feat: introduce Maybe.mapTo<R>() (#178)

Author: patrickmichalina

src/maybe/maybe.interface.ts

@@ -82,6 +82,11 @@ export interface IMaybe<T> extends IMonad<T> {
    */
   map<R>(f: (t: T) => NonNullable<R>): IMaybe<R>
 
+  /**
+   * Map to a new value while ignoring previous output value but respecting maybeness
+   */
+  mapTo<R>(v: NonNullable<R>): IMaybe<R>
+
   /**
    * Returns true if value is not empty
    */

src/maybe/maybe.spec.ts

@@ -548,6 +548,13 @@ describe('Maybe', () => {
     })
   })
 
+  describe('mapTo', () => {
+    it('should return new maybe with some', () => {
+      expect(Maybe.some(1).mapTo('deltaforce').valueOrThrowErr()).toEqual('deltaforce')
+      expect(Maybe.none().mapTo('deltaforce').valueOrNull()).toEqual(null)
+    })
+  })
+
   describe('toResult', () => {
     it('should return result object with success', () => {
       const hasSome = maybe('hi')

src/maybe/maybe.ts

@@ -86,6 +86,12 @@ export class Maybe<T> implements IMaybe<T>  {
       : new Maybe<R>()
   }
 
+  public mapTo<R>(t: NonNullable<R>): IMaybe<R> {
+    return this.isSome()
+      ? new Maybe<R>(t)
+      : new Maybe<R>()
+  }
+
   public flatMap<R>(fn: (d: NonNullable<T>) => IMaybe<R>): IMaybe<R> {
     return this.isNone() ? new Maybe<R>() : fn(this.value as NonNullable<T>)
   }