$rows = group_by([ 
    [ 'category' => 'Food', 'type' => 'pasta', 'amount' => 1, 'foo' => 10 ],
    [ 'category' => 'Food', 'type' => 'pasta', 'amount' => 1 ],
    [ 'category' => 'Food', 'type' => 'juice', 'amount' => 1 ],
    [ 'category' => 'Food', 'type' => 'juice', 'amount' => 1 ],
    [ 'category' => 'Book', 'type' => 'programming', 'amount' => 5 ],
    [ 'category' => 'Book', 'type' => 'programming', 'amount' => 2 ],
    [ 'category' => 'Book', 'type' => 'cooking', 'amount' => 6 ],
    [ 'category' => 'Book', 'type' => 'cooking', 'amount' => 2 ],
], ['category','type'], [
    'total_amount' => [
        'selector' => 'amount',
        'aggregator' => REDUCER_SUM,
    ],
    'cnt' => REDUCER_COUNT,
]);
print_r($ret);

[
    '{alias}' => [
        'selector' => '{selector}',
        'aggregator' => {constant | function},
    ],
    '{alias}' => {constant | function},
]

$ret = group_by($rows, ['category','type'], [
    'amount' => function($carry, $current) {
        return $carry + $current;
    }
]);

$result = group_by($rows, ['category','type'], [
    'total_amount' => [
        'selector'   => 'amount',
        'aggregator' => function($carry, $current) { return $carry + $current; }
    ],
]);