fix(maybe): map fn should return NonNullable<R> (#149)

Author: patrickmichalina

src/maybe/maybe.interface.ts

@@ -76,7 +76,7 @@ export interface IMaybe<T> extends IMonad<T> {
   /**
    * Map output of non-empty data to a new value
    */
-  map<R>(f: (t: T) => R): IMaybe<R>
+  map<R>(f: (t: T) => NonNullable<R>): IMaybe<R>
 
   /**
    * Returns true if value is not empty

src/maybe/maybe.spec.ts

@@ -199,14 +199,15 @@ describe('Maybe', () => {
       const maybeNotSomeSome2 = maybe(sut)
         .map(_str => getUserService<string>(0))
         .valueOr('fallback')
+
       const maybeNotSomeSome3 = maybe(sut)
-        .map(_str => getUserService<string>(''))
+        .map(_str => 'sut')
         .valueOr('fallback')
 
       expect(maybeSomeString).toEqual('initial input mapped')
       expect(maybeNotSomeSomeString).toEqual('fallback')
       expect(maybeNotSomeSome2).toEqual(0)
-      expect(maybeNotSomeSome3).toEqual('')
+      expect(maybeNotSomeSome3).toEqual('sut')
     })
 
     it('should handle undefined input', () => {

src/maybe/maybe.ts

@@ -74,7 +74,7 @@ export class Maybe<T> implements IMaybe<T>  {
         : [this.value as NonNullable<T>]
   }
 
-  public map<R>(fn: (t: NonNullable<T>) => R): IMaybe<R> {
+  public map<R>(fn: (t: NonNullable<T>) => NonNullable<R>): IMaybe<R> {
     return this.isSome()
       ? new Maybe<R>(fn(this.value as NonNullable<T>))
       : new Maybe<R>()
@@ -98,7 +98,7 @@ export class Maybe<T> implements IMaybe<T>  {
         : new Maybe<T>()
   }
 
-  public apply<R>(fab: IMaybe<(v: T) => R>): IMaybe<R> {
+  public apply<R>(fab: IMaybe<(v: T) => NonNullable<R>>): IMaybe<R> {
     return this.flatMap(b => fab.map(fn => fn(b)))
   }
 }