PHP Exercises: Create a new function that composes multiple functions into a single callable

Last update on May 27 2025 13:08:15 (UTC/GMT +8 hours)

98. Compose Multiple Functions into One

Write a PHP program to create a new function that composes multiple functions into a single callable.

Sample Solution:

PHP Code:

<?php
// Licence: https://bit.ly/2CFA5XY

// Function definition for 'compose' that takes a variable number of functions as parameters
function compose(...$functions)
{
    // Use 'array_reduce' to iterate through the functions and create a composition
    return array_reduce(
        $functions,
        function ($carry, $function) {
            // Return a new function that applies the current function to the result of the previous function
            return function ($x) use ($carry, $function) {
                return $function($carry($x));
            };
        },
        // The initial function that simply returns the input
        function ($x) {
            return $x;
        }
    );
}

// Create a composition of two functions: add 2 and multiply by 4
$compose = compose(
    // add 2
    function ($x) {
        return $x + 2;
    },
    // multiply by 4
    function ($x) {
        return $x * 4;
    }
);

// Call the composed function with an argument and display the result using 'print_r'
print_r($compose(2));

// Display a newline
echo("\n");

// Call the composed function with another argument and display the result using 'print_r'
print_r($compose(3));

?>

Explanation:

Function Definition:

The compose function is defined to take a variable number of functions as parameters using the splat operator (...$functions).

Function Composition:

It uses array_reduce to iterate over the provided functions and build a composition of them.
The callback function takes two parameters: $carry (the accumulated result) and $function (the current function being processed).

Returning a New Function:

The inner function takes an argument $x and applies the current function to the result of the previous composed function ($carry($x)).
The initial function passed to array_reduce simply returns its input, acting as the base case for composition.

Creating a Composed Function:

The $compose variable stores a new function that adds 2 and then multiplies the result by 4.
This is done by passing two anonymous functions to compose:

The first function adds 2 to the input.
The second function multiplies the result by 4.

Function Calls and Output:

The composed function is called with the argument 2, and the result is displayed using print_r, which evaluates to 16 (i.e., (2 + 2) * 4).
A newline is printed.
The composed function is called again with the argument 3, and the result is displayed, which evaluates to 20 (i.e., (3 + 2) * 4).

Output:

16
20

Flowchart:

For more Practice: Solve these Related Problems:

Write a PHP script to create a function composer that chains multiple functions into a single callable.
Write a PHP function to take an array of functions and return a new function that applies them sequentially.
Write a PHP script to implement function composition so that the output of one function is fed as the input to the next.
Write a PHP script to build a composite function from several callables using higher-order programming techniques.

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PHP Code Editor:

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