Monoids for validation

higher-order.php

<?php

function partial($function, ...$arguments)
{
    return function() use ($function, $arguments) {
        $fullArguments = array_merge($arguments, func_get_args());

        return call_user_func_array($function, $fullArguments);
    };
}

maybe.php

<?php

interface Maybe
{
}

final class Just implements Maybe
{
    private $value;

    public function __construct($value)
    {
        $this->value = $value;
    }

    public function get()
    {
        return $this->value;
    }
}

final class Nothing implements Maybe
{
}

function combine(callable $f, Maybe $m1, Maybe $m2)
{
    if ($m1 instanceof Nothing) {
        return $m2;
    } elseif ($m2 instanceof Nothing) {
        return $m1;
    } else {
        return new Just($f($m1->get(), $m2->get()));
    }
}

validation.php

<?php

require_once __DIR__ . '/higher-order.php';
require_once __DIR__ . '/maybe.php';

/**
 * lists of validation results are monoids, we only care about the validation errors:
 *
 * closure: a list of results + a list of results => a list of results
 * associativity: ([result1] + [result2]) + [result3] = [result1] + ([result2] + [result3])
 * identity: Maybe[result1] + Nothing = Maybe[result1]
 *
 * see also: http://fsharpforfunandprofit.com/posts/monoids-without-tears/
 */

function fail($message)
{
    // lift
    return new Just(array($message));
}

function validateBadWord($badWord, $string)
{
    if (stristr($string, $badWord)) {
        return fail('string contains a bad word: ' . $badWord);
    } else {
        return new Nothing();
    }
}

function validateLength($maxLength, $string)
{
    if (strlen($string) > $maxLength) {
        return fail('string is too long');
    } else {
        return new Nothing();
    }
}

function combineFails(Maybe $f1, Maybe $f2)
{
    $f = function(array $a, array $b) {
        return array_merge($a, $b);
    };

    return combine($f, $f1, $f2);
}


$validationResults = function ($string) {
    return array_map(
        function($validate) use ($string) {
            return $validate($string);
        },
        array(
            partial(@validateLength, 10),
            partial(@validateBadWord, 'monad'),
            partial(@validateBadWord, 'cobol'),
        )
    );
};

var_dump(
    array_reduce(
        $validationResults('cobol has native support for monads'),
        partial(@combineFails),
        new Nothing() // Identity
    )
);