wrap.go

     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  //
    15  // Unwrap only calls a method of the form "Unwrap() error".
    16  // In particular Unwrap does not unwrap errors returned by [Join].
    17  func Unwrap(err error) error {
    18  	u, ok := err.(interface {
    19  		Unwrap() error
    20  	})
    21  	if !ok {
    22  		return nil
    23  	}
    24  	return u.Unwrap()
    25  }
    26  
    27  // Is reports whether any error in err's tree matches target.
    28  //
    29  // The tree consists of err itself, followed by the errors obtained by repeatedly
    30  // calling Unwrap. When err wraps multiple errors, Is examines err followed by a
    31  // depth-first traversal of its children.
    32  //
    33  // An error is considered to match a target if it is equal to that target or if
    34  // it implements a method Is(error) bool such that Is(target) returns true.
    35  //
    36  // An error type might provide an Is method so it can be treated as equivalent
    37  // to an existing error. For example, if MyError defines
    38  //
    39  //	func (m MyError) Is(target error) bool { return target == fs.ErrExist }
    40  //
    41  // then Is(MyError{}, fs.ErrExist) returns true. See [syscall.Errno.Is] for
    42  // an example in the standard library. An Is method should only shallowly
    43  // compare err and the target and not call Unwrap on either.
    44  func Is(err, target error) bool {
    45  	if target == nil {
    46  		return err == target
    47  	}
    48  
    49  	isComparable := reflectlite.TypeOf(target).Comparable()
    50  	for {
    51  		if isComparable && err == target {
    52  			return true
    53  		}
    54  		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
    55  			return true
    56  		}
    57  		switch x := err.(type) {
    58  		case interface{ Unwrap() error }:
    59  			err = x.Unwrap()
    60  			if err == nil {
    61  				return false
    62  			}
    63  		case interface{ Unwrap() []error }:
    64  			for _, err := range x.Unwrap() {
    65  				if Is(err, target) {
    66  					return true
    67  				}
    68  			}
    69  			return false
    70  		default:
    71  			return false
    72  		}
    73  	}
    74  }
    75  
    76  // As finds the first error in err's tree that matches target, and if one is found, sets
    77  // target to that error value and returns true. Otherwise, it returns false.
    78  //
    79  // The tree consists of err itself, followed by the errors obtained by repeatedly
    80  // calling Unwrap. When err wraps multiple errors, As examines err followed by a
    81  // depth-first traversal of its children.
    82  //
    83  // An error matches target if the error's concrete value is assignable to the value
    84  // pointed to by target, or if the error has a method As(interface{}) bool such that
    85  // As(target) returns true. In the latter case, the As method is responsible for
    86  // setting target.
    87  //
    88  // An error type might provide an As method so it can be treated as if it were a
    89  // different error type.
    90  //
    91  // As panics if target is not a non-nil pointer to either a type that implements
    92  // error, or to any interface type.
    93  func As(err error, target any) bool {
    94  	if err == nil {
    95  		return false
    96  	}
    97  	if target == nil {
    98  		panic("errors: target cannot be nil")
    99  	}
   100  	val := reflectlite.ValueOf(target)
   101  	typ := val.Type()
   102  	if typ.Kind() != reflectlite.Ptr || val.IsNil() {
   103  		panic("errors: target must be a non-nil pointer")
   104  	}
   105  	targetType := typ.Elem()
   106  	if targetType.Kind() != reflectlite.Interface && !targetType.Implements(errorType) {
   107  		panic("errors: *target must be interface or implement error")
   108  	}
   109  	for {
   110  		if reflectlite.TypeOf(err).AssignableTo(targetType) {
   111  			val.Elem().Set(reflectlite.ValueOf(err))
   112  			return true
   113  		}
   114  		if x, ok := err.(interface{ As(any) bool }); ok && x.As(target) {
   115  			return true
   116  		}
   117  		switch x := err.(type) {
   118  		case interface{ Unwrap() error }:
   119  			err = x.Unwrap()
   120  			if err == nil {
   121  				return false
   122  			}
   123  		case interface{ Unwrap() []error }:
   124  			for _, err := range x.Unwrap() {
   125  				if As(err, target) {
   126  					return true
   127  				}
   128  			}
   129  			return false
   130  		default:
   131  			return false
   132  		}
   133  	}
   134  }
   135  
   136  var errorType = reflectlite.TypeOf((*error)(nil)).Elem()
   137
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