How to do functional programming with PHP

PHP is a widely used server-side language. One of the reasons why many web developers like to use PHP is its rich function library and simple and easy-to-use function syntax. Functional programming is a programming paradigm that well encapsulates data and behavior, making the code more modular and easy to maintain and test. In this article, we will introduce how to use PHP for functional programming.

1. Functional Programming Basics

The core idea of ​​functional programming is to treat functions as first-class citizens, and functions themselves can be passed, returned, and composed like variables. In functional programming, we don't modify mutable state, but we transform and filter it through functions.

PHP itself supports functional programming, and there are many built-in functions that can be used for functional processing. For example array_map, array_filter, etc. Below we will demonstrate how to use these functions to implement common functional programming operations.

2. Higher-order functions

Higher-order functions refer to functions that can accept functions as parameters or return functions. Such functions can be used to compose and reuse code. In PHP, commonly used higher-order functions include array_map, array_filter, array_reduce, etc.

The array_map function can accept a function and an array as parameters and return a new array. The elements of the new array are the values ​​obtained after the elements in the original array are converted by the function.

For example, we have an array $x=[1,2,3,4]$ and want to square each element in the array. You can use the following code:

function square($x) {
    return $x * $x;
}

$array = [1, 2, 3, 4];
$new_array = array_map('square', $array);
var_dump($new_array); // 输出 [1, 4, 9, 16]

## The #array_filter function can accept a function and an array as parameters and return a new array. The elements in the new array are the elements in the original array that meet the conditions.

For example, we have an array $x=[1,2,3,4]$ and want to get all the even numbers in the array. You can use the following code:

function is_even($x) {
    return $x % 2 == 0;
}

$array = [1, 2, 3, 4];
$new_array = array_filter($array, 'is_even');
var_dump($new_array); // 输出 [2, 4]

array_reduce function can accept Takes a function, an array, and an initial value as arguments and returns an accumulated result. The reduce function passes each element in the array to a function for calculation, and then gets a cumulative result.

For example, we have an array $x=[1,2,3,4]$ and want to accumulate all elements in the array. You can use the following code:

function add($a, $b) {
    return $a + $b;
}

$array = [1, 2, 3, 4];
$result = array_reduce($array, 'add');
var_dump($result); // 输出 10

Anonymous function

Anonymous function is a function without a name that can be used to define functions that are used only once or rarely. In functional programming, anonymous functions can be used for fast and lightweight function definition and composition.

In PHP, you can use the keyword function to define anonymous functions, or you can use arrow functions for quick definition.

For example, we can use an anonymous function to quickly define a square function:

$square = function($x) {
    return $x * $x;
};

$result = $square(3);
var_dump($result); // 输出 9

Arrow function syntax can further simplify the above code:

$square = fn($x) => $x * $x;
$result = $square(3);
var_dump($result); // 输出 9

Curi Currying is a technique that converts a function with multiple parameters into a function with a single parameter. In functional programming, currying can be used to reuse and simplify functions.
4. In PHP, you can use closures and higher-order functions to implement currying. For example, we have a function add(x,y,z) and hope to implement a curried version:

function add($x, $y, $z) {
    return $x + $y + $z;
}

$curried_add = function($x) use ($add) {
    return function($y) use ($x, $add) {
        return function($z) use ($x, $y, $add) {
            return $add($x, $y, $z);
        };
    };
};

$result = $curried_add(1)(2)(3);
var_dump($result); // 输出 6

Combined function

Combined function refers to combining multiple Functions are combined together to form new functions, which can be used to simplify the code and enhance the readability and maintainability of the code.
5. In PHP, you can use closures and higher-order functions to achieve function combination. For example, we have two functions $f(x)$ and $g(x)$, and want to implement a combined function $h(x)$ such that $h(x) = f(g(x))$:

function f($x) {
    return $x + 1;
}

function g($x) {
    return $x * 2;
}

$compose = function($f, $g) {
    return function($x) use ($f, $g) {
        return $f($g($x));
    };
};

$h = $compose('f', 'g');
$result = $h(2);
var_dump($result); // 输出 5

Summary

This article introduces how to use PHP for functional programming, including common techniques such as higher-order functions, anonymous functions, currying, and function composition. Functional programming can make code more modular, easier to test and maintain. For web developers, understanding functional programming is very valuable.

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