Functional, immutable and extensible enumerations and collections for PHP 5.3
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Xi Collections

Functional, immutable and extensible enumerations and collections for PHP 5.3.

Design Philosophy

PHP has always lacked solid collections support, with the vast majority of programmers making do with arrays and the related built-in functions. With the introduction of SPL in PHP 5.0 and the consequent extensions in 5.3, there are currently more choices than ever if all you want for is speed and answers to specific use cases. Array processing, however, is not significantly better than ten years ago, with the API about as comfortable and handy for everyday tasks as picking at your dinner with a shovel.

Xi Collections aims to rectify the situation and inject your workflow with a hearty dose of functional and declarative aspects. This is intended to result in more clarity in expressing and understanding processing collections of objects or data, allowing you to work faster and deliver more self-documenting code.

Design Principles

  • Object immutability. The collections' methods do not manipulate the collections' contents, but return a new collection instead.

  • API chainability. Most operations will return a new collection except specific reduces and size informations.

  • Embracing functional programming. The Collections API is chosen to facilitate a functional workflow. Existing FP-compatible functionality in PHP is prioritized for inclusion to the API, and other important concepts that are missing are stolen from other languages and libraries.

  • Out-of-the-box extensibility. Decorators for separating the concrete Collection implementations from your modifications to the API are included. The interfaces tend to the minimal rather than extensive, making new implementations easier. Specifically, the whole of PHP's array functions is not built-in, but can be readily used if you need to.


From imperative to functional

Let's assume a simple loop that filters and transforms a set of data:

public function getMatchingInterestingParts() {
	$result = array();
	foreach ($this->getFoos() as $key => $value) {
		if ($this->match($value)) {
			$result[$key] = $value->getInterestingParts();
	return $result;

Here's the same expressed with Collections:

public function getMatchingInterestingParts() {
    return $this->getFoos()
        ->filter(function(Foo $foo) {
            return $this->matches($foo);
        })->map(function(Foo $foo) {
            return $foo->getInterestingParts();

The latter bit of code is not much shorter, and for someone unfamiliar with functional constructs it may be more difficult to process. It does, however, have a few interesting qualities. The code communicates its intent better - filter values, then map the result, and nothing else. This is especially beneficial when considering code with a significantly more complex set of transformations. There's less room for error; index associations are automatically maintained. This also means you can focus on the interesting bits instead of boilerplate, which helps both when reading and when writing the code. A third benefit is that you can take full advantage of type hints and the safety they can bring, something which will be lacking with a simple foreach loop.

Simplifying common access patterns

One of the most common use cases for looping over an array is collecting the results of a member access or method invocation from every item. Collections makes that easy.

public function getBarsByFoos() {
    $bars = array();
    foreach ($this->getFoos() as $key => $foo) {
        $bars[$key] = $foo->getBar();
    return $bars;
// becomes
public function getBarsByFoos() {
    return $this->getFoos()->invoke('getBar');

public function getFooTrivialities() {
    $trivialities = array();
    foreach ($this->getFoos() as $key => $foo) {
        $trivialities[$key] = $foo->triviality;
    return $trivialities;
// becomes
public function getFooTrivialities() {
    return $this->getFoos()->pick('triviality');

Picking even works for arrays (or objects implementing ArrayAccess) as well, and you don't need to care about which type the input is.

Inspecting intermediate steps of complex operations

Suppose you have a pipeline where data is transformed according to complex rules.

public function getAliveQuxen() {
    return $this->getFoos()
        ->filter(function($bar) { return $bar->isAlive(); })

Suppose further that you want to inspect the data as it passes from one step to another. This is where you'd introduce temporary variables, were the code imperatively structured. With Collections, all you need is tap. It accepts a function that takes the contents of the collection as its parameter - and does nothing but call the function.

public function getAliveQuxen() {
    return $this->getFoos()
        ->filter(function($bar) { return $bar->isAlive(); })
        ->tap(function($bars) { $this->log($bars); })

A reader of your code will be able to immediately recognize that the part in tap is only being executed for its side effects and that it has nothing to do with the transformation itself. We could've used each in a similar fashion if we were instead interested in the individual units of computation.

public function getAliveQuxen() {
    return $this->getFoos()
        ->filter(function($bar) { return $bar->isAlive(); })
        ->each(function($bar) { $this->logBar($bar); })

Delaying computation using views

In some cases you may wish to expose a certain Collection to a consumer, but are not certain whether the Collection is going to be used, and generating one is potentially costly. In such a case you can apply a CollectionView, which is a set of transformation operations that haven't yet been applied to an underlying base Collection. Upon access, the operations will be applied and the resulting values provided to the consumer.

A Collection is transformed into a view backed by itself by invoking view. Best effort is made to apply all Collection method calls lazily. Forcing the view into actual values happens when accessing any Enumerator methods.

public function getEnormouslyExpensiveCollection() {
    return $this->getStuff()->view()->map(function(Stuff $s) {
        return enormouslyExpensiveComputation($s);

There's a caveat, however. It is not guaranteed that the transformation from lazy to strict should happen exactly once per CollectionView object. If you need that, you should force the view object to get a strict one.

Using an extended API on the fly

In any given PHP environment there tends to be an amount of existing functionality around for processing data in a Traversable format. PHP itself has a plethora of built-in array functions that aren't feasible to support in the Collection API if it is supposed to be kept minimal. This can potentially change with the introduction of traits in PHP 5.4, but for now you'll have to figure out ways to use these functions manually. At the core of this facility is apply. It accepts a function that applies a transformation of some kind to the collection, the result of which is taken in as a new collection.

Let's assume you want to sort your values. Here's a way to do it using apply.

public function getSortedFoos() {
    return $this->getFoos()
        ->apply(function($collection) {
            $foos = $collection->toArray();
            return $foos;

The argument is a collection, which will have to be converted to an array first to be accepted by ksort. The function also operates on references, not values, so a temporary variable is necessary. There's an amount of cruft with this use case, but you're likely to be using raw PHP functions rarely. If you're using apply with functions that have a more reasonable API, eg. accept Traversable objects instead of necessitating arrays, the footprint becomes much more palatable. In such a fictional scenario for ksort, for instance:

public function getSortedFoos() {
    return $this->getFoos()

API basics

Collections has two core interfaces. Enumerable implements a set of collection operations relying only on traversability. Collection extends the Enumerable operations to a superset that includes operations that yield other collections in return. This means collections can be transformed into other collections.

Every concrete class has a static create method that can be used for fluently constructing and accessing a collection. For instance:

ArrayCollection::create($values)->invoke('getBar')->each(function(Bar $bar) { $bar->engage(); });

Below is a short description of the APIs provided by Enumerable and Collection. For more thorough information, you'll need to consult the source.


Element retrieval

  • first: Returns the first element in the collection
  • last: Returns the last element in the collection
  • find: Returns the first value that satisfies a given predicate

Element conditions

  • exists: Checks whether the collection has at least one element satisfying a given predicate
  • forAll: Checks whether all of the elements in the collection satisfy a given predicate
  • countAll: Counts the amount of elements in the collection that satisfy a given predicate

Size information

  • count: Counts the amount of elements in the collection


  • reduce: Uses a given callback to reduce the collection's elements to a single value, starting from a provided initial value


  • tap: Calls a provided callback with this object as a parameter
  • each: Performs an operation once per key-value pair



  • map: Applies a callback for each value-key-pair in the Collection and returns a new one with values replaced by the return values from the callback
  • flatMap: Applies a callback for each key-value-pair in the Collection assuming that the callback result value is iterable and returns a new one with values from those iterable
  • pick: Get a Collection with a key or member property picked from each value
  • values: Get a Collection with just the values from this Collection
  • keys: Get a Collection with the keys from this one as values
  • invoke: Map this Collection by invoking a method on every value
  • apply: Creates a new Collection of this type from the output of a given callback that takes this Collection as its argument


  • take: Creates a new Collection with up to $number first elements from this one
  • rest: Creates a new Collection with the rest of the elements except first
  • filter: Creates a Collection with the values of this collection that match a given predicate
  • filterNot: Creates a collection with the values of this collection that do not match a given predicate
  • unique: Get a Collection with only the unique values from this one


  • partition: Split a collection into a pair of two collections; one with elements that match a given predicate, the other with the elements that do not.
  • groupBy: Group the values in the Collection into nested Collections according to a given callback


  • concatenate: Creates a Collection with elements from this and another one
  • union: Creates a Collection with key-value pairs in the $other Collection overriding ones in $this Collection
  • flatten: Flatten nested arrays and Traversables
  • add: Get a new Collection with given value and optionally key appended

Size information

  • isEmpty: Checks whether the collection is empty


  • indexBy: Reindex the Collection using a given callback
  • sortWith: Get this Collection sorted with a given comparison function
  • sortBy: Get this Collection sorted with a given metric


  • view: Provides a Collection where transformer operations are applied lazily

Specific reduces

  • min: Returns the minimum value in the collection
  • max: Returns the maximum value in the collection
  • sum: Returns the sum of values in the collection
  • product: Returns the product of values in the collection


  • force: Coerces this view back into the underlying Collection type

Collection implementations

  • ArrayCollection: Basic Collection backed by a plain PHP array.
  • OuterCollection: A decorator for a Collection. Can easily be extended to provide more collection operations without locking down the implementation specifics.

Running the unit tests

phpunit -c tests


  • Collection implementations backed by SPL (SplFixedArray, SplDoublyLinkedList?)
  • Use traits?