From f008c002915ee19b5bf93b6cae648f10ca861a22 Mon Sep 17 00:00:00 2001
From: Elizabeth Hunt <me@liz.coffee>
Date: Thu, 3 Jul 2025 13:14:29 -0700
Subject: Adds join functions to Eithers

---
 u/types/fn/either.ts | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

(limited to 'u/types')

diff --git a/u/types/fn/either.ts b/u/types/fn/either.ts
index 6140ada..aa67d41 100644
--- a/u/types/fn/either.ts
+++ b/u/types/fn/either.ts
@@ -1,4 +1,4 @@
-import { IOptional, type Mapper, Optional, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
+import { BiMapper, IOptional, type Mapper, Optional, type Supplier, Tagged, isTagged } from '@emprespresso/pengueno';
 
 export const IEitherTag = 'IEither' as const;
 export type IEitherTag = typeof IEitherTag;
@@ -82,6 +82,25 @@ 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);
+    }
+
+    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),
+        );
+    }
+
     static left<E, T>(e: E): IEither<E, T> {
         return new Either({ err: e, _tag: ELeftTag });
     }
-- 
cgit v1.2.3-70-g09d2