Pure functions are concepts in functional programming. They can be implemented in PHP by following specific rules. The core features include: 1. No side effects, no modification of global variables, object states or performing I/O operations; 2. The same input always returns the same output, and does not rely on external data such as time and random numbers; 3. Keep simple and focused, only process inputs and return results, and do not use reference modifications or static variables to retain state. For example, sum(int $a, int $b): int is a typical pure function that only depends on parameters and has no external influence.
A pure function is a concept from functional programming that refers to a function which, given the same input, will always return the same output and has no side effects. This means it doesn't modify any external state or rely on external variables — it only depends on its parameters.
In PHP, while not strictly functional, you can still write pure functions by following some basic rules.
No Side Effects
A pure function shouldn't change anything outside of itself. That includes:
- Not modifying global variables
- Not changing object states (if passed in)
- Not performing I/O operations like writing to a file or making API calls
Example:
// Impure example – modifies an external variable
$counter = 0;
function increment() {
global $counter;
return $counter;
} // Pure version – take value as input, returns new value
function increment($value) {
return $value 1;
}The second version is better because it's predictable and easier to test. You know what goes in and exactly what comes out.
Consistent Output for Same Input
A pure function must always return the same result when given the same arguments. It shouldn't rely on randomness, current time, or any hidden data.
Avoid this:
function getCurrentTime() {
return time(); // Different every time – impure
}Instead, if you need time-based logic, pass the time as an argument:
function formatTime($timestamp) {
return date('Ymd H:i:s', $timestamp); // Pure
}This makes your code more testable and less dependent on environment state.
Keep It Simple and Focused
Pure functions tend to be small and focused on one task. They take input, process it, and return output — nothing else.
Here are a few tips:
- Use scalar types (
int,string,bool) or arrays as inputs and outputs where possible. - Avoid using
&$variablesfor reference changes. - Don't use static variables inside the function to retain state.
Example of a pure PHP function:
function sum(int $a, int $b): int {
return $a $b;
}This function is completely self-contained. It doesn't touch anything else, and it gives a reliable result every time.
That's basically how you write a pure function in PHP. It's not hard, but it does require being mindful of dependencies and side effects.
The above is the detailed content of What is a 'pure function' and how to write one in PHP?. For more information, please follow other related articles on the PHP Chinese website!
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