rfc:iteration-tools

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rfc:iteration-tools [2008/10/29 20:55]
amenthes reorganized the sections
rfc:iteration-tools [2017/09/22 13:28]
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-====== Request for Comments: Functional tools in PHP ====== 
-  * Version: 1.0 
-  * Date: 2008-10-29 
-  * Author: Ionut Gabriel Stan <​ionut.dot.g.dot.stan.at.gmail.com>​ 
-  * Status: RFC not yet completed 
-  * First Published at: http://​wiki.php.net/​rfc/​functional-tools 
- 
-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 returns 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 
- 
-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. 
- 
- 
- 
-==== Why do we need RFCs? ==== 
- 
-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. Thankfully, these patterns were observed almost fourty years ago by some very smart people. 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 
- 
-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 be processed 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 
- 
-====Shortcomings of current tools==== 
-While these classes do their job they have some shortcomings:​ 
-    * They only iterate over Traversables. Supporting arrays would be nice (Indeed, we have array_map(),​ array_walk() and some other similar functions for array processing, yet, I think we should unify these similar tools. Not to mention that array_* function do not adhere to some param order standard. 
-    * FilterIterator,​ RecursiveFilterIterator and SearchIterator require declaration of an aditional classes that extend them 
-    * Although not a shortcoming,​ we need a few more abstraction 
-    * CallbackFilterIterator,​ although I haven'​t had the chance to use it, it misses one important thing. It's not passing the iterator itself as a third parameter to the callback. 
- 
-What I'm proposing is introducing in the language of the following functions, which are similar to those existing in JavaScript 1.8: 
-    * map() 
-    * forEach() 
-    * reduce() - an alternative name - in some languages the name is fold() 
-    * reduceRight() - an alternative name in some languages the name is foldr() 
-    * filter() 
-    * some() 
-    * every() 
- 
- 
-==== Use cases ==== 
- 
-This an example PHP script for reading files with .php extension from a certain directory. 
- 
-<php> 
-<?php 
- 
-// 1.1 how could it 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';​ 
-}); 
- 
-</​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. Another difference is that CallbackFilterIterator may also be used as a virtual CallbackMapIterator. 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 translates 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 ===== 
- 
-RFCs do not in any way replace discussions on the mailing list. 
- 
-===== Proposal and Patch ===== 
- 
-Pages from Mozilla Developer Center wiki documenting these kind of functions can be found here: 
-    * https://​developer.mozilla.org/​en/​new_in_javascript_1.6#​section_2 
-    * https://​developer.mozilla.org/​en/​new_in_javascript_1.8#​section_4 
- 
-Some PHP function signatures, mostly identical to the JavaScript versions, 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) 
- 
-==== Rejected Features ==== 
- 
-To be completed 
- 
-==== Similar implementations ==== 
- 
-JavaScript 1.8: 
-    * https://​developer.mozilla.org/​en/​new_in_javascript_1.6#​section_2 
-    * https://​developer.mozilla.org/​en/​new_in_javascript_1.8#​section_4 
- 
-Python: 
-    * http://​www.python.org/​doc/​2.5.2/​tut/​node7.html#​SECTION007130000000000000000 
-    * http://​www.python.org/​doc/​2.5.2/​lib/​itertools-functions.html 
- 
- 
-===== Changelog ===== 
- 
  
rfc/iteration-tools.txt · Last modified: 2017/09/22 13:28 (external edit)