1 // Copyright 2018 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package errors 6 7 import ( 8 "internal/reflectlite" 9 ) 10 11 // Unwrap returns the result of calling the Unwrap method on err, if err's 12 // type contains an Unwrap method returning error. 13 // Otherwise, Unwrap returns nil. 14 func Unwrap(err error) error { 15 u, ok := err.(interface { 16 Unwrap() error 17 }) 18 if !ok { 19 return nil 20 } 21 return u.Unwrap() 22 } 23 24 // Is reports whether any error in err's chain matches target. 25 // 26 // The chain consists of err itself followed by the sequence of errors obtained by 27 // repeatedly calling Unwrap. 28 // 29 // An error is considered to match a target if it is equal to that target or if 30 // it implements a method Is(error) bool such that Is(target) returns true. 31 // 32 // An error type might provide an Is method so it can be treated as equivalent 33 // to an existing error. For example, if MyError defines 34 // 35 // func (m MyError) Is(target error) bool { return target == fs.ErrExist } 36 // 37 // then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for 38 // an example in the standard library. 39 func Is(err, target error) bool { 40 if target == nil { 41 return err == target 42 } 43 44 isComparable := reflectlite.TypeOf(target).Comparable() 45 for { 46 if isComparable && err == target { 47 return true 48 } 49 if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) { 50 return true 51 } 52 // TODO: consider supporting target.Is(err). This would allow 53 // user-definable predicates, but also may allow for coping with sloppy 54 // APIs, thereby making it easier to get away with them. 55 if err = Unwrap(err); err == nil { 56 return false 57 } 58 } 59 } 60 61 // As finds the first error in err's chain that matches target, and if so, sets 62 // target to that error value and returns true. Otherwise, it returns false. 63 // 64 // The chain consists of err itself followed by the sequence of errors obtained by 65 // repeatedly calling Unwrap. 66 // 67 // An error matches target if the error's concrete value is assignable to the value 68 // pointed to by target, or if the error has a method As(interface{}) bool such that 69 // As(target) returns true. In the latter case, the As method is responsible for 70 // setting target. 71 // 72 // An error type might provide an As method so it can be treated as if it were a 73 // different error type. 74 // 75 // As panics if target is not a non-nil pointer to either a type that implements 76 // error, or to any interface type. 77 func As(err error, target interface{}) bool { 78 if target == nil { 79 panic("errors: target cannot be nil") 80 } 81 val := reflectlite.ValueOf(target) 82 typ := val.Type() 83 if typ.Kind() != reflectlite.Ptr || val.IsNil() { 84 panic("errors: target must be a non-nil pointer") 85 } 86 targetType := typ.Elem() 87 if targetType.Kind() != reflectlite.Interface && !targetType.Implements(errorType) { 88 panic("errors: *target must be interface or implement error") 89 } 90 for err != nil { 91 if reflectlite.TypeOf(err).AssignableTo(targetType) { 92 val.Elem().Set(reflectlite.ValueOf(err)) 93 return true 94 } 95 if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) { 96 return true 97 } 98 err = Unwrap(err) 99 } 100 return false 101 } 102 103 var errorType = reflectlite.TypeOf((*error)(nil)).Elem() 104