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errors: document Is and As methods

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Add brief descriptions of why one might implement
an Is or As method.

Fixes #33364.

Change-Id: I81a091bf564c654ddb9ef3997e780451a01efb7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/191338
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Jonathan Amsterdam 2019-08-22 12:29:29 -04:00
parent 95c3c43072
commit fc4663d56f
1 changed files with 12 additions and 1 deletions
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13
src/errors/wrap.go
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@ -28,6 +28,14 @@ func Unwrap(err error) error {
// //
// An error is considered to match a target if it is equal to that target or if // 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. // 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 == os.ErrExist }
//
// then Is(MyError{}, os.ErrExist) returns true. See syscall.Errno.Is for
// an example in the standard library.
func Is(err, target error) bool { func Is(err, target error) bool {
if target == nil { if target == nil {
return err == target return err == target
@ -61,7 +69,10 @@ func Is(err, target error) bool {
// As(target) returns true. In the latter case, the As method is responsible for // As(target) returns true. In the latter case, the As method is responsible for
// setting target. // setting target.
// //
// As will panic if target is not a non-nil pointer to either a type that implements // An error type might provide an As method so it can be treated as if it were a
// 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. As returns false if err is nil. // error, or to any interface type. As returns false if err is nil.
func As(err error, target interface{}) bool { func As(err error, target interface{}) bool {
if target == nil { if target == nil {