I tried to implement a call-with-current-continuation function, that would allow us to do some "early returns" in recursive functions. It works fantastically, and i can now write functions in continuation passing style, which opens up a whole new dimention to functional programming.
The only problem i have with this, is the way of implementing it. I implemented it using a throw/catch scheme in php, which is of course completely hidden behind function calls. What are your opinions?
function let_cc($f)
{
return function(...$params) use ($f) {
$continuation = function($v) {
throw new Continuation($v);
};
try {
$result = call($f, cons($continuation), al($params)));
} catch(Continuation $c) {
$result = $c->getContinuation();
}
return $result;
};
}
final class Continuation extends Exception {
private $continuation;
public function __construct($continuation)
{
$this->continuation = $continuation;
parent::__construct('Continuation');
}
public function getContinuation()
{
return $this->continuation;
}
}
An example of using it in a function:
// Get a list of numbers that appear after te last 1
function after_1($l)
{
// We can break out of this whole let_cc() block by calling the $skip function,
// that let_cc injects in our function.
$after_1_ = let_cc(function($skip, $l) {
$after_1__ = function($l) use ($skip, &$after_1__) {
if (isempty($l)) {
return l();
}
elseif (head($l) == 1) {
// using the $skip function we can "forget" everything we already cons'ed
// in previous recursions
return $skip($after_1__($l));
} else {
return cons($first, $after_1__($l));
}
};
return $after_1__($l);
});
return $after_1_($l);
}
$foo = after_1(l(1, 2, 3, 1, 4, 5)); // l(4, 5)
$bar = after_1(l(1, 2, 3, 4, 5)); // l(2, 3, 4, 5)
$baz = after_1(l(2, 3, 4, 5)); // l(2, 3, 4, 5)
I tried to implement a
call-with-current-continuation
function, that would allow us to do some "early returns" in recursive functions. It works fantastically, and i can now write functions in continuation passing style, which opens up a whole new dimention to functional programming.The only problem i have with this, is the way of implementing it. I implemented it using a throw/catch scheme in php, which is of course completely hidden behind function calls. What are your opinions?
An example of using it in a function: