Added functor map method and unit tests

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

@@ -15,7 +15,8 @@ import java.util.function.Function;
 public sealed interface Either<L, R> permits Left, Right {
     /**
      * <p>
-     *      Executing an anonymous function by discriminating the Either data type value
+     *      Executing an anonymous function by discriminating against
+     *      the <i>Either</i> data type value
      * </p>
      *
      * @param onLeft The function to execute on the Left case
@@ -27,7 +28,7 @@ public sealed interface Either<L, R> permits Left, Right {
 
     /**
      * <p>
-     *     Returns true if the Either type is instantiated with the Left subtype,
+     *     Returns true if the <i>Either</i> type is instantiated with the Left subtype,
      *     false otherwise
      * </p>
      * @return Boolean value
@@ -36,10 +37,30 @@ public sealed interface Either<L, R> permits Left, Right {
 
     /**
      * <p>
-     *     Returns true if the Either type is instantiated with the Right subtype,
+     *     Returns true if the Either type is instantiated with the <i>Right</i> subtype,
      *     false otherwise
      * </p>
      * @return Boolean value
      */
     boolean isRight();
+
+    /**
+     * <p>
+     *     Defines a functor. That is, a data type that supports
+     *     a mapping operation defined by the map method.
+     *     <br /><br />
+     *     This method
+     *     applies a function(<i>fn</i>) to the values inside the data type,
+     *     returning a new data type(i.e., a new functor) if and only if the the Either
+     *     type is instantiated with the <i>Right</i> subtype. Otherwise it leaves the functor
+     *     unchanged.
+     *     <br /><br />
+     *     The type of the resulting functor is the return type specified on the <i>fn</i>
+     *     function
+     * </p>
+     * @param fn The function to applies to the Either data type
+     * @return An <i>Either</i> functor
+     * @param <T> The return type of the <i>fn</i> function
+     */
+    <T> Either<L, T> map(Function<R, T> fn);
 }

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

@@ -26,4 +26,9 @@ public record Left<L, R>(L value) implements Either<L, R> {
     public boolean isRight() {
         return false;
     }
+
+    @Override
+    public <T> Either<L, T> map(Function<R, T> fn) {
+        return new Left<>(this.value);
+    }
 }

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

@@ -26,4 +26,9 @@ public record Right<L, R>(R value) implements Either<L, R> {
     public boolean isRight() {
         return true;
     }
+
+    @Override
+    public <T> Either<L, T> map(Function<R, T> fn) {
+        return new Right<>(fn.apply(this.value));
+    }
 }

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

@@ -9,10 +9,12 @@ import static org.junit.jupiter.api.Assertions.assertFalse;
 
 public class LeftTests {
     private Either<Integer, String> resEither;
+    private Either<Error, Integer> numEither;
 
     @BeforeEach
     public void tearUp() {
         this.resEither = new Left<>(19);
+        this.numEither = new Left<>(new Error("Undefined variable"));
     }
 
     @Test
@@ -36,4 +38,14 @@ public class LeftTests {
     public void testIsRight() {
         assertFalse(this.resEither.isRight());
     }
+
+    @Test
+    public void testFunctorMapLeft() {
+        Either<Error, Integer> res = this.numEither.map(x -> x * x);
+
+        assertEquals(res.match(
+            Throwable::getMessage,
+            _ -> "Undefined variable"
+        ), "Undefined variable");
+    }
 }

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

@@ -9,10 +9,12 @@ import static org.junit.jupiter.api.Assertions.assertFalse;
 
 public class RightTests {
     private Either<Integer, String> resEither;
+    private Either<Error, Integer> numEither;
 
     @BeforeEach
     public void tearUp() {
         this.resEither = new Right<>("Query executed successfully");
+        this.numEither = new Right<>(4);
     }
 
     @Test
@@ -36,4 +38,14 @@ public class RightTests {
     public void testIsRight() {
         assertTrue(this.resEither.isRight());
     }
+
+    @Test
+    public void testFunctorMapRight() {
+        Either<Error, Integer> res = this.numEither.map(x -> x * x);
+
+        assertEquals((int)res.match(
+            _ -> 0,
+            x -> x
+        ), 16);
+    }
 }