PHP Streams

stream.php

<?php
// Now I remember why I don't use PHP anymore.

function repeat($n) {
	return function() use ($n) {
		return [$n, repeat($n)];
	};
}

function countFrom($n) {
	return function() use ($n) {
		return [$n, countFrom($n+1)];
	};
}

function mapOn($s, $fn) {
	return function() use ($s, $fn){
		list($head, $tail) = $s();
		return [$fn($head), mapOn($tail, $fn)];
	};
}

function take($s, $n) {
	if($n <= 0) {
		return [];
	}

	list($head, $tail) = $s();

	$arr = take($tail, $n-1);
	array_unshift($arr, $head);
	return $arr;
}

function combine($s1, $s2, $fn) {
	return function() use ($s1, $s2, $fn) {
		list($h1, $t1) = $s1();
		list($h2, $t2) = $s2();
		return [$fn($h1, $h2), combine($t1, $t2, $fn)];
	};
}

$test1 = countFrom(1);

$test2 = mapOn($test1, function($n) {
	return $n+$n;
});

$test3 = combine($test1, $test2, function($x, $y) {
	return $x + $y;
});

echo "\n\tTest 1\n";
foreach(take($test1, 5) as $i) {
	echo $i."\n";
}

echo "\n\tTest 2\n";
foreach(take($test2, 5) as $i) {
	echo $i."\n";
}

echo "\n\tTest 3\n";
foreach(take($test3, 20) as $i) {
	echo $i."\n";
}