func.php

<?php
function map($collection) : \Generator
{
    if (is_array($collection) || $collection instanceof \Traversable)
    {
        foreach($collection as $element) {
            yield $element;
        }
    }
    throw new \InvalidArgumentException();
}

function reduce($collection, callable $callback, $initializeValue = null)
{
    if (is_array($collection) || $collection instanceof \Traversable)
    {
        foreach($collection as $element) {
            $initializeValue = $callback($element, $initializeValue);
        }
        return $initializeValue;
    }
    throw new \InvalidArgumentException();
}

function filter($collection, callable $callback) : \Generator
{
    if (is_array($collection) || $collection instanceof \Traversable)
    {
        foreach($collection as $element) {
            if ($callback($element)) {
                yield $element;
            }
        }
    }
    throw new \InvalidArgumentException();
}