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Learn Functional Programming in PHP

Referential Transparency

Learn about referentially transparent functions with different useful examples.

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Overview

A referentially transparent function is one in which the function call is interchangeable with the output of the function. This interchangeability shouldn’t affect a program’s side effects or side causes. Like immutability, referential transparency is associable with pure functions. In fact, the latter is a metric for measuring the former.

PHP
<?php
$insertUnderscores = fn (string $text): string => ( 
    implode('_', explode(' ', $text))
);
var_dump($insertUnderscores('hello world') === 'hello_world');  // evaluates to true

The lambda function above replaces spaces in a text with underscores. This function’s output, with a "hello world" input, is compared with the underscore-separated string "hello_world". The comparison yields a positive result, implying that this function call could’ve been replaced with the ...