なんかもうめっちゃ変えた

packages/backend/src/misc/prelude/maybe.ts

@@ -0,0 +1,20 @@
+export interface IMaybe<T> {
+	isJust(): this is IJust<T>;
+}
+
+export interface IJust<T> extends IMaybe<T> {
+	get(): T;
+}
+
+export function just<T>(value: T): IJust<T> {
+	return {
+		isJust: () => true,
+		get: () => value,
+	};
+}
+
+export function nothing<T>(): IMaybe<T> {
+	return {
+		isJust: () => false,
+	};
+}