Added test for ary function

docs/functional-php.md

@@ -356,6 +356,26 @@ $partiallyAppliedSubtractor = partial_right($subtractor, 10);
 $partiallyAppliedSubtractor(20); // -> 10
 ```
 
+## ary()
+
+`Functional\ary` (as in arity) takes a `callable` and a count and calls the `callable` with 
+that many arguments using `take_left` if positive, or `take_right` if negative. 
+Throws if passed 0.
+
+For example:
+
+```php
+use function Functional\ary;
+use function Functional\map;
+
+# This fails because map calls it's callable with the element, 
+# index and the whole collection
+# map($array, 'ucfirst');
+
+# Using `ary`
+map($array, ary('ucfirst', 1')); # Passes only the first argument to `ucfirst`
+```
+
 ## partial_any()
 
 There is a third function in the family called `partial_any`. Unlike its siblings it doesn’t automatically merge but it

tests/Functional/AryTest.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * @package   Functional-php
+ * @author    Hugo Sales <hugo@fc.up.pt>
+ * @copyright 2020 Hugo Sales
+ * @license   https://opensource.org/licenses/MIT MIT
+ * @link      https://github.com/lstrojny/functional-php
+ */
+
+namespace Functional\Tests;
+
+use function Functional\ary;
+
+class AryTest extends AbstractTestCase
+{
+
+    public function test()
+    {
+        $this->assertSame(5, ary(function ($a = 0, $b = 0) { return $a + $b; }, 1)(5));
+        $this->assertSame(5, ary(function ($a = 0, $b = 0, $c = 0) { return $a + $b + $c; }, 1)(5));
+        $this->assertSame(6, ary(function ($a = 0, $b = 0, $c = 0) { return $a + $b + $c; }, -1)(6));
+        $this->assertSame(7, ary(function ($a = 0, $b = 0, $c = 0) { return $a + $b + $c; }, 2)(5, 2));
+        $this->assertNull(ary(function ($a = 0, $b = 0) { return $a + $b; }, 0)(5));
+    }
+}