Lazy sequences using PHP 5.5 generators.

php-lazy-seq.php

<?php

// thanks to metaultralurker on reddit for inspiration

function map(callable $fn, \Traversable $data) {
    foreach ($data as $v) {
        yield $fn($v);
    }
}

function filter(callable $fn, \Traversable $data) {
    foreach ($data as $v) {
        if ($fn($v)) {
            yield $v;
        }
    }
}

function infinite_range() {
    $i = 0;
    while (true) {
        yield $i++;
    }
}

function take($n, \Traversable $data) {
    $i = 0;
    foreach ($data as $v) {
        if ($i++ === $n) {
            break;
        }

        yield $v;
    }
}

function reduce(callable $fn, \Traversable $data, $runningTotal = null) {
    $first = true;
    foreach ($data as $v) {
        if (null === $runningTotal && $first) {
            $runningTotal = $v;
            continue;
        }
        $runningTotal = $fn($runningTotal, $v);
        $first = false;
    }

    return $runningTotal;
}

$isEven = function ($x) {
    return $x % 2 == 0;
};

$data = take(10, map('floatval', filter($isEven, infinite_range())));

/*
var_dump($data->current());
$data->next();
var_dump($data->current());
$data->next();
*/

// var_dump(iterator_to_array($data));

$concat = function ($sep, $a, $b) {
    return $a.$sep.$b;
};

$concatComma = function ($a, $b) use ($concat) {
    return $concat(',', $a, $b);
};

var_dump(reduce($concatComma, $data));