Functional Programming in PHP

  • Gilles Crettenand

Functional programming has gained a lot of traction those 3 to 5 last years.

First, there are some success stories around it: Twitter move to Scala,
Whatsapp being written in Erlang. Then we have some new kick-ass languages like Rust. Finally, it seems the new hype is to create a functional language compiling to javascript: Elm, Purescript. On a more academic front, there is also research on new concepts like dependant typing, see Idris for example.

Those are all cool and shiny new toys, but we can benefit from some techniques without having to learn a new tool, just by applying some principles to our everyday PHP! But first of all, what exactly is functional programming?

Update 17.02.2017

On February the 17th 2017, my book about the topic was published by Packt Publishing. You can find all relevant information on its dedicated page: Functional PHP. The book is available in a printed edition and in most digital formats.

Functional programming

Functional programming is a paradigm as are imperative, logical and OO programming. The main point is that everything is a function, in the mathematical sense. The corollary of that is that you cannot have side effects since all outputs depends solely on the input of the function. It also means you cannot have any mutable state, once something was computed, it cannot be changed to something else.

This can be cumbersome a time, so a distinction was made between pure functional languages (like Haskel or Ocaml) and other function languages, Scala for example, where you can introduce mutable variables or even do plain old Java. Another issue are input and outputs which are by definition mutable, the “problem” has been resolved in most functional languages by wrapping them into a Monad, the rest of this blog post will not talk about that, so I won't explain this barbaric word, but feel free to do some research on the internet, it is really interesting.

Despite all those lengthy explanation, functional programming is first and for all a way of thinking and solving issues than can be used in any languages.

It is just easier to do in some that are designed for it.

The advantages

Since the output depends solely on the input, reasoning about a function is made a whole lot easier, you have everything under the eyes, there is no need to keep in mind any kind of global or shared state, thus reducing complexity a lot. Testing is made easier, you don't have to mock a lot of dependencies or create big ass objects, you just need to pass the arguments your function need.

A functional style also often result in writing small and self-contained functions used later as building blocks for bigger functionality, resulting in clearly decoupled code and extended reusability. A nice little bonus of a pure function is that you can cache its result, this is called memoization but more about that later. A perhaps less appealing argument for us, is that calls to pure function can easily be distributed across threads or even computers because you have the guarantee that the inputs are enough to perform the computation and that there will be no side effects.

It is simply a matter of combining the results again at the end.

Functional programming in PHP: The basics

Functions in PHP

The first important thing is that functions are, more or less, first class citizen in the PHP world :

// Function as a variable
$func = function() {
  return 42;
};

// Function as a return type
function createFunction() {
  return function() { return "Thanks for all the fish"; };
};
$func2 = createFunction();

// Function as a parameter
function display($func) {
  echo $func()."\n\n";
}

// Call a function by name
call_user_func_array('strtoupper', $someString);

// objects as functions
class SomeClass {
  public function __invoke($param1, $param2) {
    [...]
  }
}
$instance = new SomeClass();
$instance('First', 'Second'); // call the __invoke() method

Mapping

Knowing that, we can start playing around with basic functional concepts. For each example, the “common” way of doing things will be presented first followed by the functional one. As it is often the case in functional programming, we will most of the time manipulate an array.

Applying a function to all elements :

// The imperative way :
foreach($stringArray as $k => $v) {
  $stringArray[$k] = strtoupper($v);
}

// The functional way :
$result = array_map('strtoupper', $stringArray);

Reducing

“Reduce” an array, often called “fold” in functional languages :

// The imperative way :
$result = 0;
foreach($intArray as $v) {
  $result += $v;
}

// The functional way :
$result = array_reduce($intArray, function($a, $b) { return $a + $b; }, 0)

This particular example might not be a good one, but keep in mind that the applied function can be of any particular complexity, and that you can declare and use it in other places. It is also important to note that PHP offers an array_sum function for this particular case :

$result = array_sum($intArray);

Filtering

Filter an array :

// The imperative way :
$result = array();
foreach($intArray as $v) {
  if($v % 2 === 0) {
    $result[] = $v;
  }
}

// The functional way :
$result = array_filter($intArray, function($a) { return ($a % 2 === 0); })

A good example on how you can leverage the array_filter function is given in “ PHP The Right Way“.

Some utility functions

In most functional languages and library, you will also find functions that operates on predicates (a test that return a boolean) and arrays. The most common examples are :

  • first / last which returns the first, respectively last, element of an array matching the predicate
  • any which returns true as soon as one element matches the predicate
  • all which returns true if all elements matches the predicate

Those don't exists in PHP but you can find various implementations on the github gist I created for the occasion. You can use this kind of function in rights management. For example to check if the connected user has at least one role with a particular set of permission or to verify that you can apply a particular transformation on all elements in an array :

// check for an administrative right
if(any($roles, function($r) { return $r->isAllowedToTranslate(); }) {
  [do something]
}

// check if you can apply an operation to a set of elements
if(all($elements, function($e) { return $e->canBeDeleted(); }) {
  [do something]
}

Once you have created all those little building blocks to manipulate your data, you can then compose them to perform more complex operations. Sadly PHP don't propose a compose function, but you can again find an implementation in the gist linked above. For example, say I want to do a reverse natural sort. PHP already proposes most of the sort function in both “direction”, but this is not the case for natsort, we could however easily achieve our goal with the following :

$natrsort = compose('array_reverse', 'natsort');
$natrsort($myArray);

There would be a lot of others thing to say or show, but I don't want to overwhelm you too much and there is already a lot of really interesting resources available on the web:

There is even a book about this exact subject: “ Functional Programming in PHP“. I didn't had the opportunity to read it as of now, but the recorded talk available is another take at explaining functional programming with a bit of PHP history at the beginning. I found it a bit too long, but still interesting.

Memoization

The last thing I want to talk about is memoization or, put in other words, function results caching. Remember when we said that the result is solely dependent on the input, now say we have a computational heavy function that is called multiple times with the same parameters. Knowing that this is a pure function means you can just cache the result the first time and then simply return hit the cache. This is exactly the same thing as using caching for web URLs.

You can do that really easily using static local variables in PHP:

function factorial($n) {
  static $cache = array();

  if($n == 1) return 1;

  if(! array_key_exists($n, $cache)) {
    $cache[$n] = $n * factorial($n - 1);
  }

  return $cache[$n];
}

You can also generalize this mechanism using a nifty function that will return a memoized version of any pure function :

function memoize($func) {
  return function() use($func) {
    static $cache = array();

    $args = func_get_args();
    $key = serialize($args);
    if(! array_key_exists($key, $cache)) {
      $cache[$key] = call_user_func_array($func, $args);
    }
    return $cache[$key];
  }
}

You can then simply do the following :

$factorial = function($n) use(&$factorial) {
  if($n == 1) return 1;
  return $n * $factorial($n -1);
}
$mem_factorial = memoize($factorial);

Et voilà! You just built a cached version of a pure function.

Conclusion

Functional programming in PHP is not necessarily this difficult thing we are something lead to believe it is. You don't need to learn a brand new language to start applying the principle in your daily work, you can even start with tiny little steps and take your time to build up your expertise!

In most cases, having a “functional mindset” makes your code easier to reason with and automatically promotes common best practices like “Single Responsibility Principle” and “Reusability”. You even have a total absence of side effect for free if you write pure functions!

Someone once said Functional languages enforce what is simply “good code” in other languages.

Also, learning different ways to do things can only open your mind and make you a better developer. This is why I encourage you to try techniques discussed in this post and why not take the more radical decision to try one of those obscure pure functional languages that some bearded guy are talking about in conferences. I can't promise you that you will fall in love and don't come back to PHP, but I am sure that you will learn a lot in the process!

If I could only say one last thing, it would be : “Always strive to learn new things and don't stay in your comfort zone”.


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