1 files changed, 22 insertions, 6 deletions
diff --git a/u/fn/either.ts b/u/fn/either.ts
index 916bb71..9dc1027 100644
--- a/u/fn/either.ts
+++ b/u/fn/either.ts
@@ -1,10 +1,16 @@
-import type { Mapper, Supplier } from "@emprespresso/pengueno";
+import type { BiMapper, Mapper, Supplier } from "@emprespresso/pengueno";
+import { isObject } from "../leftpadesque/mod.ts";
+
+type IEitherTag = "IEither";
+const iEitherTag: IEitherTag = "IEither";
 
 export interface IEither<E, T> {
+  readonly _tag: IEitherTag;
   mapBoth: <Ee, Tt>(
     errBranch: Mapper<E, Ee>,
     okBranch: Mapper<T, Tt>,
   ) => IEither<Ee, Tt>;
+  fold: <Tt>(folder: BiMapper<E | null, T | null, Tt>) => 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>;
@@ -15,14 +21,20 @@ export interface IEither<E, T> {
 }
 
 export class Either<E, T> implements IEither<E, T> {
-  private constructor(private readonly err?: E, private readonly ok?: T) {}
+  private constructor(
+    private readonly err?: E,
+    private readonly ok?: T,
+    public readonly _tag: IEitherTag = iEitherTag,
+  ) {}
 
-  public moveRight<Tt>(
-    t: Tt,
-  ) {
+  public moveRight<Tt>(t: Tt) {
     return this.mapRight(() => t);
   }
 
+  public fold<R>(folder: BiMapper<E | null, T | null, R>): R {
+    return folder(this.err ?? null, this.ok ?? null);
+  }
+
   public mapBoth<Ee, Tt>(
     errBranch: Mapper<E, Ee>,
     okBranch: Mapper<T, Tt>,
@@ -37,7 +49,7 @@ export class Either<E, T> implements IEither<E, T> {
   }
 
   public mapRight<Tt>(mapper: Mapper<T, Tt>): IEither<E, Tt> {
-    if (this.ok !== undefined) return Either.right(mapper(this.ok));
+    if (this.ok !== undefined) return Either.right<E, Tt>(mapper(this.ok));
     return Either.left<E, Tt>(this.err!);
   }
 
@@ -79,3 +91,7 @@ export class Either<E, T> implements IEither<E, T> {
     }
   }
 }
+
+export const isEither = <E, T>(o: unknown): o is IEither<E, T> => {
+  return isObject(o) && "_tag" in o && o._tag === "IEither";
+};