Adding more tests

training/src/main/java/info/jab/fp/util/Either.java

@@ -91,7 +91,7 @@ default <U> Either<L, U> map(Function<? super R, ? extends U> mapper) {
  * @param <U> the type of the new Right value
  * @return a new Either instance
  */
- default <U> Either<L, U> flatMap(Function<? super R, Either<L, U>> mapper) {
+ default <U> Either<L, U> flatMap(Function<? super R, ? extends Either<L, U>> mapper) {
  if (isRight()) {
  return mapper.apply(((Right<L, R>) this).value());
  } else {

training/src/test/java/info/jab/fp/util/EitherMonadLawsTest.java

@@ -0,0 +1,45 @@
+package info.jab.fp.util;
+
+import java.util.function.Function;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import org.junit.jupiter.api.Test;
+
+class EitherMonadLawsTest {
+
+ // Left Identity: return a >>= f ≡ f a
+ @Test
+ void testLeftIdentity() {
+ Function<Integer, Either<String, Integer>> f = x -> Either.right(x + 1);
+ Integer a = 42;
+
+ Either<String, Integer> leftIdentity = Either.<String, Integer>right(a).flatMap(f);
+ Either<String, Integer> expected = f.apply(a);
+
+ assertEquals(expected, leftIdentity);
+ }
+
+ // Right Identity: m >>= return ≡ m
+ @Test
+ void testRightIdentity() {
+ Either<String, Integer> m = Either.right(42);
+
+ Either<String, Integer> rightIdentity = m.flatMap(Either::right);
+
+ assertEquals(m, rightIdentity);
+ }
+
+ // Associativity: (m >>= f) >>= g ≡ m >>= (\x -> f x >>= g)
+ @Test
+ void testAssociativity() {
+ Function<Integer, Either<String, Integer>> f = x -> Either.right(x + 1);
+ Function<Integer, Either<String, Integer>> g = x -> Either.right(x * 2);
+
+ Either<String, Integer> m = Either.right(42);
+
+ Either<String, Integer> leftAssociative = m.flatMap(f).flatMap(g);
+ Either<String, Integer> rightAssociative = m.flatMap(x -> f.apply(x).flatMap(g));
+
+ assertEquals(leftAssociative, rightAssociative);
+ }
+}

training/src/test/java/info/jab/fp/util/EitherTest.java

@@ -9,12 +9,6 @@
 
 public class EitherTest {
 
- @Test
- public void testFold() {
- Either<String, Integer> success = Either.right(42);
- Either<String, Integer> failure = Either.left("Error occurred");
- }
-
  @Test
  void testLeft() {
  Either<String, Integer> left = Either.left("Error");