diff --git a/composer.json b/composer.json
index 93763b4..0e7cad6 100644
--- a/composer.json
+++ b/composer.json
@@ -14,8 +14,9 @@
     "phpunit/phpunit": "*"
   },
   "autoload": {
-    "psr-4": {
-      "Pairs\\": "src"
-    }
+    "files": [
+        "src/Pairs.php",
+        "src/Lists.php"
+    ]
   }
 }
diff --git a/composer.lock b/composer.lock
index 19ce563..4c8c5cc 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,8 +4,8 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "hash": "86509b22058b7ffc55258e8448472c86",
-    "content-hash": "9213c47d564505429c56200c93133fed",
+    "hash": "142419ec950cea250c7fb31febfe42b1",
+    "content-hash": "2baadab544344aec474df09d2b1710a4",
     "packages": [],
     "packages-dev": [
         {
diff --git a/phpunit b/phpunit
new file mode 100644
index 0000000..ed435c9
--- /dev/null
+++ b/phpunit
@@ -0,0 +1,27 @@
+#!/usr/bin/env php
+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+if (version_compare('5.6.0', PHP_VERSION, '>')) {
+    fwrite(
+        STDERR,
+        'This version of PHPUnit requires PHP 5.6; using the latest version of PHP is highly recommended.' . PHP_EOL
+    );
+
+    die(1);
+}
+
+if (!ini_get('date.timezone')) {
+    ini_set('date.timezone', 'UTC');
+}
+
+require __DIR__ . '/vendor/autoload.php';
+
+PHPUnit_TextUI_Command::main();
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
new file mode 100644
index 0000000..3404601
--- /dev/null
+++ b/phpunit.xml.dist
@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit backupGlobals="false"
+         backupStaticAttributes="false"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false"
+         syntaxCheck="false"
+         bootstrap="vendor/autoload.php"
+>
+  <testsuites>
+    <testsuite name="Test Suite">
+      <directory>./tests/</directory>
+    </testsuite>
+  </testsuites>
+</phpunit>
diff --git a/src/Lists.php b/src/Lists.php
new file mode 100644
index 0000000..ffa5ff2
--- /dev/null
+++ b/src/Lists.php
@@ -0,0 +1,102 @@
+<?php
+
+namespace Lists;
+use function Pairs\cons;
+use function Pairs\car;
+use function Pairs\cdr;
+use function Pairs\toString;
+
+
+/**
+ * Applies callable function $func to list $list
+ * @param  callable $func function
+ * @param  callable $list list
+ * @return result list
+ */
+function map(callable $func, callable $list) {
+    $map = function ($items, $acc) use (&$map, $func) {
+        if (is_null($items)) {
+            return reverse($acc);
+        }
+        return $map(cdr($items), cons($func(car($items)), $acc));
+    };
+
+    return $map($list, null);
+}
+
+/**
+ * Filters list $list using callable function $func
+ * @param  callable $func function
+ * @param  callable $list list
+ * @return result list
+ */
+function filter(callable $func, callable $list) {
+    $map = function ($func, $items) use (&$map) {
+        if ($items === null) {
+            return null;
+        } else {
+            $curr = car($items);
+            $rest = $map($func, cdr($items));
+            // filter
+            return $func($curr) ? cons($curr, $rest) : $rest;
+        }
+    };
+
+    return $map($func, $list);
+}
+
+/**
+ * Collapses the list $list using callable function $func
+ * @param  callable $func function
+ * @param  callable $list list
+ * @param  mixed   $acc
+ * @return result
+ */
+function reduce(callable $func, callable $list, $acc = null) {
+    $iter = function ($items, $acc) use (&$iter, $func) {
+        return is_null($items) ? $acc : $iter(cdr($items), $func(car($items), $acc));
+    };
+
+    return $iter($list, $acc);
+}
+
+/**
+ * Concatenates two lists
+ * @param  pair $list1
+ * @param  pair $list2
+ * @return new list
+ */
+function append(callable $list1, callable $list2)
+{
+     if ($list1 === null) {
+        return $list2;
+    } else {
+        return cons(car($list1), append(cdr($list1), $list2));
+    }
+}
+
+/**
+ * Reverse list $list
+ * @param  callable $list list
+ * @return result
+ */
+function reverse(callable $list) {
+    $iter = function ($items, $acc) use (&$iter) {
+        return is_null($items) ? $acc : $iter(cdr($items), cons(car($items), $acc));
+    };
+
+    return $iter($list, null);
+}
+
+/**
+ * Returns length of list
+ * @param  callable $list list
+ * @return integer  length
+ */
+function length($list) {
+    if ($list === null || !is_callable($list)) {
+        return 0;
+    } else {
+        return 1 + length(cdr($list));
+    }
+}
diff --git a/src/Pairs.php b/src/Pairs.php
index d840ed4..2185585 100644
--- a/src/Pairs.php
+++ b/src/Pairs.php
@@ -33,7 +33,7 @@ function toString($list)
     $arr = [];
     $iter = function ($items) use (&$arr, &$iter) {
         if ($items != null) {
-            $arr[] = listToString(car($items));
+            $arr[] = toString(car($items));
             $iter(cdr($items));
         }
 
diff --git a/tests/ListsTest.php b/tests/ListsTest.php
new file mode 100644
index 0000000..a6f07b2
--- /dev/null
+++ b/tests/ListsTest.php
@@ -0,0 +1,66 @@
+<?php
+
+namespace Pairs\tests;
+
+
+use function Pairs\cons;
+use function Pairs\car;
+use function Pairs\cdr;
+use function Pairs\toString;
+
+
+use function Lists\length;
+use function Lists\reverse;
+use function Lists\map;
+use function Lists\filter;
+use function Lists\reduce;
+
+class ListsTest extends \PHPUnit_Framework_TestCase
+{
+
+    public function testLength()
+    {
+        $list = cons(1,cons(2, cons(3, null)));
+        $this->assertEquals(3, length($list));
+
+    }
+
+    public function testReverse()
+    {
+        $list = cons(1, cons(2, cons(3, null)));
+        $expected = toString(cons(3, cons(2, cons(1, null))));
+        $this->assertEquals($expected, toString(reverse($list)));
+    }
+
+    public function testMap()
+    {
+        $list = cons(1, cons(2, cons(3, null)));
+        $expected = toString(cons(3, cons(4, cons(5, null))));
+        $map = map(function ($x) {
+            return $x + 2;
+        }, $list);
+        $this->assertEquals($expected, toString($map));
+    }
+
+    public function testFilter()
+    {
+        $list = cons(2, cons(3, cons(4, null)));
+        $expected = toString(cons(2, cons(4, null)));
+        $filter = filter(function ($x) {
+            return $x % 2 == 0;
+        }, $list);
+
+        $this->assertEquals(2, length($filter));
+        $this->assertEquals($expected, toString($filter));
+    }
+
+    public function testReduce()
+    {
+        $list = cons(1, cons(2, cons(3, null)));
+        $expected = 6;
+        $map = reduce(function ($x, $acc) {
+            return $x + $acc;
+        }, $list, 0);
+        $this->assertEquals($expected, $map);
+    }
+}
diff --git a/tests/PairsTest.php b/tests/PairsTest.php
index 4a3d2c0..7e4fa80 100644
--- a/tests/PairsTest.php
+++ b/tests/PairsTest.php
@@ -2,8 +2,6 @@
 
 namespace Pairs\tests;
 
-require_once 'Pairs.php';
-
 use function Pairs\cons;
 use function Pairs\car;
 use function Pairs\cdr;