1 files changed, 93 insertions, 0 deletions
diff --git a/lib/types/fn/optional.ts b/lib/types/fn/optional.ts
new file mode 100644
index 0000000..504e496
--- /dev/null
+++ b/lib/types/fn/optional.ts
@@ -0,0 +1,93 @@
+import { type Mapper, Predicate, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
+
+export type MaybeGiven<T> = T | undefined | null;
+
+export const IOptionalTag = 'IOptional' as const;
+export type IOptionalTag = typeof IOptionalTag;
+export const isOptional = <T>(o: unknown): o is IOptional<T> => isTagged(o, IOptionalTag);
+export class IOptionalEmptyError extends Error {}
+export interface IOptional<t, T extends NonNullable<t> = NonNullable<t>> extends Tagged<IOptionalTag>, Iterable<T> {
+    readonly move: <_T>(t: MaybeGiven<_T>) => IOptional<_T>;
+    readonly map: <_T>(mapper: Mapper<T, MaybeGiven<_T>>) => IOptional<_T>;
+    readonly filter: (mapper: Predicate<T>) => IOptional<T>;
+    readonly flatMap: <_T>(mapper: Mapper<T, MaybeGiven<IOptional<_T>>>) => IOptional<_T>;
+    readonly orSome: (supplier: Supplier<MaybeGiven<t>>) => IOptional<T>;
+    readonly get: Supplier<T>;
+    readonly present: Supplier<boolean>;
+}
+
+type OSomeTag = typeof OSomeTag;
+const OSomeTag = 'O.Some' as const;
+interface Some<T> extends Tagged<OSomeTag> {
+    value: NonNullable<T>;
+}
+
+const ONoneTag = 'O.None' as const;
+type ONoneTag = typeof ONoneTag;
+interface None extends Tagged<ONoneTag> {}
+
+const isNone = (o: unknown): o is None => isTagged(o, ONoneTag);
+const isSome = <T>(o: unknown): o is Some<T> => isTagged(o, OSomeTag);
+
+class _Tagged implements Tagged<IOptionalTag> {
+    protected constructor(public readonly _tag = IOptionalTag) {}
+}
+
+export class Optional<t, T extends NonNullable<t> = NonNullable<t>> extends _Tagged implements IOptional<T> {
+    private constructor(private readonly self: Some<T> | None) {
+        super();
+    }
+
+    public move<_T>(t: MaybeGiven<_T>): IOptional<_T> {
+        return this.map(() => t);
+    }
+
+    public orSome(supplier: Supplier<MaybeGiven<t>>): IOptional<T> {
+        if (isNone(this.self)) return Optional.from(supplier());
+        return this;
+    }
+
+    public get(): T {
+        if (isNone(this.self)) throw new IOptionalEmptyError('called get() on None optional');
+        return this.self.value;
+    }
+
+    public filter(mapper: Predicate<T>): IOptional<T> {
+        if (isNone(this.self) || !mapper(this.self.value)) return Optional.none();
+        return Optional.some(this.self.value);
+    }
+
+    public map<_T>(mapper: Mapper<T, MaybeGiven<_T>>): IOptional<_T> {
+        if (isNone(this.self)) return Optional.none();
+        return Optional.from(mapper(this.self.value)) as IOptional<_T>;
+    }
+
+    public flatMap<_T>(mapper: Mapper<T, MaybeGiven<IOptional<_T>>>): IOptional<_T> {
+        if (isNone(this.self)) return Optional.none();
+        return Optional.from(mapper(this.self.value))
+            .orSome(() => Optional.none())
+            .get();
+    }
+
+    public present() {
+        return isSome(this.self);
+    }
+
+    *[Symbol.iterator]() {
+        if (isSome(this.self)) yield this.self.value;
+    }
+
+    static some<t, T extends NonNullable<t> = NonNullable<t>>(value: T): IOptional<T> {
+        return new Optional({ value, _tag: OSomeTag });
+    }
+
+    private static readonly _none = new Optional({ _tag: ONoneTag });
+    static none<T>(): IOptional<T> {
+        return this._none as unknown as IOptional<T>;
+    }
+
+    static from<t, T extends NonNullable<t> = NonNullable<t>>(value: MaybeGiven<t>): IOptional<T> {
+        if (value === null || value === undefined) return Optional.none<T>();
+        return Optional.some(<T>value);
+    }
+}