Added swap method and unit tests

src/main/java/com/ceticamarco/lambdatonic/Either.java

@@ -70,7 +70,7 @@ public sealed interface Either<L, R> permits Left, Right {
      *     Applies <i>onLeft</i> function to the <i>Left</i> subtype or the
      *     <i>onRight</i> function to the <i>Right</i> subtype.
      * </p>
-     * @param onLeft The function to apply to the <i>Left</i> subtyp
+     * @param onLeft The function to apply to the <i>Left</i> subtype
      * @param onRight The function to apply to the <i>Right</i> subtype
      * @return An <i>Either</i> functor
      * @param <T> The return type of the <i>onLeft</i> function
@@ -109,4 +109,12 @@ public sealed interface Either<L, R> permits Left, Right {
      * @return An <i>Optional</i> data type
      */
     Optional<R> toOptional();
+
+    /**
+     * <p>
+     *     Flips the <i>Left</i> and the <i>Right</i> data types.
+     * </p>
+     * @return An <i>Either</i> monad with <i>Left</i> and <i>Right</i> flipped
+     */
+    Either<R, L> swap();
 }

src/main/java/com/ceticamarco/lambdatonic/Left.java

@@ -52,4 +52,9 @@ public record Left<L, R>(L value) implements Either<L, R> {
     public Optional<R> toOptional() {
         return Optional.empty();
     }
+
+    @Override
+    public Either<R, L> swap() {
+        return new Right<>(this.value);
+    }
 }

src/main/java/com/ceticamarco/lambdatonic/Right.java

@@ -52,4 +52,9 @@ public record Right<L, R>(R value) implements Either<L, R> {
     public Optional<R> toOptional() {
         return Optional.of(this.value);
     }
+
+    @Override
+    public Either<R, L> swap() {
+        return new Left<>(this.value);
+    }
 }

src/test/java/com/ceticamarco/lambdatonic/LeftTests.java

@@ -103,4 +103,12 @@ public class LeftTests {
             Optional.empty()
         );
     }
+
+    @Test
+    public void testSwapFromLeft() {
+        assertEquals(
+                this.resEither.swap(),
+                new Right<>(19)
+        );
+    }
 }

src/test/java/com/ceticamarco/lambdatonic/RightTests.java

@@ -103,4 +103,12 @@ public class RightTests {
             Optional.of(4)
         );
     }
+
+    @Test
+    public void testSwapFromRight() {
+        assertEquals(
+            this.numEither.swap(),
+            new Left<>(4)
+        );
+    }
 }