1 files changed, 25 insertions, 6 deletions

diff --git a/u/fn/either.ts b/u/fn/either.ts
index 12240d0..916bb71 100644
--- a/u/fn/either.ts
+++ b/u/fn/either.ts
@@ -5,37 +5,56 @@ export interface IEither<E, T> {
     errBranch: Mapper<E, Ee>,
     okBranch: Mapper<T, Tt>,
   ) => IEither<Ee, Tt>;
-  flatMap: <Tt>(mapper: Mapper<T, IEither<E, Tt>>) => IEither<E, Tt>;
+  moveRight: <Tt>(t: Tt) => IEither<E, Tt>;
   mapRight: <Tt>(mapper: Mapper<T, Tt>) => IEither<E, Tt>;
   mapLeft: <Ee>(mapper: Mapper<E, Ee>) => IEither<Ee, T>;
+  flatMap: <Tt>(mapper: Mapper<T, IEither<E, Tt>>) => IEither<E, Tt>;
+  flatMapAsync: <Tt>(
+    mapper: Mapper<T, Promise<IEither<E, Tt>>>,
+  ) => Promise<IEither<E, Tt>>;
 }
 
 export class Either<E, T> implements IEither<E, T> {
   private constructor(private readonly err?: E, private readonly ok?: T) {}
 
+  public moveRight<Tt>(
+    t: Tt,
+  ) {
+    return this.mapRight(() => t);
+  }
+
   public mapBoth<Ee, Tt>(
     errBranch: Mapper<E, Ee>,
     okBranch: Mapper<T, Tt>,
   ): Either<Ee, Tt> {
-    if (this.err) return Either.left(errBranch(this.err));
+    if (this.err !== undefined) return Either.left(errBranch(this.err));
     return Either.right(okBranch(this.ok!));
   }
 
   public flatMap<Tt>(mapper: Mapper<T, Either<E, Tt>>): Either<E, Tt> {
-    if (this.ok) return mapper(this.ok);
+    if (this.ok !== undefined) return mapper(this.ok);
     return Either.left<E, Tt>(this.err!);
   }
 
   public mapRight<Tt>(mapper: Mapper<T, Tt>): IEither<E, Tt> {
-    if (this.ok) return Either.right(mapper(this.ok));
+    if (this.ok !== undefined) return Either.right(mapper(this.ok));
     return Either.left<E, Tt>(this.err!);
   }
 
-  public mapLeft<Ee>(mapper: Mapper<E, Ee>) {
-    if (this.err) return Either.left<Ee, T>(mapper(this.err));
+  public mapLeft<Ee>(mapper: Mapper<E, Ee>): IEither<Ee, T> {
+    if (this.err !== undefined) return Either.left<Ee, T>(mapper(this.err));
     return Either.right<Ee, T>(this.ok!);
   }
 
+  public async flatMapAsync<Tt>(
+    mapper: Mapper<T, Promise<IEither<E, Tt>>>,
+  ): Promise<IEither<E, Tt>> {
+    if (this.err !== undefined) {
+      return Promise.resolve(Either.left<E, Tt>(this.err));
+    }
+    return await mapper(this.ok!).catch((err) => Either.left<E, Tt>(err as E));
+  }
+
   static left<E, T>(e: E) {
     return new Either<E, T>(e);
   }