3 files changed, 43 insertions, 23 deletions

diff --git a/u/types/fn/callable.ts b/u/types/fn/callable.ts
index 51756d7..60d747b 100644
--- a/u/types/fn/callable.ts
+++ b/u/types/fn/callable.ts
@@ -10,6 +10,8 @@ export interface Mapper<T, U> extends Callable<U, T> {
     (t: T): U;
 }
 
+export interface Predicate<T> extends Mapper<T, boolean> {}
+
 export interface BiMapper<T, U, R> extends Callable {
     (t: T, u: U): R;
 }
diff --git a/u/types/fn/either.ts b/u/types/fn/either.ts
index aa67d41..80f32b4 100644
--- a/u/types/fn/either.ts
+++ b/u/types/fn/either.ts
@@ -1,18 +1,36 @@
-import { BiMapper, IOptional, type Mapper, Optional, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
+import {
+    BiMapper,
+    IOptional,
+    type Mapper,
+    Optional,
+    Predicate,
+    type Supplier,
+    Tagged,
+    isTagged,
+} from '@emprespresso/pengueno';
 
 export const IEitherTag = 'IEither' as const;
 export type IEitherTag = typeof IEitherTag;
 export const isEither = <E, T>(o: unknown): o is IEither<E, T> => isTagged(o, IEitherTag);
 export interface IEither<E, T> extends Tagged<IEitherTag> {
-    readonly mapBoth: <_E, _T>(errBranch: Mapper<E, _E>, okBranch: Mapper<T, _T>) => IEither<_E, _T>;
-    readonly fold: <_T>(leftFolder: Mapper<E, _T>, rightFolder: Mapper<T, _T>) => _T;
     readonly left: Supplier<IOptional<E>>;
     readonly right: Supplier<IOptional<T>>;
-    readonly moveRight: <_T>(t: _T) => IEither<E, _T>;
+
     readonly mapRight: <_T>(mapper: Mapper<T, _T>) => IEither<E, _T>;
+    readonly filter: (mapper: Predicate<T>) => IEither<E, T>;
     readonly mapLeft: <_E>(mapper: Mapper<E, _E>) => IEither<_E, T>;
+    readonly mapBoth: <_E, _T>(errBranch: Mapper<E, _E>, okBranch: Mapper<T, _T>) => IEither<_E, _T>;
+
     readonly flatMap: <_T>(mapper: Mapper<T, IEither<E, _T>>) => IEither<E, _T>;
     readonly flatMapAsync: <_T>(mapper: Mapper<T, Promise<IEither<E, _T>>>) => Promise<IEither<E, _T>>;
+
+    readonly moveRight: <_T>(t: _T) => IEither<E, _T>;
+    readonly fold: <_T>(leftFolder: Mapper<E, _T>, rightFolder: Mapper<T, _T>) => _T;
+    readonly joinRight: <O, _T>(other: IEither<E, O>, mapper: BiMapper<O, T, _T>) => IEither<E, _T>;
+    readonly joinRightAsync: <O, _T>(
+        other: Supplier<Promise<IEither<E, O>>> | Promise<IEither<E, O>>,
+        mapper: BiMapper<O, T, _T>,
+    ) => Promise<IEither<E, _T>>;
 }
 
 const ELeftTag = 'E.Left' as const;
@@ -62,6 +80,11 @@ export class Either<E, T> extends _Tagged implements IEither<E, T> {
         return Either.left<E, _T>(this.self.err);
     }
 
+    public filter(mapper: Predicate<T>): IEither<E, T> {
+        if (isLeft(this.self)) return Either.left<E, T>(this.self.err);
+        return Either.fromFailable<E, T>(() => this.right().filter(mapper).get());
+    }
+
     public async flatMapAsync<_T>(mapper: Mapper<T, Promise<IEither<E, _T>>>): Promise<IEither<E, _T>> {
         if (isLeft(this.self)) return Promise.resolve(Either.left(this.self.err));
         return await mapper(this.self.ok).catch((err) => Either.left(err));
@@ -82,23 +105,18 @@ export class Either<E, T> extends _Tagged implements IEither<E, T> {
         return Optional.none();
     }
 
-    static joinRight<K, E, T>(
-        arr: Array<K>,
-        mapper: BiMapper<K, T, IEither<E, T>>,
-        init: IEither<E, T>,
-    ): IEither<E, T> {
-        return arr.reduce((acc: IEither<E, T>, x: K) => acc.flatMap((t) => mapper(x, t)), init);
+    public joinRight<O, _T>(other: IEither<E, O>, mapper: BiMapper<O, T, _T>) {
+        return this.flatMap((t) => other.mapRight((o) => mapper(o, t)));
     }
 
-    static joinRightAsync<K, E, T>(
-        arr: Array<K>,
-        mapper: BiMapper<K, T, Promise<IEither<E, T>>>,
-        init: IEither<E, T>,
-    ): Promise<IEither<E, T>> {
-        return arr.reduce(
-            (acc: Promise<IEither<E, T>>, x: K) => acc.then((res) => res.flatMapAsync((t) => mapper(x, t))),
-            Promise.resolve(init),
-        );
+    public joinRightAsync<O, _T>(
+        other: Supplier<Promise<IEither<E, O>>> | Promise<IEither<E, O>>,
+        mapper: BiMapper<O, T, _T>,
+    ) {
+        return this.flatMapAsync(async (t) => {
+            const o = typeof other === 'function' ? other() : other;
+            return o.then((other) => other.mapRight((o) => mapper(o, t)));
+        });
     }
 
     static left<E, T>(e: E): IEither<E, T> {
diff --git a/u/types/fn/optional.ts b/u/types/fn/optional.ts
index 3396a45..504e496 100644
--- a/u/types/fn/optional.ts
+++ b/u/types/fn/optional.ts
@@ -1,4 +1,4 @@
-import { type Mapper, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
+import { type Mapper, Predicate, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
 
 export type MaybeGiven<T> = T | undefined | null;
 
@@ -9,7 +9,7 @@ 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: Mapper<T, boolean>) => 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>;
@@ -48,11 +48,11 @@ export class Optional<t, T extends NonNullable<t> = NonNullable<t>> extends _Tag
     }
 
     public get(): T {
-        if (isNone(this.self)) throw new IOptionalEmptyError('empty value');
+        if (isNone(this.self)) throw new IOptionalEmptyError('called get() on None optional');
         return this.self.value;
     }
 
-    public filter(mapper: Mapper<T, boolean>): IOptional<T> {
+    public filter(mapper: Predicate<T>): IOptional<T> {
         if (isNone(this.self) || !mapper(this.self.value)) return Optional.none();
         return Optional.some(this.self.value);
     }