Merge pull request #179 from Kevin-Lee/task/170/add-Either.forEachLeft

Close #170 - Add Either.forEachLeft

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

@@ -40,6 +40,8 @@ public abstract <C> C fold(
 
   public abstract void forEach(Consumer<? super B> f);
 
+  public abstract void forEachLeft(Consumer<? super A> f);
+
   public abstract Optional<B> toOptional();
 
   public Maybe<B> toMaybe() {
@@ -151,6 +153,11 @@ public A getLeftOrElse(final Supplier<A> alternative) {
     public void forEach(final Consumer<? super B> f) {
     }
 
+    @Override
+    public void forEachLeft(final Consumer<? super A> f) {
+      f.accept(this.value);
+    }
+
     @Override
     public Optional<B> toOptional() {
       return Optional.empty();
@@ -249,6 +256,10 @@ public void forEach(final Consumer<? super B> f) {
       f.accept(this.value);
     }
 
+    @Override
+    public void forEachLeft(final Consumer<? super A> f) {
+    }
+
     @Override
     public Optional<B> toOptional() {
       return Optional.ofNullable(value);

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

@@ -423,6 +423,35 @@ public void forEach_RightCase() {
       );
   }
 
+  @Test
+  public void forEachLeft_LeftCase() {
+    final String input = "Some error";
+    final String expected = input;
+    final String[] updated = { null };
+    test("Either.forEachLeft on Left", "Either.forEachLeft on Left should apply the given function with side-effect")
+      .when(() ->
+        Either.<String, Integer>left(input)
+          .forEachLeft(err -> updated[0] = err)
+      )
+      .then(() ->
+        assertThat(updated[0]).isEqualTo(expected)
+      );
+  }
+
+  @Test
+  public void forEachLeft_RightCase() {
+    final Integer input = 999;
+    final String[] updated = { null };
+    test("Either.forEachLeft on Right", "Either.forEachLeft on Right should do nothing")
+      .when(() ->
+        Either.<String, Integer>right(input)
+          .forEachLeft(err -> updated[0] = err)
+      )
+      .then(() ->
+        assertThat(updated[0]).isNull()
+      );
+  }
+
   @Test
   public void toOptional_LeftCase() {
     final String leftValue = "Some Error";