PHP で reduce と reduceRight

reduce_and_reduceRight.php

<?php
// Haskell の foldl と foldr で同じ値の計算: http://blog.sarabande.jp/post/13599950587

function reduce(array $input, $callable, $initial = NULL) {
    $ret = $initial;
    foreach ($input as $v) {
        $ret = call_user_func($callable, $ret, $v);
    }
    return $ret;
}

function reduceRight(array $input, $callable, $initial = NULL) {
    $ret = $initial;
    foreach (array_reverse($input) as $v) {
        $ret = call_user_func($callable, $v, $ret);
    }
    return $ret;
}

function f($x, $y) {
    return $x + 2 * $y;
}

$array = array(1, 2, 3);

var_dump(
reduce($array, 'f', 4), // 16
reduceRight($array, 'f', 4), // 49
f(f(f(4, 1), 2), 3), // 16
f(1, f(2, f(3, 4)))  // 49
);