Adding MaybeT::filter

JonWoolwine · jnape · commit 71214d3630b0 · 2020-07-30T14:32:21.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/MaybeT.java b/src/main/java/com/jnape/palatable/lambda/monad/transformer/builtin/MaybeT.java
@@ -44,6 +44,17 @@ public <MMA extends MonadRec<Maybe<A>, M>> MMA runMaybeT() {
         return mma.coerce();
     }
 
+    /**
+     * If the embedded value is present and satisfies <code>predicate</code>
+     * then return <code>just</code> the embedded value
+     *
+     * @param predicate the predicate to apply to the embedded value
+     * @return maybe the satisfied value embedded under M
+     */
+    public MaybeT<M, A> filter(Fn1<? super A, ? extends Boolean> predicate) {
+        return maybeT(mma.fmap(ma -> ma.filter(predicate)));
+    }
+
     /**
      * {@inheritDoc}
      */
diff --git a/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/MaybeTTest.java b/src/test/java/com/jnape/palatable/lambda/monad/transformer/builtin/MaybeTTest.java
@@ -22,6 +22,8 @@
 import static com.jnape.palatable.lambda.adt.Either.right;
 import static com.jnape.palatable.lambda.adt.Maybe.just;
 import static com.jnape.palatable.lambda.adt.Maybe.nothing;
+import static com.jnape.palatable.lambda.functions.builtin.fn2.GT.gt;
+import static com.jnape.palatable.lambda.functions.builtin.fn2.LT.lt;
 import static com.jnape.palatable.lambda.functor.builtin.Identity.pureIdentity;
 import static com.jnape.palatable.lambda.functor.builtin.Lazy.lazy;
 import static com.jnape.palatable.lambda.io.IO.io;
@@ -70,4 +72,11 @@ public void composedZip() {
                 .unsafePerformAsyncIO(Executors.newFixedThreadPool(2))
                 .join();
     }
+
+    @Test
+    public void filter() {
+        MaybeT<Identity<?>, Integer> maybeT = pureMaybeT(pureIdentity()).apply(1);
+        assertEquals(maybeT(new Identity<>(just(1))), maybeT.filter(gt(0)));
+        assertEquals(maybeT(new Identity<>(nothing())), maybeT.filter(lt(0)));
+    }
 }
