add option

Signed-off-by: Xe Iaso <me@christine.website>

option.go

@@ -0,0 +1,47 @@
+package gonads
+
+import "errors"
+
+var (
+	ErrOptionIsNone = errors.New("gonads: Option[T] has no value")
+)
+
+// Option is a container that might contain a value.
+type Option[T any] struct {
+	val *T
+}
+
+// IsSome returns true if the Option has some value in it.
+func (o Option[T]) IsSome() bool {
+	return o.val != nil
+}
+
+// IsNone returns true if the Option has no value in it.
+func (o Option[T]) IsNone() bool {
+	return !o.IsSome()
+}
+
+// Take a value out of the option if it exists.
+func (o Option[T]) Take() (*T, error) {
+	if o.IsNone() {
+		return nil, ErrOptionIsNone
+	}
+
+	return o.val, nil
+}
+
+// Set the value of the option to a given value.
+func (o *Option[T]) Set(val T) {
+	o.val = &val
+}
+
+// Clear removes the value from an Option.
+func (o *Option[T]) Clear() {
+	o.val = nil
+}
+
+// NewOption creates a new Option with the given type. All Options
+// Start out as nil.
+func NewOption[T any]() *Option[T] {
+	return &Option[T]{}
+}

option_test.go

@@ -0,0 +1,24 @@
+package gonads
+
+import (
+	"testing"
+)
+
+func TestOption(t *testing.T) {
+	o := NewOption[string]()
+	val, err := o.Take()
+	if err == nil {
+		t.Fatalf("[unexpected] wanted no value out of Option[T], got: %v", val)
+	}
+
+	o.Set("hello friendos")
+	_, err = o.Take()
+	if err != nil {
+		t.Fatalf("[unexpected] wanted no value out of Option[T], got: %v", err)
+	}
+
+	o.Clear()
+	if o.IsSome() {
+		t.Fatal("Option should have none, but has some")
+	}
+}