php 5.3 y combinator and other musings

Whenever I get excited about php 5.3 and start foaming at the mouth about lambdas/closures etc. I think I probably just come off like a wing nut. So here is the "canonical example" converted from the work of this dudes javascript example: http://rayfd.wordpress.com/2007/05/06/y-combinator-for-dysfunctional-non-schemers/

function apply(){
  $args = func_get_args();
  return call_user_func_array(array_shift($args), $args);
}

function Y($f) {
    $recur = function($r) use($f) {
        return function($n) use ($f, $r) {
            return apply($f($r($r)), $n);
        };
    };
    return $recur($recur);
}

$fibo = Y(function($f) {
    return function($n) use ($f) {
        if($n <= 2) {
            return 1;
        } else {
            return $f($n - 1) + $f($n - 2);
        }
    };
});

echo $fibo(15); #610

You'll notice the main trick is the explicit closures using the "use" keyword. I think I almost DO like this solution as it is rather explicit about what vars you are closing over.

Here is another example taken from the first chapter of SICP:

define('tolerance', 0.00001);

function fixed_point($f, $first_guess){
    $close_enough = function($v1, $v2){
        return (abs($v1 - $v2) < tolerance);
    };
  
    $try = function($guess) use(&$try, $f, $close_enough){
        $next = $f($guess);
        return $close_enough($guess, $next) ? $next : $try($next);
    };

    return $try($first_guess);
}

echo fixed_point('cos', 1.0); #0.7390822985224
echo fixed_point(function($y){return sin($y) + cos($y);}, 1.0); #1.2587315962971

Other than the ugliness that is the mandatory return statements and semi-colons even for the last/only statement in a function... I actually sort of dig this.

The only trick in this example is "use(&$try, ..." clause which passes a ref to $try so the function can call itself.

Here are some more cool scheme tricks. Creating cons, car, cdr, lst etc. from closures:

function cons($x, $y){
    return function($m) use(&$x, &$y){
        return $m ? $x : $y;
    };
}

function car($z){ return $z(true); }

function cdr($z){ return $z(false); }

define('nil', null);

function lst(){
    $args = func_get_args();
    return cons(
        array_shift($args),
        empty($args) ? nil : call_user_func_array('lst', $args));
}

function length($items){
    return $items == nil ?
        0 :
        1 + length(cdr($items));
}

function append($list1, $list2){
    return $list1 == nil ?
        $list2 :
        cons(car($list1), append(cdr($list1), $list2));
}

Obviously the performance implications here have not been explored - it's purely academic but interesting to see what is now possible.

Want to implement your own objects? Here is a perl-esque approach returning a hash of functions and using a ref to $self for accessing methods within the object:

function person($age){
    $self = array(
        'get_age' => function() use(&$age){
            return $age;
        },
        'set_age' => function($new_age) use(&$self, &$age){
            $age = $new_age;
            return $self['get_age']();
        }
    );
    return $self;
}
$peter = person(20);
echo $peter['get_age'](); #20
echo $peter['set_age'](50); #50
echo $peter['get_age']() #50

Another thought that came to mind was "ghetto macros" by either abusing the $GLOBALS var to create functions or returning an array one could extract into an environment of their choice. Behold:

function many_methods($name, $x){
    $GLOBALS['get_'.$name] = function() use(&$x){
        return puts($x);
    };
  
    $GLOBALS['set_'.$name] = function($y) use(&$x){
        $x = $y;
    };
}
many_methods('tony', 20);
$get_tony(); #20
$set_tony(500); #500
$get_tony(); #500

function clean_many($name, $x){
    return array(
        'get_'.$name => function() use(&$x){
            return puts($x);
        },
        'set_'.$name => function($y) use(&$x){
            $x = $y;
        });
}

function inner_scope(){
    extract(clean_many('walter', 400));
    $get_walter();
    $set_walter(500);
}

etc. etc. etc.