Merge pull request #64 from Kevin-Lee/task/61/add-from-maybe

Close #61 - Add fromMaybe to Either

src/main/java/j8plus/types/Either.java

@@ -59,6 +59,13 @@ public static <A, B> Either<A, B> fromOptional(
     }
   }
 
+  public static <A, B> Either<A, B> fromMaybe(
+    final Maybe<? extends B> maybe
+  , final Supplier<? extends A> ifNone
+  ) {
+    return maybe.fold(Either::right, () -> Either.left(ifNone.get()));
+  }
+
   static final class Left<A, B> extends Either<A, B> {
     final A value;
 

src/test/java/j8plus/types/EitherTest.java

@@ -65,6 +65,32 @@ public void fromOptional_EmptyCase() {
       );
   }
 
+  @Test
+  public void fromMaybe_JustCase() {
+    final Integer expectedValue = 999;
+    final Maybe<Integer> input = Maybe.just(expectedValue);
+    test("Either.fromMaybe(just)", "Either.fromMaybe(Just(someValue)) should create Right")
+      .when(() ->
+        Either.fromMaybe(input, () -> "Some error")
+      )
+      .then(actual ->
+        assertThat(actual).isEqualTo(Either.right(expectedValue))
+      );
+  }
+
+  @Test
+  public void fromMaybe_NothingCase() {
+    final String expectedValue =  "Some error";
+    final Maybe<Integer> input = Maybe.nothing();
+    test("Either.fromMaybe(Nothing)", "Either.fromMaybe(Nothing) should create Left")
+      .when(() ->
+        Either.fromMaybe(input, () -> expectedValue)
+      )
+      .then(actual ->
+        assertThat(actual).isEqualTo(Either.left(expectedValue))
+      );
+  }
+
   @Test
   public void isLeft_LeftCase() {
     test("Either.isLeft", "Either.isLeft should return true for Left")