add partial application util

phpunit.xml

@@ -0,0 +1,11 @@
+<phpunit bootstrap="tests/bootstrap.php">
+  <testsuites>
+    <testsuite name="all">
+      <directory>tests</directory>
+    </testsuite>
+  </testsuites><filter>
+  <whitelist processUncoveredFilesFromWhitelist="true">
+    <directory suffix=".php">src</directory>
+  </whitelist>
+</filter>
+</phpunit>

src/Utils/PartiallyAppliedFunction.php

@@ -0,0 +1,53 @@
+<?php
+/**
+ * Copyright 2018 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+namespace Kadet\Functional\Utils;
+
+use const Kadet\Functional\_ as PLACEHOLDER;
+
+class PartiallyAppliedFunction
+{
+    protected $callable;
+    protected $arguments = [];
+
+    public function __construct(callable $callable, ...$args)
+    {
+        switch (true) {
+            case $callable instanceof PartiallyAppliedFunction:
+                $this->callable  = $callable->callable;
+                $this->arguments = $callable->prepareArguments($args);
+                break;
+
+            default:
+                $this->callable  = $callable;
+                $this->arguments = array_values($args);
+        }
+    }
+
+    public function __invoke(...$args)
+    {
+        $arguments = $this->prepareArguments($args);
+        return ($this->callable)(...$arguments);
+    }
+
+    private function prepareArguments($args)
+    {
+        $result = [];
+        foreach ($this->arguments as $argument) {
+            $result[] = $argument === PLACEHOLDER
+                ? array_shift($args)
+                : $argument;
+        }
+
+        return array_merge(
+            array_values($result),
+            array_values($args)
+        );
+    }
+}

src/Utils/index.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * Copyright 2018 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+namespace Kadet\Functional;
+
+use Kadet\Functional\Utils\PartiallyAppliedFunction;
+
+function partial(callable $callable, ...$args)
+{
+    return new PartiallyAppliedFunction($callable, ...$args);
+}

src/functions.php

@@ -7,6 +7,8 @@ use Kadet\Functional\Predicate\AnyOfPredicate;
 use Kadet\Functional\Predicate\ClosurePredicate;
 use Kadet\Functional\Predicate\ConstantPredicate;
 
+require_once __DIR__.'/Utils/index.php';
+
 /**
  * @param $predicate
  *
@@ -57,3 +59,11 @@ function any(...$predicates)
     $predicates = unpack($predicates);
     return new AnyOfPredicate(...array_map(predicate::class, $predicates));
 }
+
+function symbol($name = null)
+{
+    $id = random_bytes(64);
+    return sprintf("%s(%s)", $name ?: 'symbol', $id);
+}
+
+define(__NAMESPACE__.'\\_', symbol());

tests/PartialApplicationTest.php

@@ -0,0 +1,57 @@
+<?php
+/**
+ * Copyright 2018 Kacper Donat
+ *
+ * @author Kacper "Kadet" Donat <kacper@kadet.net>
+ *
+ * Full license available in separate LICENSE file
+ */
+
+use \Kadet\Functional as f;
+use const Kadet\Functional\_;
+
+function subtract($a, $b) {
+    return $a - $b;
+}
+
+class PartialApplicationTest extends \PHPUnit\Framework\TestCase
+{
+    public function testPartialApplicationOfOneArgument()
+    {
+        $applied = f\partial('subtract', 10);
+
+        $this->assertEquals(0, $applied(10));
+        $this->assertEquals(10, $applied(0));
+        $this->assertEquals(20, $applied(-10));
+    }
+
+    public function testPartialApplicationOfAllArguments()
+    {
+        $applied = f\partial('subtract', 10, 20);
+
+        $this->assertEquals(-10, $applied());
+    }
+
+    public function testPartialApplicationOfFewArguments()
+    {
+        $applied = f\partial('array_merge', ['a'], ['b'], ['c']);
+
+        $this->assertEquals(['a', 'b', 'c', 'd', 'e'], $applied(['d'], ['e']));
+    }
+
+    public function testPartialApplicationWithPlaceholders()
+    {
+        $applied = f\partial('array_merge', ['a'], _, ['c']);
+
+        $this->assertEquals(['a', 'd', 'c'], $applied(['d']));
+    }
+
+    public function testPartialApplicationOfAppliedFunction()
+    {
+
+        $applied = f\partial('array_merge', ['a'], _, ['c']);
+        $double  = f\partial($applied, _, ['d']);
+
+        $this->assertEquals(['a', 'b', 'c', 'd', 'e'], $double(['b'], ['e']));
+    }
+}