xerrors.go

// Copyright 2018 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 xerrors

import (
	"errors"
	"reflect"
)

// Is reports whether any error in err's chain matches target.
//
// The chain consists of err itself followed by the sequence of errors obtained by
// repeatedly calling Unwrap.
//
// 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.
func Is(err, target error) bool {
	type predicate interface {
		Is(error) bool
	}
	if target == nil {
		return target == nil
	}
	isComparable := reflect.TypeOf(target).Comparable()
	for err != nil {
		if isComparable && err == target {
			return true
		}

		if x, ok := err.(predicate); ok && x.Is(target) {
			return true
		}

		// TODO: consider supporing target.Is(err). This would allow
		// user-definable predicates, but also may allow for coping with sloppy
		// APIs, thereby making it easier to get away with them.
		err = Unwrap(err)
	}
	return false
}

// 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 errors.New(text)
}

// Unwrap returns the result of calling the Unwrap or Cause method on err, if err's
// type contains an Unwrap method returning error.
// Otherwise, Unwrap returns nil.
func Unwrap(err error) error {
	type unwrapper interface {
		Unwrap() error
	}
	type causer interface {
		Cause() error
	}
	switch err := err.(type) {
	case unwrapper:
		return err.Unwrap()
	case causer:
		return err.Cause()
	default:
		return nil
	}
}

// As finds the first error in err's chain that matches target, and if so, sets
// target to that error value and returns true.
//
// The chain consists of err itself followed by the sequence of errors obtained by
// repeatedly calling Unwrap.
//
// An error matches target if the error's concrete value is assignable to the value
// pointed to by target, or if the error has a method As(interface{}) bool such that
// As(target) returns true. In the latter case, the As method is responsible for
// setting target.
//
// As will panic 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.
func As(err error, target interface{}) bool {
	if target == nil {
		panic("errors: target cannot be nil")
	}
	val := reflect.ValueOf(target)
	typ := val.Type()
	if typ.Kind() != reflect.Ptr || val.IsNil() {
		panic("errors: target must be a non-nil pointer")
	}
	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
		panic("errors: *target must be interface or implement error")
	}
	targetType := typ.Elem()
	for err != nil {
		if reflect.TypeOf(err).AssignableTo(targetType) {
			val.Elem().Set(reflect.ValueOf(err))
			return true
		}
		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
			return true
		}
		err = Unwrap(err)
	}
	return false
}

var errorType = reflect.TypeOf((*error)(nil)).Elem()