feat: add Result type

Also derived from Either

src/monads/Maybe.ts

@@ -9,9 +9,10 @@ import {
   isLeft,
   isRight,
 } from "./Either"
+import { Result, fold as resultFold } from "./Result"
 
-type Just<T> = Right<T>
-type Nothing<T = unknown> = Left<null>
+export type Just<T> = Right<T>
+export type Nothing<T = unknown> = Left<null>
 export type Maybe<T> = Nothing<T> | Just<T>
 
 export const Just = <T>(value: T): Maybe<T> => Right(value)
@@ -35,6 +36,9 @@ export const cata = <T, U>(handlers: { Nothing: () => U; Just: (v: T) => U }) =>
 export const fold = <T, U>(nothingFn: () => U, justFn: (v: T) => U) =>
   fold_<null, T, U>(nothingFn, justFn) as (maybe: Maybe<T>) => U
 
+export const fromResult = <A, B>(result: Result<A, B>) =>
+  resultFold(Nothing, Just)(result) as Maybe<B>
+
 // Chain
 export const chain = <T, U>(fn: (a: T) => Maybe<U>) =>
   chain_(fn) as (maybe: Maybe<T>) => Maybe<U>

src/monads/Result.ts

@@ -0,0 +1,50 @@
+import { identity, pipe } from "../core/index"
+import {
+  Right,
+  Left,
+  cata as cata_,
+  fold as fold_,
+  chain as chain_,
+  map as map_,
+  isLeft,
+  isRight,
+} from "./Either"
+
+export type Ok<B> = Right<B>
+export type Err<A> = Left<A>
+export type Result<A, B> = Err<A> | Ok<B>
+
+export const Ok = <B, A = unknown>(value: B): Result<A, B> => Right(value)
+export const Err = <A, B = unknown>(value: A): Result<A, B> => Left(value)
+
+export const of = Ok
+
+export const cata = <A, B, U>(handlers: {
+  Err: (a: A) => U
+  Ok: (v: B) => U
+}) =>
+  cata_({
+    Left: handlers.Err,
+    Right: handlers.Ok,
+  }) as (result: Result<A, B>) => U
+
+export const fold = <A, B, U>(errorFn: (a: A) => U, okFn: (b: B) => U) =>
+  fold_(errorFn, okFn) as (result: Result<A, B>) => U
+
+// Chain
+export const chain = <A, B, U>(fn: (a: B) => Result<A, U>) =>
+  chain_(fn) as (result: Result<A, B>) => Result<A, U>
+
+// Functor
+export const map = <A, B, U>(fn: (a: B) => U) =>
+  map_(fn) as (result: Result<A, B>) => Result<A, U>
+
+// Error handling
+export const orElse = <A, B>(fn: (a: A) => B) =>
+  pipe(fold<A, B, B>(fn, identity), (v: B) => Ok<B, A>(v))
+
+export const isOk = <A, B>(result: Result<A, B>): result is Ok<B> =>
+  isRight(result)
+
+export const isError = <A, B>(result: Result<A, B>): result is Err<A> =>
+  isLeft(result)

src/monads/__tests__/Maybe.spec.ts

@@ -11,7 +11,9 @@ import {
   orElse,
   isJust,
   isNothing,
+  fromResult,
 } from "../Maybe"
+import { Err, Ok } from "../Result"
 
 describe("Maybe", () => {
   test("Just", () => {
@@ -41,6 +43,10 @@ describe("Maybe", () => {
     expect(fromNullable(null)).toEqual(Nothing()))
   test("fromNullable(5)", () => expect(fromNullable(5)).toEqual(Just(5)))
 
+  test("fromResult(Err)", () =>
+    expect(fromResult(Err("whoops"))).toEqual(Nothing()))
+  test("fromResult(Ok)", () => expect(fromResult(Ok(5))).toEqual(Just(5)))
+
   test("cata(Just)", () => {
     const J = jest.fn()
     const N = jest.fn()

src/monads/__tests__/Result.spec.ts

@@ -0,0 +1,74 @@
+import {
+  Ok,
+  Err,
+  cata,
+  fold,
+  of,
+  map,
+  chain,
+  orElse,
+  isOk,
+  isError,
+} from "../Result"
+
+describe("Result", () => {
+  test("Ok", () => {
+    const O = jest.fn()
+    const E = jest.fn()
+
+    fold(E, O)(Ok(5))
+
+    expect(O).toBeCalledWith(5)
+    expect(E).not.toHaveBeenCalled()
+  })
+
+  test("Err", () => {
+    const O = jest.fn()
+    const E = jest.fn()
+
+    fold(E, O)(Err("whoops"))
+
+    expect(O).not.toHaveBeenCalled()
+    expect(E).toHaveBeenCalled()
+  })
+
+  test("of", () => expect(of(5)).toEqual(Ok(5)))
+
+  test("cata(Ok)", () => {
+    const O = jest.fn()
+    const E = jest.fn()
+
+    cata({ Ok: O, Err: E })(Ok(5))
+
+    expect(O).toBeCalledWith(5)
+    expect(E).not.toHaveBeenCalled()
+  })
+
+  test("cata(Err)", () => {
+    const O = jest.fn()
+    const E = jest.fn()
+
+    cata({ Ok: O, Err: E })(Err("whoops"))
+
+    expect(O).not.toHaveBeenCalled()
+    expect(E).toHaveBeenCalled()
+  })
+
+  test("map(Ok)", () =>
+    expect(map((v: number) => v * 2)(Ok(5))).toEqual(Ok(10)))
+  test("map(Err)", () =>
+    expect(map(jest.fn())(Err("whoops"))).toEqual(Err("whoops")))
+
+  test("chain(Ok)", () =>
+    expect(chain((v: number) => Ok(v * 2))(Ok(5))).toEqual(Ok(10)))
+  test("chain(Err)", () =>
+    expect(chain(jest.fn())(Err("whoops"))).toEqual(Err("whoops")))
+
+  test("orElse", () => expect(orElse(() => 10)(Err("whoops"))).toEqual(Ok(10)))
+
+  test("isOk(Ok)", () => expect(isOk(Ok(5))).toBe(true))
+  test("isOk(Err)", () => expect(isOk(Err("whoops"))).toBe(false))
+
+  test("isError(Ok)", () => expect(isError(Ok(5))).toBe(false))
+  test("isError(Err)", () => expect(isError(Err("whoops"))).toBe(true))
+})