Add do notation for either

src/lib/util/basic/either.ts

@@ -14,15 +14,55 @@ export const right = <A>(a: A): Either<A, never> => ({
 
 // Operations
 export const eitherMap =
-  <E, A, B>(f: (a: A) => B) => (fa: Either<A, E>): Either<B, E> =>
+  <A, B, E>(f: (a: A) => B) => (fa: Either<A, E>): Either<B, E> =>
     fa._tag === 'Right' ? right(f(fa.right)) : fa
 
 export const eitherChain =
-  <E, A, B>(f: (a: A) => Either<B, E>) =>
+  <A, B, E>(f: (a: A) => Either<B, E>) => (fa: Either<A, E>): Either<B, E> =>
-  (fa: Either<A, E>): Either<B, E> =>
     fa._tag === 'Right' ? f(fa.right) : fa
 
 export const fold =
   <A, B, E>(onLeft: (e: E) => B, onRight: (a: A) => B) =>
   (fa: Either<A, E>): B =>
     fa._tag === 'Left' ? onLeft(fa.left) : onRight(fa.right)
+
+// Do notation
+export class DoEither<T extends Record<string, unknown>, E> {
+  constructor(private readonly either: Either<T, E>) {}
+
+  bind<K extends string, A>(
+    key: K,
+    fa: Either<A, E>,
+  ): DoEither<T & Record<K, A>, E> {
+    const newEither = eitherChain<T, T & Record<K, A>, E>((scope: T) =>
+      eitherMap<A, T & Record<K, A>, E>((a: A) => ({...scope, [key]: a}))(fa)
+    )(this.either)
+    return new DoEither(newEither)
+  }
+
+  return<B>(f: (scope: T) => B): Either<B, E> {
+    return eitherMap<T, B, E>(f)(this.either)
+  }
+
+  done(): Either<T, E> {
+    return this.either
+  }
+
+  do(fa: Either<unknown, E>): DoEither<T, E> {
+    const newEither = eitherChain<T, T, E>((scope: T) =>
+      eitherMap<unknown, T, E>(() => scope)(fa)
+    )(this.either)
+    return new DoEither(newEither)
+  }
+
+  doL(f: (scope: T) => Either<unknown, E>): DoEither<T, E> {
+    const newEither = eitherChain<T, T, E>((scope: T) =>
+      eitherMap<unknown, T, E>(() => scope)(f(scope))
+    )(this.either)
+    return new DoEither(newEither)
+  }
+
+  static start<E>(): DoEither<{}, E> {
+    return new DoEither(right({}))
+  }
+}