Byte-sized functional programming: Filter first

Submitted by Larry on 3 August 2020 - 1:47pm

Often when working with a list, we only want to work with a subset of a list that meets some criteria. All non-zero values, for example, or all users that have a given role. The procedural way to do that is to stick an if statement inside a foreach loop:

foreach ($list as $value) {
    If (!
meets_criteria($value)) {
// ...

That mixes up the filtering with the iteration, though. It also doesn't work if we're using `array_map()`.

Instead, we can make stripping down the list a separate operation called "filter." PHP offers the array_filter() function for that purpose.

= fn(User $user): bool => $user->hasRole('moderator');

$filtered = array_filter($users, $criteria);

Now we can work with the `$filtered` list, which has only the values we want. That could be a simple foreach<code> loop, or, better, it's now ideally suited for use with <code>array_map().

Want to know more about functional programming and PHP? Read the whole book on the topic: Thinking Functionally in PHP.

Thinking Functionally in PHP