feat: add Either

Refactor Maybe on top of Either

src/monads/Either.ts

@@ -0,0 +1,60 @@
+import { pipe } from "../core/index"
+
+export interface Left<A> {
+  readonly type: "Left"
+  readonly value: A
+}
+
+export interface Right<B> {
+  readonly type: "Right"
+  readonly value: B
+}
+
+export type Either<A, B> = Left<A> | Right<B>
+
+export const Left = <A, B = unknown>(value: A): Either<A, B> => ({
+  type: "Left",
+  value,
+})
+
+export const Right = <B, A = unknown>(value: B): Either<A, B> => ({
+  type: "Right",
+  value,
+})
+
+export const cata = <A, B, R>(handlers: {
+  Left: (v: A) => R
+  Right: (v: B) => R
+}) => (either: Either<A, B>): R => {
+  switch (either.type) {
+    case "Left":
+      return handlers.Left(either.value)
+
+    case "Right":
+      return handlers.Right(either.value)
+  }
+}
+
+export const fold = <A, B, U>(LeftFn: (v: A) => U, RightFn: (v: B) => U) =>
+  cata({ Left: LeftFn, Right: RightFn })
+
+// Bimap
+export const bimap = <A, B, C, D>(LeftFn: (v: A) => B, RightFn: (v: C) => D) =>
+  fold<A, C, Either<B, D>>(
+    pipe<A, B, Either<B, D>>(LeftFn, Left),
+    pipe<C, D, Either<B, D>>(RightFn, Right),
+  )
+
+// Chain
+export const chain = <A, B, B2>(fn: (a: B) => Either<A, B2>) =>
+  fold<A, B, Either<A, B2>>(Left, fn)
+
+// Functor
+export const map = <A, B, B2>(fn: (a: B) => B2) =>
+  chain<A, B, B2>(pipe<B, B2, Either<A, B2>>(fn, Right))
+
+export const isLeft = <A, B>(either: Either<A, B>): either is Left<A> =>
+  either.type === "Left"
+
+export const isRight = <A, B>(either: Either<A, B>): either is Right<B> =>
+  either.type === "Right"

src/monads/Maybe.ts

@@ -1,18 +1,21 @@
 import { identity, pipe } from "../core/index"
-
-export interface Just<T> {
-  readonly type: "Just"
-  readonly value: T
-}
-
-export interface Nothing<T = unknown> {
-  readonly type: "Nothing"
-}
-
-export type Maybe<T> = Just<T> | Nothing<T>
-
-export const Just = <T>(value: T): Maybe<T> => ({ type: "Just", value })
-export const Nothing = <T>(): Maybe<T> => ({ type: "Nothing" })
+import {
+  Right,
+  Left,
+  cata as cata_,
+  fold as fold_,
+  chain as chain_,
+  map as map_,
+  isLeft,
+  isRight,
+} from "./Either"
+
+type Just<T> = Right<T>
+type Nothing<T = unknown> = Left<null>
+export type Maybe<T> = Nothing<T> | Just<T>
+
+export const Just = <T>(value: T): Maybe<T> => Right(value)
+export const Nothing = <T>(): Maybe<T> => Left(null)
 
 // Monoid
 export const empty = Nothing
@@ -21,40 +24,29 @@ export const empty = Nothing
 export const of = Just
 
 export const fromNullable = <T>(valueOrNull: T | null | undefined) =>
-  valueOrNull ? Just<T>(valueOrNull) : Nothing()
-
-export const cata = <T, U>(handlers: {
-  Nothing: () => U
-  Just: (v: T) => U
-}) => (maybe: Maybe<T>): U => {
-  switch (maybe.type) {
-    case "Just":
-      return handlers.Just(maybe.value)
-
-    case "Nothing":
-      return handlers.Nothing()
-  }
-}
+  valueOrNull ? Just<T>(valueOrNull) : Nothing<T>()
 
-export const fold = <T, U>(justFn: (v: T) => U, nothingFn: () => U) =>
-  cata({ Just: justFn, Nothing: nothingFn })
+export const cata = <T, U>(handlers: { Nothing: () => U; Just: (v: T) => U }) =>
+  cata_({
+    Left: handlers.Nothing,
+    Right: handlers.Just,
+  }) as (maybe: Maybe<T>) => U
 
-// Bimap
-export const bimap = <T, U>(justFn: (v: T) => U, nothingFn: () => U) =>
-  pipe(fold(justFn, nothingFn), Just)
+export const fold = <T, U>(nothingFn: () => U, justFn: (v: T) => U) =>
+  fold_<null, T, U>(nothingFn, justFn) as (maybe: Maybe<T>) => U
 
 // Chain
 export const chain = <T, U>(fn: (a: T) => Maybe<U>) =>
-  fold<T, Maybe<U>>(fn, Nothing)
+  chain_(fn) as (maybe: Maybe<T>) => Maybe<U>
 
 // Functor
-export const map = <T, U>(fn: (a: T) => U) => chain<T, U>(pipe(fn, Just))
+export const map = <T, U>(fn: (a: T) => U) =>
+  map_(fn) as (maybe: Maybe<T>) => Maybe<U>
 
 // Error handling
-export const orElse = <T>(fn: () => T) => bimap<T, T>(identity, fn)
+export const orElse = <T>(fn: () => T) => pipe(fold<T, T>(fn, identity), Just)
 
-export const isJust = <T>(maybe: Maybe<T>): maybe is Just<T> =>
-  maybe.type === "Just"
+export const isJust = <T>(maybe: Maybe<T>): maybe is Just<T> => isRight(maybe)
 
 export const isNothing = <T>(maybe: Maybe<T>): maybe is Nothing<T> =>
-  maybe.type === "Nothing"
+  isLeft(maybe)

src/monads/__tests__/Either.spec.ts

@@ -0,0 +1,83 @@
+import {
+  Right,
+  Left,
+  cata,
+  fold,
+  bimap,
+  map,
+  chain,
+  isRight,
+  isLeft,
+} from "../Either"
+
+describe("Either", () => {
+  test("Right", () => {
+    const L = jest.fn()
+    const R = jest.fn()
+
+    fold(L, R)(Right(5))
+
+    expect(R).toBeCalledWith(5)
+    expect(L).not.toHaveBeenCalled()
+  })
+
+  test("Left", () => {
+    const L = jest.fn()
+    const R = jest.fn()
+
+    fold(L, R)(Left(5))
+
+    expect(R).not.toHaveBeenCalled()
+    expect(L).toHaveBeenCalledWith(5)
+  })
+
+  test("cata(Right)", () => {
+    const L = jest.fn()
+    const R = jest.fn()
+
+    cata({ Left: L, Right: R })(Right(5))
+
+    expect(R).toBeCalledWith(5)
+    expect(L).not.toHaveBeenCalled()
+  })
+
+  test("cata(Left)", () => {
+    const L = jest.fn()
+    const R = jest.fn()
+
+    cata({ Left: L, Right: R })(Left(5))
+
+    expect(R).not.toHaveBeenCalled()
+    expect(L).toHaveBeenCalledWith(5)
+  })
+
+  test("bimap(Right)", () => {
+    const L = jest.fn()
+
+    expect(bimap(L, (v: number) => v * 2)(Right(5))).toEqual(Right(10))
+
+    expect(L).not.toHaveBeenCalled()
+  })
+
+  test("bimap(Left)", () => {
+    const R = jest.fn()
+
+    expect(bimap(() => 10, R)(Left(5))).toEqual(Left(10))
+
+    expect(R).not.toHaveBeenCalled()
+  })
+
+  test("map(Right)", () =>
+    expect(map((v: number) => v * 2)(Right(5))).toEqual(Right(10)))
+  test("map(Left)", () => expect(map(jest.fn())(Left(5))).toEqual(Left(5)))
+
+  test("chain(Right)", () =>
+    expect(chain((v: number) => Right(v * 2))(Right(5))).toEqual(Right(10)))
+  test("chain(Left)", () => expect(chain(jest.fn())(Left(5))).toEqual(Left(5)))
+
+  test("isRight(Right)", () => expect(isRight(Right(5))).toBe(true))
+  test("isRight(Left)", () => expect(isRight(Left(5))).toBe(false))
+
+  test("isLeft(Right)", () => expect(isLeft(Right(5))).toBe(false))
+  test("isLeft(Left)", () => expect(isLeft(Left(5))).toBe(true))
+})

src/monads/__tests__/Maybe.spec.ts

@@ -6,7 +6,6 @@ import {
   empty,
   of,
   fromNullable,
-  bimap,
   map,
   chain,
   orElse,
@@ -19,7 +18,7 @@ describe("Maybe", () => {
     const J = jest.fn()
     const N = jest.fn()
 
-    fold(J, N)(Just(5))
+    fold(N, J)(Just(5))
 
     expect(J).toBeCalledWith(5)
     expect(N).not.toHaveBeenCalled()
@@ -29,7 +28,7 @@ describe("Maybe", () => {
     const J = jest.fn()
     const N = jest.fn()
 
-    fold(J, N)(Nothing())
+    fold(N, J)(Nothing())
 
     expect(J).not.toHaveBeenCalled()
     expect(N).toHaveBeenCalled()
@@ -62,22 +61,6 @@ describe("Maybe", () => {
     expect(N).toHaveBeenCalled()
   })
 
-  test("bimap(Just)", () => {
-    const N = jest.fn()
-
-    expect(bimap((v: number) => v * 2, N)(Just(5))).toEqual(Just(10))
-
-    expect(N).not.toHaveBeenCalled()
-  })
-
-  test("bimap(Nothing)", () => {
-    const J = jest.fn()
-
-    expect(bimap(J, () => 10)(Nothing())).toEqual(Just(10))
-
-    expect(J).not.toHaveBeenCalled()
-  })
-
   test("map(Just)", () =>
     expect(map((v: number) => v * 2)(Just(5))).toEqual(Just(10)))
   test("map(Nothing)", () =>