Closures in PHP, Javascript and Elixir

Closures are one of the fundamental building blocks of functional programming techniques.
A Closure is a construct that permits to bind the definition of a function together a particular scope, normally implemented via anonymous functions.
They are particularly important, for example in the implementation of event callbacks.
Let’s see how they work in three different languages: PHP, Javascript and Elixir.

PHP is not -traditionally- a programming language strictly functional and to code following the functional programming paradigm we need some of the features added in the last versions, specifically PHP 5.3.

Consider the following example:

<?php

$a = 10;

$closure = function() use ($a) {

    echo $a."\n";

};

$closure();

$a = 20;

$closure();

// Output:
// 10
// 10

The $a variable value is copied to the closure function scope via the ‘use’ keyword.
Note that we must explicit declare the variable we want to inherit inside the closure scope. From PHP 5.4 the $this special variable is inherited automatically.

The Javascript example:

var a = 10;

var closure = function() {

    console.log(a);
    
}

closure();

a = 20;

closure();

// Output:
// 10
// 20

We don’t need to explicitly “pass in” the variables same as in PHP: the closure “grabs” automatically all the variables in the outer scope.
And there is another difference, more important: the variables are bound by reference, if we mutate the data in the outer scope, the closure value will change.

And now Elixir:

a = 10

closure = fn() -> IO.puts a end

closure.()

a = 20

closure.()

// Output:
// 10
// 10

The closure creates an instance that refers to immutable values by default.
It inherits automatically all the value in the outer scope.