source: code/trunk/vendor/golang.org/x/xerrors/wrap.go@ 822

// 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 (
        "reflect"
)

// A Wrapper provides context around another error.
type Wrapper interface {
        // Unwrap returns the next error in the error chain.
        // If there is no next error, Unwrap returns nil.
        Unwrap() error
}

// Opaque returns an error with the same error formatting as err
// but that does not match err and cannot be unwrapped.
func Opaque(err error) error {
        return noWrapper{err}
}

type noWrapper struct {
        error
}

func (e noWrapper) FormatError(p Printer) (next error) {
        if f, ok := e.error.(Formatter); ok {
                return f.FormatError(p)
        }
        p.Print(e.error)
        return nil
}

// Unwrap returns the result of calling the Unwrap method on err, if err implements
// Unwrap. Otherwise, Unwrap returns nil.
func Unwrap(err error) error {
        u, ok := err.(Wrapper)
        if !ok {
                return nil
        }
        return u.Unwrap()
}

// Is reports whether any error in err's chain matches target.
//
// 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 {
        if target == nil {
                return err == target
        }

        isComparable := reflect.TypeOf(target).Comparable()
        for {
                if isComparable && err == target {
                        return true
                }
                if x, ok := err.(interface{ Is(error) bool }); 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.
                if err = Unwrap(err); err == nil {
                        return false
                }
        }
}

// As finds the first error in err's chain that matches the type to which target
// points, and if so, sets the target to its value and returns true. An error
// matches a type if it is assignable to the target type, or if it has a method
// As(interface{}) bool such that As(target) returns true. As will panic if target
// is not a non-nil pointer to a type which implements error or is of interface type.
//
// The As method should set the target to its value and return true if err
// matches the type to which target points.
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()