feat(transform): add mapOrElse transform function

operators/transform.ts

@@ -46,3 +46,24 @@ export function mapOr<T, U>(
 
   return defaultValue;
 }
+
+/** Computes a default function result (if none), or applies a different function to the contained value (if any).
+ *
+ * @example
+ * ```ts
+ * import { mapOrElse, Option, Some } from "https://deno.land/x/optio/mod.ts";
+ * import { assertEquals } from "https://deno.land/std/testing/asserts.ts";
+ *
+ * const option: Option<string> = Some.of("Hello");
+ * assertEquals(mapOrElse(option, () => 2 ** 3, ({ length }) => length), 5);
+ * ```
+ */
+export function mapOrElse<T, U>(
+  option: Option<T>,
+  defaultFn: () => U,
+  fn: (value: T) => U,
+): U {
+  if (isSome(option)) return fn(option.get);
+
+  return defaultFn();
+}

operators/transform_test.ts

@@ -1,6 +1,6 @@
 // Copyright © 2023 Tomoki Miyauchi. All rights reserved. MIT license.
 
-import { map, mapOr } from "./transform.ts";
+import { map, mapOr, mapOrElse } from "./transform.ts";
 import { None, Option, Some } from "../spec.ts";
 import {
   assertEquals,
@@ -54,3 +54,29 @@ describe("mapOr", () => {
     assertSpyCalls(fn, 0);
   });
 });
+
+describe("mapOrElse", () => {
+  it("should call mapper if it is some", () => {
+    const INPUT = "Hello, World!";
+    const option: Option<string> = Some.of(INPUT);
+    const fn = spy((v: string) => v.length);
+    const defaultFn = spy(() => 0);
+    const optionLen = mapOrElse(option, defaultFn, fn);
+
+    assertEquals(optionLen, INPUT.length);
+    assertSpyCalls(fn, 1);
+    assertSpyCallArgs(fn, 0, [INPUT]);
+    assertSpyCalls(defaultFn, 0);
+  });
+
+  it("should return default value if it is none", () => {
+    const option: Option<string> = None;
+    const fn = spy((v: string) => v.length);
+    const defaultFn = spy(() => 0);
+    const optionLen = mapOrElse(option, defaultFn, fn);
+
+    assertEquals(optionLen, 0);
+    assertSpyCalls(fn, 0);
+    assertSpyCalls(defaultFn, 1);
+  });
+});