Maybe monad

If a previous contraint outputs a Maybe and you want to access the inner value you can do:

use Innmind\Validation\Is;
use Innmind\Immutable\Maybe;

$validate = Is::int()
    ->or(Is::null())
    ->map(Maybe::of(...))
    ->and(Is::just());

In this example the input can be an int or null but it will fail the validation in case the value is null because Maybe::of(...) will move the null as a Nothing and we say we want a Just.