Merge pull request #67 from grakic/gh-pages_functional

Extends the functional paradigm section.

_posts/03-02-01-Programming-Paradigms.md

@@ -5,8 +5,8 @@ isChild: true
 ## Programming Paradigms
 
 PHP is a flexible, dynamic language that supports a variety of programming techniques. It has evolved dramatically over
-the years, notably adding a solid object-oriented model in PHP 5.0 (2004), anonymous functions and namespaces in PHP
-5.3 (2009), and traits in PHP 5.4 (2012).
+the years, notably adding a solid object-oriented model in PHP 5.0 (2004), anonymous functions and namespaces in PHP 5.3 
+(2009), and traits in PHP 5.4 (2012). 
 
 ### Object-oriented Programming
 
@@ -18,23 +18,22 @@ interfaces, inheritence, constructors, cloning, exceptions, and more.
 
 ### Functional Programming
 
-PHP has had support for anonymous functions and closures since PHP 5.3:
+PHP supports first-class function, meaning that a function can be assigned to a variable. Both user defined and built-in 
+functions can be referenced by a variable and invoked dynamically. Functions can be passed as arguments to other
+functions (feature called Higher-order functions) and function can return other functions.
 
-{% highlight php %}
-<?php
-$greet = function($name)
-{
-    print("Hello {$name}");
-};
+Recursion, a feature that allows a function to call itself is supported by the language, but most of the PHP code focus
+on iteration.
 
-$greet('World');
-{% endhighlight %}
+New anonymous functions (with support for closures) are present since PHP 5.3 (2009).
 
 PHP 5.4 added the ability to bind closures to an object's scope and also improved support for callables such that they
 can be used interchangeably with anonymous functions in almost all cases.
 
+* Continue reading on [Functional Programming in PHP](/pages/Functional-Programming.html)
 * [Read about Anonymous Functions][anonymous-functions]
 * [Read about the Closure class][closure-class]
+* [More details in the Closures RFC][closures-rfc]
 * [Read about Callables][callables]
 * [Read about dynamically invoking functions with `call_user_func_array`][call-user-func-array]
 
@@ -58,3 +57,4 @@ available as `__call()` and `__callStatic()`.
 [reflection]: http://www.php.net/manual/en/intro.reflection.php
 [traits]: http://www.php.net/traits
 [call-user-func-array]: http://php.net/manual/en/function.call-user-func-array.php
+[closures-rfc]: https://wiki.php.net/rfc/closures

pages/Functional-Programming.md

@@ -0,0 +1,87 @@
+---
+layout: page
+title: Functional Programming in PHP
+---
+
+# Functional Programming in PHP
+
+PHP supports first-class function, meaning that a function can be assigned to a variable. Both user defined and built-in 
+functions can be referenced by a variable and invoked dynamically. Functions can be passed as arguments to other
+functions (feature called Higher-order functions) and function can return other functions.
+
+Recursion, a feature that allows a function to call itself is supported by the language, but most of the PHP code focus
+on iteration.
+
+New anonymous functions (with support for closures) are present since PHP 5.3 (2009).
+
+PHP 5.4 added the ability to bind closures to an object's scope and also improved support for callables such that they
+can be used interchangeably with anonymous functions in almost all cases.
+
+The most common usage of higher-order functions is when implementing a strategy pattern. Built-in `array_filter`
+function asks both for the input array (data) and a function (a strategy or a callback) used as a filter function on
+each array item.
+
+{% highlight php %}
+<?php
+$input = array(1, 2, 3, 4, 5, 6);
+
+// Creates a new anonymous function and assigns it to a variable
+$filter_even = function($item) {
+    return ($item % 2) == 0;
+};
+
+// Built-in array_filter accepts both the data and the function
+$output = array_filter($input, $filter_even);
+
+// The function doesn't need to be assigned to a variable. This is valid too:
+$output = array_filter($input, function($item) {
+    return ($item % 2) == 0;
+});
+
+print_r($output);
+{% endhighlight %}
+
+Closure is an anonymous function that can access variables imported from the outside scope without using any global
+variables. Theoretically, a closure is a function with some arguments closed (e.g. fixed) by the environment when it is 
+defined. Closures can work around variable scope restrictions in a clean way.
+
+In the next example we use closures to define a function returning a single filter function for `array_filter`, out of
+a family of filter functions.
+
+{% highlight php %}
+<?php
+/**
+ * Creates an anonymous filter function accepting items > $min
+ *
+ * Returns a single filter out of a family of "greater than n" filters
+ */
+function criteria_greater_than($min)
+{
+    return function($item) use ($min) {
+        return $item > $min;
+    };
+}
+
+$input = array(1, 2, 3, 4, 5, 6);
+
+// Use array_filter on a input with a selected filter function
+$output = array_filter($input, criteria_greater_than(3));
+
+print_r($output); // items > 3
+{% endhighlight %}
+
+Each filter function in the family accepts only elements greater than some minimum value. Single filter returned by 
+`criteria_greater_than` is a closure whith `$min` argument closed by the value in the scope (given as an argument when 
+`criteria_greater_than` is called).
+
+Early binding is used by default for importing `$min` variable into the created function. For true closures with late
+binding one should use a reference when importing. Imagine a templating or input validation libraries, where closure is 
+defined to capture variables in scope and access them later when the anonymous function is evaluated.
+
+* [Read about Anonymous functions][anonymous-functions]
+* [More details in the Closures RFC][closures-rfc]
+* [Read about dynamically invoking functions with `call_user_func_array`][call-user-func-array]
+
+[anonymous-functions]: http://www.php.net/manual/en/functions.anonymous.php
+[call-user-func-array]: http://php.net/manual/en/function.call-user-func-array.php
+[closures-rfc]: https://wiki.php.net/rfc/closures