feat: add Result.fromMaybe

Author: tdreyno

src/monads/Result.ts

@@ -9,6 +9,7 @@ import {
   isLeft,
   isRight,
 } from "./Either"
+import { Maybe, fold as maybeFold } from "./Maybe"
 
 export type Ok<B> = Right<B>
 export type Err<A> = Left<A>
@@ -31,6 +32,9 @@ export const cata = <A, B, U>(handlers: {
 export const fold = <A, B, U>(errorFn: (a: A) => U, okFn: (b: B) => U) =>
   fold_(errorFn, okFn) as (result: Result<A, B>) => U
 
+export const fromMaybe = <B>(maybe: Maybe<B>) =>
+  maybeFold(() => Err(null), Ok)(maybe) as Result<null, B>
+
 // Chain
 export const chain = <A, B, U>(fn: (a: B) => Result<A, U>) =>
   chain_(fn) as (result: Result<A, B>) => Result<A, U>

src/monads/__tests__/Result.spec.ts

@@ -9,7 +9,9 @@ import {
   orElse,
   isOk,
   isError,
+  fromMaybe,
 } from "../Result"
+import { Nothing, Just } from "../Maybe"
 
 describe("Result", () => {
   test("Ok", () => {
@@ -64,6 +66,10 @@ describe("Result", () => {
   test("chain(Err)", () =>
     expect(chain(jest.fn())(Err("whoops"))).toEqual(Err("whoops")))
 
+  test("fromMaybe(Nothing)", () =>
+    expect(fromMaybe(Nothing())).toEqual(Err(null)))
+  test("fromMaybe(Just)", () => expect(fromMaybe(Just(5))).toEqual(Ok(5)))
+
   test("orElse", () => expect(orElse(() => 10)(Err("whoops"))).toEqual(Ok(10)))
 
   test("isOk(Ok)", () => expect(isOk(Ok(5))).toBe(true))