Add These::fromMaybes (palatable#117)

7h3kk1d · web-flow · commit 31ad72636a68 · 2021-05-09T13:39:12.000-05:00
diff --git a/src/main/java/com/jnape/palatable/lambda/adt/These.java b/src/main/java/com/jnape/palatable/lambda/adt/These.java
@@ -189,6 +189,25 @@ public static <A, B> These<A, B> both(A a, B b) {
         return new Both<>(tuple(a, b));
     }
 
+    /**
+     * Convenience method for converting a pair of {@link Maybe}s into a {@link Maybe} of {@link These}. If both
+     * {@link Maybe}s are {@link Maybe#just} then the result is a {@link Maybe#just} {@link These#both}. If only one
+     * {@link Maybe} is {@link Maybe#just} then it will be {@link Maybe#just} {@link These#a} or
+     * {@link Maybe#just} {@link These#b}. If both  {@link Maybe}s are {@link Maybe#nothing} then the result will be
+     * {@link Maybe#nothing}.
+     *
+     * @param maybeA   the first optional value
+     * @param maybeB   the second optional value
+     * @param <A> the first possible type
+     * @param <B> the second possible type
+     * @return the wrapped values as a <code>{@link Maybe}&lt;{@link These}&lt;A,B&gt;&gt;</code>
+     */
+    public static <A, B> Maybe<These<A, B>> fromMaybes(Maybe<A> maybeA, Maybe<B> maybeB) {
+        return maybeA.fmap(a -> maybeB.fmap(b -> both(a, b)).orElse(a(a)))
+                .fmap(Maybe::just)
+                .orElse(maybeB.fmap(These::b));
+    }
+
     /**
      * The canonical {@link Pure} instance for {@link These}.
      *
diff --git a/src/test/java/com/jnape/palatable/lambda/adt/TheseTest.java b/src/test/java/com/jnape/palatable/lambda/adt/TheseTest.java
@@ -12,9 +12,12 @@
 import testsupport.traits.MonadRecLaws;
 import testsupport.traits.TraversableLaws;
 
+import static com.jnape.palatable.lambda.adt.Maybe.just;
+import static com.jnape.palatable.lambda.adt.Maybe.nothing;
 import static com.jnape.palatable.lambda.adt.These.a;
 import static com.jnape.palatable.lambda.adt.These.b;
 import static com.jnape.palatable.lambda.adt.These.both;
+import static com.jnape.palatable.lambda.adt.These.fromMaybes;
 import static com.jnape.palatable.lambda.functor.builtin.Lazy.lazy;
 import static com.jnape.palatable.traitor.framework.Subjects.subjects;
 import static org.junit.Assert.assertEquals;
@@ -48,4 +51,12 @@ public void staticPure() {
         These<String, Integer> these = These.<String>pureThese().apply(1);
         assertEquals(b(1), these);
     }
+
+    @Test
+    public void fromMaybesPermutations() {
+        assertEquals(nothing(), fromMaybes(nothing(), nothing()));
+        assertEquals(just(These.a(1)), fromMaybes(just(1), nothing()));
+        assertEquals(just(These.b(1)), fromMaybes(nothing(), just(1)));
+        assertEquals(just(These.both(1, "hello")), fromMaybes(just(1), just("hello")));
+    }
 }
