Maybe is not foldable

Author: widmogrod

src/Monad/Maybe/Just.php

@@ -81,4 +81,15 @@ public function extract()
     {
         return $this->value;
     }
+
+    /**
+     * foldl _ z Nothing = z
+     * foldl f z (Just x) = f z x
+     *
+     * @inheritdoc
+     */
+    public function reduce(callable $function, $accumulator)
+    {
+        return $function($accumulator, $this->value);
+    }
 }

src/Monad/Maybe/Maybe.php

@@ -9,6 +9,7 @@
 
 interface Maybe extends
     FantasyLand\Monad,
+    FantasyLand\Foldable,
     Common\ValueOfInterface,
     FantasyLand\Monoid
 {

src/Monad/Maybe/Nothing.php

@@ -73,4 +73,15 @@ public function extract()
     {
         return null;
     }
+
+    /**
+     * foldl _ z Nothing = z
+     * foldl f z (Just x) = f z x
+     *
+     * @inheritdoc
+     */
+    public function reduce(callable $function, $accumulator)
+    {
+        return $accumulator;
+    }
 }

test/Functional/ConcatTest.php

@@ -5,6 +5,8 @@
 namespace test\Functional;
 
 use Widmogrod\Functional as f;
+use Widmogrod\Monad\Maybe\Just;
+use Widmogrod\Monad\Maybe\Nothing;
 
 class ConcatTest extends \PHPUnit\Framework\TestCase
 {
@@ -59,6 +61,14 @@ public function provideData()
                     3
                 ]),
             ],
+            'Just of lists' => [
+                '$array' => Just::of(f\fromIterable(['a', 1, 3])),
+                '$expected' => f\fromIterable(['a', 1, 3]),
+            ],
+            'Nothing of lists' => [
+                '$array' => Nothing::mempty(),
+                '$expected' => f\fromNil()
+            ],
         ];
     }
 }