feat(spec): add option spec interface and impl

spec.ts

@@ -0,0 +1,50 @@
+// Copyright © 2023 Tomoki Miyauchi. All rights reserved. MIT license.
+// This module is browser compatible.
+
+/** The option type. */
+export enum OptionType {
+  Some = "some",
+  None = "none",
+}
+
+/** Common type. */
+interface Container {
+  /** Option type. */
+  type: OptionType;
+}
+
+/** The {@link None} API. */
+export interface None extends Container {
+  type: OptionType.None;
+}
+
+/** The {@link Some} API. */
+export interface Some<T> extends Container {
+  type: OptionType.Some;
+
+  /** Return contained {@link T}. */
+  get get(): T;
+}
+
+export interface SomeConstructor {
+  /** {@link Some} value of type {@link T } . */
+  of<const T>(value: T): Some<T>;
+}
+
+/** No value. */
+export const None: None = { type: OptionType.None };
+
+/** {@link Some} constructor. */
+export const Some: SomeConstructor = {
+  of<T>(value: T): Some<T> {
+    return {
+      type: OptionType.Some,
+      get get(): T {
+        return value;
+      },
+    };
+  },
+};
+
+/** Representation of {@link Some} or {@link None}. */
+export type Option<T> = Some<T> | None;