feat: add Or method.

Author: ghosind

error.go

@@ -3,5 +3,6 @@ package optional
 import "errors"
 
 var (
+	ErrNilFunction = errors.New("nil function")
 	ErrNoSuchValue = errors.New("no such value")
 )

optional.go

@@ -120,6 +120,20 @@ func (opt *Optional[T]) IsPresent() bool {
 	return opt.val != nil
 }
 
+// Or returns the Optional instance if the value is present, otherwise returns an Optional produced
+// by the supplying function.
+func (opt *Optional[T]) Or(supplier func() *Optional[T]) *Optional[T] {
+	if supplier == nil {
+		panic(ErrNilFunction)
+	}
+
+	if opt.IsPresent() {
+		return opt
+	}
+
+	return supplier()
+}
+
 // OrElse returns the value if present, otherwise returns other.
 func (opt *Optional[T]) OrElse(other T) T {
 	if opt.IsEmpty() {

optional_test.go

@@ -227,6 +227,30 @@ func TestOptionalIsEmpty(t *testing.T) {
 	a.NotTrueNow(opt.IsEmpty())
 }
 
+func TestOptionalOr(t *testing.T) {
+	a := assert.New(t)
+
+	opt := optional.New[string](nil)
+	a.PanicOfNow(func() {
+		opt.Or(nil)
+	}, optional.ErrNilFunction)
+
+	out := opt.Or(func() *optional.Optional[string] { return optional.Of("or value") })
+	a.NotNilNow(out)
+	a.TrueNow(out.IsPresent())
+	a.NotPanicNow(func() {
+		a.EqualNow(out.GetPanic(), "or value")
+	})
+
+	opt = optional.Of("Hello world")
+	out = opt.Or(func() *optional.Optional[string] { return optional.Of("or value") })
+	a.NotNilNow(out)
+	a.TrueNow(out.IsPresent())
+	a.NotPanicNow(func() {
+		a.EqualNow(out.GetPanic(), "Hello world")
+	})
+}
+
 func TestOptionalOrElse(t *testing.T) {
 	a := assert.New(t)