Adding MaybeT#or for choosing the first present effect result

jnape · jnape · commit 75b1fb70fe17 · 2020-02-08T16:16:13.000-06:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -11,6 +11,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 ### Added
 - `MergeHMaps`, a `Monoid` that merges `HMap`s by merging the values via key-specified `Semigroup`s
+- `Id#id` overload that accepts an argument and returns it
+- `MaybeT#or`, choose the first `MaybeT` that represents an effect around `just` a value
+- `StateMatcher, StateTMatcher, WriterTMatcher`
 
 ## [5.1.0] - 2019-10-13
 ### Changed
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
@@ -55,6 +55,18 @@ public MaybeT<M, A> filter(Fn1<? super A, ? extends Boolean> predicate) {
         return maybeT(mma.fmap(ma -> ma.filter(predicate)));
     }
 
+    /**
+     * Returns the first {@link MaybeT} that is an effect around {@link Maybe#just(Object) just} a result.
+     *
+     * @param other the other {@link MaybeT}
+     * @return the first present {@link MaybeT}
+     */
+    public MaybeT<M, A> or(MaybeT<M, A> other) {
+        MonadRec<Maybe<A>, M> mMaybeA = runMaybeT();
+        return maybeT(mMaybeA.flatMap(maybeA -> maybeA.match(constantly(other.runMaybeT()),
+                                                             a -> mMaybeA.pure(just(a)))));
+    }
+
     /**
      * {@inheritDoc}
      */
@@ -145,7 +157,7 @@ public <B> MaybeT<M, A> discardR(Applicative<B, MaybeT<M, ?>> appB) {
 
     @Override
     public boolean equals(Object other) {
-        return other instanceof MaybeT<?, ?> && Objects.equals(mma, ((MaybeT) other).mma);
+        return other instanceof MaybeT<?, ?> && Objects.equals(mma, ((MaybeT<?, ?>) other).mma);
     }
 
     @Override
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
@@ -27,9 +27,7 @@
 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;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.MaybeT.liftMaybeT;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.MaybeT.maybeT;
-import static com.jnape.palatable.lambda.monad.transformer.builtin.MaybeT.pureMaybeT;
+import static com.jnape.palatable.lambda.monad.transformer.builtin.MaybeT.*;
 import static com.jnape.palatable.traitor.framework.Subjects.subjects;
 import static org.junit.Assert.assertEquals;
 
@@ -79,4 +77,16 @@ public void filter() {
         assertEquals(maybeT(new Identity<>(just(1))), maybeT.filter(gt(0)));
         assertEquals(maybeT(new Identity<>(nothing())), maybeT.filter(lt(0)));
     }
+
+    @Test
+    public void orSelectsFirstPresentValueInsideEffect() {
+        assertEquals(maybeT(new Identity<>(just(1))),
+                     maybeT(new Identity<>(just(1))).or(maybeT(new Identity<>(nothing()))));
+
+        assertEquals(maybeT(new Identity<>(just(1))),
+                     maybeT(new Identity<>(nothing())).or(maybeT(new Identity<>(just(1)))));
+
+        assertEquals(maybeT(new Identity<>(just(1))),
+                     maybeT(new Identity<>(just(1))).or(maybeT(new Identity<>(just(2)))));
+    }
 }
