rune) { // implements fmt.Formatterxerrors.FormatError(m, f, c)}func (m *MyError2) FormatError(p xerrors.Printer) error { // implements xerrors.Formatterp.Print(m.Message)if p.Detail() {m.frame.Format(p)}return nil}func main() {err := &MyError2{Message: "oops", frame: xerrors.Caller(1)}fmt.Printf("%v\n", err)fmt.Println()fmt.Printf("%+v\n", err)}

func Is(err, targeterror)bool

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

The tree consists of err itself, followed by the errors obtained by repeatedlycalling its Unwrap() error or Unwrap() []error method. When err wraps multipleerrors, 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 ifit 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 equivalentto 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. Seesyscall.Errno.Is foran example in the standard library. An Is method should only shallowlycompare err and the target and not callUnwrap on either.

Deprecated: As of Go 1.13, this function simply callserrors.Is.

package mainimport ("fmt""golang.org/x/xerrors")func main() {err := xerrors.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 formattingfeatures to create descriptive error messages.

package mainimport ("fmt")func main() {const name, id = "bimmler", 17err := 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(errerror)error

Unwrap returns the result of calling the Unwrap method on err, if err implementsUnwrap. Otherwise, Unwrap returns nil.

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

Deprecated: As of Go 1.13, this function simply callserrors.Unwrap.