// 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.

//

// An error e wraps another error if e's type has one of the methods

//

// Unwrap() error

// Unwrap() []error

//

// If e.Unwrap() returns a non-nil error w or a slice containing w,

// then we say that e wraps w. A nil error returned from e.Unwrap()

// indicates that e does not wrap any error. It is invalid for an

// Unwrap method to return an []error containing a nil error value.

//

// An easy way to create wrapped errors is to call [fmt.Errorf] and apply

// the %w verb to the error argument:

//

// wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)

//

// Successive unwrapping of an error creates a tree. The [Is] and [As]

// functions inspect an error's tree by examining first the error

// itself followed by the tree of each of its children in turn

// (pre-order, depth-first traversal).

//

// See https://go.dev/blog/go1.13-errors for a deeper discussion of the

// philosophy of wrapping and when to wrap.

//

// [Is] examines the tree of its first argument 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 [io/fs.ErrExist].

//

// [As] examines the tree of its first argument 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 [*io/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 New(text string) error {

return &errorString{text}

}

// errorString is a trivial implementation of error.

type errorString struct {

s string

}

func (e *errorString) Error() string {

return e.s

}

// ErrUnsupported indicates that a requested operation cannot be performed,

// because it is unsupported. For example, a call to [os.Link] when using a

// file system that does not support hard links.

//

// Functions and methods should not return this error but should instead

// return an error including appropriate context that satisfies

//

// errors.Is(err, errors.ErrUnsupported)

//

// either by directly wrapping ErrUnsupported or by implementing an [Is] method.

//

// Functions and methods should document the cases in which an error

// wrapping this will be returned.

var ErrUnsupported = New("unsupported operation")