fp.php

<?php

declare(strict_types = 1);

function id($x)
{
    return $x;
}

function anyPass(array $preds)
{
    return function ($x) use ($preds) {
        return array_reduce($preds, function ($prev, callable $pred) use ($x) {
            return true === $prev
                ? $prev
                : $pred($x);
        }, false);
    };
}

function isEmpty($x)
{
    return anyPass([
        'is_null',
        equals(''),
        equals(0),
        equals(0.0),
        equals('0'),
        equals([]),
        equals(false),
    ])($x);
}

function ifElse(callable $pred)
{
    return function (callable $succfn) use ($pred) {
        return function (callable $failfn) use ($pred, $succfn) {
            return function ($x = null) use ($pred, $succfn, $failfn) {
                return $pred($x)
                    ? $succfn($x)
                    : $failfn($x);
            };
        };
    };
}

function equals($x)
{
    return function ($y) use ($x) {
        return $y === $x;
    };
}

function always($x)
{
    return function () use ($x) {
        return $x;
    };
}

function has($x)
{
    return function (array $xss) use ($x) {
        return isset($xss[$x]);
    };
}

function cond(array $pairs)
{
    return function ($x) use ($pairs) {
        $pair = head($pairs);

        if (!isset($pair[1])) {
            throw new \InvalidArgumentException();
        }

        $lazyfn = function ($x) use ($pairs) {
            return cond(tail($pairs))($x);
        };

        return ifElse($pair[0])
            ($pair[1])
            ($lazyfn)
            ($x);
    };
}

function head(array $xss)
{
    if ([] === $xss) {
        throw new \InvalidArgumentException();
    }
    
    return array_values(array_slice($xss, 0, 1))[0];
}

function tail(array $xss) {
    return array_slice($xss, 1);
}

function get(array $xss)
{
    return function ($x, $notfound = false) use ($xss) {
        return $xss[$x] ?? $notfound;
    };
}

function getIn(array $xss)
{
    return function (array $ks, $notfound = false) use ($xss) {
        $lazyfn = function (array $ks) use ($xss, $notfound) {
            $xs = get($xss)(head($ks), $notfound);

            return is_array($xs)
                ? getIn($xs)(tail($ks), $notfound)
                : $xs;
        };

        return cond([
            [has(0), $lazyfn],
            [always(true), always($notfound)]
        ])($ks);
    };
}

$numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0];
assert(head($numbers) === 1);
assert(tail($numbers) === [2, 3, 4, 5, 6, 7, 8, 9, 0]);

$user = [
    'username' => 'sally',
    'profile' => [
        'name' => 'Sally Clojurian',
        'address' => [
            'city' => 'Austin',
            'state' => 'TX'
        ]
    ]
];

$xuser = get($user);
assert($xuser('name', null) === null);
assert($xuser('username') === 'sally');

$yuser = getIn($user);
assert($yuser([], null) === null);
assert($yuser(['profile', 'name']) === 'Sally Clojurian');
assert($yuser(['profile', 'address', 'city']) === 'Austin');

$xcond = cond([
    [equals(0), always('water freezes at 0°C')],
    [equals(100), always('water boils at 100°C')],
    [always(true), function ($temp) {
        return 'nothing special happens at '.$temp.'°C';
    }]
]);
assert($xcond(0) === 'water freezes at 0°C');
assert($xcond(100) === 'water boils at 100°C');
assert($xcond(50) === 'nothing special happens at 50°C');

$xanyPass = anyPass([has('telephone'), has('name')]);
$yanyPass = anyPass([has('username')]);
assert($xanyPass($user) === false);
assert($yanyPass($user) === true);

assert(isEmpty('') === true);
assert(isEmpty(null) === true);
assert(isEmpty(0) === true);
assert(isEmpty(0.1) === false);