Byte-sized functional programming: Filter first

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)) {
        continue;
    }
    // ...
}

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.

$criteria = 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 loop, or, better, it's now ideally suited for use with array_map().

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