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errors: mention Is methods should not call Unwrap

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errors.Is internally unwraps the error until the error matches the
target. Because of this, a user-authored Is method on an error type
need not call errors.Unwrap on itself or the target, because that would
make the unwrapping operation O(N^2). It is a subtle detail to remind
authors for resource efficiency reasons.

Change-Id: Ic1ba59a5bdbfe2c7cb51a2cba2537ab6de4a13ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/356789
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Jean de Klerk <deklerk@google.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Jean de Klerk <deklerk@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Matt T. Proud 2021-10-19 10:09:06 +02:00 committed by Damien Neil
parent 6c3cd5d2eb
commit 7999fd4710
1 changed files with 2 additions and 1 deletions
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3
src/errors/wrap.go
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@ -35,7 +35,8 @@ func Unwrap(err error) error {
// 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 example in the standard library. An Is method should only shallowly
// compare err and the target and not call Unwrap on either.
func Is(err, target error) bool {
if target == nil {
return err == target