Copyright 2011 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
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.
package errors
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.
func ( string) error {
	return &errorString{}
}
errorString is a trivial implementation of error.
type errorString struct {
	s string
}

func ( *errorString) () string {
	return .s