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FunctorLaws.php
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FunctorLaws.php
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<?php
declare(strict_types=1);
namespace FunctionalPHP\FantasyLand\Helpful;
use FunctionalPHP\FantasyLand\Functor;
use const FunctionalPHP\FantasyLand\identity;
use function FunctionalPHP\FantasyLand\compose;
use function FunctionalPHP\FantasyLand\map;
class FunctorLaws
{
/**
* Generic test to verify if a type obey the functor laws.
*
* @param callable $assertEqual Asserting function (Functor $f1, Functor $f2, $message)
* @param callable $f (a -> b)
* @param callable $g (a -> b)
* @param Functor $x f a
*/
public static function test(
callable $assertEqual,
callable $f,
callable $g,
Functor $x
) {
// identity: fmap id == id
$assertEqual(
map(identity, $x),
$x,
'identity'
);
// composition: fmap (f . g) == fmap f . fmap g
$assertEqual(
map(compose($f, $g), $x),
compose(map($f), map($g))($x),
'composition'
);
}
}