Function As

errutil/as.go:36–72

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

(err error, target interface{})

// - it also supports recursing through causes with Cause().
// - if it detects an API use error, its panic object is a valid error.
func As(err error, target interface{}) bool {
	if target == nil {
		panic(AssertionFailedf("errors.As: target cannot be nil"))
	}

	// We use introspection for now, of course when/if Go gets generics
	// all this can go away.
	val := reflect.ValueOf(target)
	typ := val.Type()
	if typ.Kind() != reflect.Ptr || val.IsNil() {
		panic(AssertionFailedf("errors.As: target must be a non-nil pointer, found %T", target))
	}
	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
		panic(AssertionFailedf("errors.As: *target must be interface or implement error, found %T", target))
	}

	targetType := typ.Elem()
	for c := err; c != nil; c = errbase.UnwrapOnce(c) {
		if reflect.TypeOf(c).AssignableTo(targetType) {
			val.Elem().Set(reflect.ValueOf(c))
			return true
		}
		if x, ok := c.(interface{ As(interface{}) bool }); ok && x.As(target) {
			return true
		}

		// If at any point in the single cause chain including the top,
		// we encounter a multi-cause chain, recursively explore it.
		for _, cause := range errbase.UnwrapMulti(c) {
			if As(cause, target) {
				return true
			}
		}
	}

	return false
}

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