Source: wrap.go in package errors


package errors

import (
	"internal/reflectlite"
)

Unwrap returns the result of calling the Unwrap method on err, if err's type contains an Unwrap method returning error. Otherwise, Unwrap returns nil.

func Unwrap(err error) error {
	u, ok := err.(interface {
		Unwrap() error
	})
	if !ok {
		return nil
	}
	return u.Unwrap()
}

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. An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines func (m MyError) Is(target error) bool { return target == fs.ErrExist } then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library.

func Is(err, target error) bool {
	if target == nil {
		return err == target
	}

	isComparable := reflectlite.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 supporting 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 target, and if so, sets target to that error value and returns true. Otherwise, it returns false. 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. An error type might provide an As method so it can be treated as if it were a different error type. As panics if target is not a non-nil pointer to either a type that implements error, or to any interface type.

func As(err error, target interface{}) bool {
	if target == nil {
		panic("errors: target cannot be nil")
	}
	val := reflectlite.ValueOf(target)
	typ := val.Type()
	if typ.Kind() != reflectlite.Ptr || val.IsNil() {
		panic("errors: target must be a non-nil pointer")
	}
	if e := typ.Elem(); e.Kind() != reflectlite.Interface && !e.Implements(errorType) {
		panic("errors: *target must be interface or implement error")
	}
	targetType := typ.Elem()
	for err != nil {
		if reflectlite.TypeOf(err).AssignableTo(targetType) {
			val.Elem().Set(reflectlite.ValueOf(err))
			return true
		}
		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
			return true
		}
		err = Unwrap(err)
	}
	return false
}