rfc:iteration-tools

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Request for Comments: Iteration tools in PHP

This RFC proposes a series of functions or classes to facilitate easy processing of data sets represented as either arrays or Traversables. The assumed PHP version is 5.3 because of the new lambda structures needed for this proposal.

Introduction

Most of the programs we write, invariable of the programming language we use, have as their purpose data processing. While this data can be represented in various formats it is as true that quite often this data comes grouped as sets of data. For example it is a common situation to issue a database query which resturns a result set or just as well we might need to read the contents of a directory or the structure of an XML document. Oblivious of the data source these result sets are represented in PHP in three ways:

  • array. Ex: scandir()
  • Traversable. Ex: DirectoryIterator
  • resource. Ex: mysql_query()

In order to read these data structures PHP offers us 3 looping structures:

  • for
  • while
  • foreach

Given that this is such a recurrent situation and conforming to the DRY principle but also in total respect with common sense an abstraction is required.

Why do we need RFCs?

Depending on the task at hand the processing involved inside these loops may be ridiculously easy or painfully hard. With time, the more you do this the more you realize there's a pattern emerging out there. Thankfully, these patterns were observed almost fourty years ago. These people found out that iterative processes can be abstracted away in a handful of functions. For example:

  • some of the functions modify data in the set
  • some of them filter data and keep only what passes the filter thus resulting a filtered data set
  • some use all the data in the result to create a combined result

…and the list may go on with a few other abstracted use cases. It turns out that separating the iteration from the inner data calculations is a good thing and people came up with what they called higher order functions, that took at least two parameters, the data set to process and the *function* that did the processing (which in some of the cases were “unnamed” functions or lambdas). Languages that did not have possibilities for higher order functions made use of their best features and provided different alternatives if any. For example in PHP we have at our disposal the following SPL classes revolving around the same idea:

  • FilterIterator
  • RecursiveFilterIterator
  • CallbackFilterIterator
  • SearchIterator

While these classes do their job they have some shortcomings:

  • they only iterate over Traversables. Supporting arrays would be nice
  • FilterIterator, RecursiveFilterIterator and SearchIterator require declaration of an aditional classes that extend them
  • Although not a shortcoming, we need a few more abstraction

What I'm proposing is introduction in the language the following functions, which are similar to those existing in JavaScript 1.8:

  • map
  • forEach
  • reduce (an alternative name in some languages is fold)
  • reduceRight (an alternative name in some languages is foldr)
  • filter
  • some
  • every

Pages in the Mozilla Developer Center wiki documenting these functions can be found here:

Some documentation about these functions, most of it pasted from MDC modified where necessary because of PHP related aspects:

map()

  • Signature: array|Iterator map(array|Traversable iter, callback callback)
  • Description: runs a function on every item in the array|Traversable and returns the results in an array|Iterator
  • Callback signature: mixed callback(mixed value, mixed key, array|Traversable iter)

forEach()

  • Signature: void forEach(array|Traversable iter, callback callback)
  • Description: runs a function on every item in the array|Traversable and returns nothing (just like a foreach construct with scope)
  • Callback signature: void callback(mixed value, mixed key, array|Traversable iter)

reduce()

  • Signature: mixed reduce(array|Traversable iter, callback callback[, mixed initialValue[, mixed initialKey]])
  • Description: runs a function on every item in the array|Traversable and collects the results from previous calls.
  • Callback signature: void callback(mixed previousValue, mixed previousKey, mixed currentValue, mixed currentKey, array|Traversable iter) or
  • Callback signature: void callback(mixed previousValue, mixed currentValue, mixed previousKey, mixed currentKey, array|Traversable iter)
  • The callback function receives different arguments oposed to the JS version due to the fact that PHP's arrays and Iterators can have value whom data type are not necesarilly of type integer, i.e. they aren't always numerically indexed

reduceRight()

  Same as reduce() but in reverse.

filter()

  • Signature: array|Iterator filter(array|Traversable iter, callback callback)
  • Description: runs a function on every item in the array|Traversable and returns an array of all items for which the function returns true.
  • Callback signature: bool callback(mixed value, mixed key, array|Traversable iter)

some()

  • Signature: bool some(array|Traversable iter, callback callback)
  • Description: runs a function on items in the array|Traversable while that function returns false. It returns true if the function returns true for *any* item it could visit
  • Callback signature: bool callback(mixed value, mixed key, array|Traversable iter)

every()

  • Signature: bool every(array|Traversable iter, callback callback)
  • Description: runs a function on items in the array|Traversable while that function is returning true. It returns true if the function returns true for *every* item it could visit
  • Callback signature: bool callback(mixed value, mixed key, array|Traversable iter)

Use cases

This an example of a piece of code that reads all PHP files from a certain directory.

<?php
 
// 1.1 how could be done right now -------------------------------------------------
class OnlyPHPFiles extends FilterIterator {
    public function accept() {
        $ext = strtolower(pathinfo($this->getRealPath(), PATHINFO_EXTENSION));
        return $ext === 'php';
    }
}
 
$dirs = array();
foreach (new OnlyPHPFiles(new DirectoryIterator(__DIR__)) as $file) {
    $dirs[] = $file;
}
 
// 1.2 or with the CallbackFilterIterator which I don't know with which PHP
// version it will be shipped
$dirs = new CallbackFilterIterator(new DirectoryIterator(__DIR__), function($value) {
    $ext = strtolower(pathinfo($this->getRealPath(), PATHINFO_EXTENSION));
    return $ext === 'php';
});
 
 
// 2.1 how could be done right now with my proposal ------------------------------------
$dirs = filter(new DirectoryIterator(__DIR__), function($current, $key, $iterator) {
    $ext = strtolower(pathinfo($this->getRealPath(), PATHINFO_EXTENSION));
    return $ext === 'php';
});
 
?>

While the 1.2 example is very similar to 2.1 it differs from it in that it's not passing the iterator to the callback function. Furthermore it does more than it should do. For example, it's not only filtering the elements of the iterator into a new iterator, but it MAY also change those values. In my proposal, the function that changes values is map() which… maps a certain value to another depending on the callback function. filter() only keeps items that validate agains certain criteria inside the callback function.

Common Misconceptions

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rfc/iteration-tools.1225245613.txt.gz · Last modified: 2017/09/22 13:28 (external edit)