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Date: Fri, 15 Nov 2013 11:07:56 +0100
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Subject: RE: [PHP-DEV] Proposal: Type Casting User Classes
From: php@tutteli.ch ("Robert Stoll")

Hi Chris

> -----Original Message-----
> From: Chris London [mailto:me@chrislondon.co]
> Sent: Thursday, November 14, 2013 7:40 PM
> To: internals@lists.php.net
> Subject: [PHP-DEV] Proposal: Type Casting User Classes
> 
> Hey everyone,
> 
> I want to get a feel for something I would like to implement into PHP
core.
>  I would like to be able to cast objects to my own class types.  Similar
to
> already available PHP syntax:
> 
> $foo = (string) $bar;
> 
> The syntax would look like this:
> 
> $foo = (\My\FooObject) $bar;
> 
> It would also support:
> 
> $foo = ( (\My\FooObject) $bar )->myFooObjectFunction();
> 
> I'm still trying to decide what would happen internally and I would like
some
> feedback.  I'm thinking of 3 different things it could do:
> 
> 1) Directly change the class type, functionally equivalent to:
> 
>     function castType($object, $type) {
>         return unserialize(preg_replace("/^O:[0-9]+:\"[^\"]+\":/i",
>     "O:".strlen($type).":\"$type\":", serialize($object)));
>     }

[Robert Stoll]
Seems a bad idea to me. Would be slow and is not really a cast but a
conversion. 

> 2) Throw a fatal error if the object doesn't match the type trying to be
cast.

[Robert Stoll]
That's what I would expect from a runtime type checking mechanism

> 3) Add a new static magic method called __cast() that would allow the
> developer to decide what happens. Like this:
> 
> namespace \My;
> 
> class FooObject {
>     public static function __cast($foo) {
>         // Do stuff here
>         return new FooObject;
>     }
> }

[Robert Stoll] 
This is interesting but goes beyond casts and is a conversion mechanism
again. But would be truly helpful for utility classes like Email etc. (there
was already another discussion about this feature as far as I remember)

> I'm willing to implement this myself if everybody else likes this idea.
>  Thanks!
> 
> Chris London

You might be interested in my project Type-Safe PHP (TSPHP):
http://tsphp.tutteli.ch

It is far from a real release candidate but that's mostly because I am the
only developer contributing to this project so far (and unfortunately I do
not really have time lately).
Other contributors are welcome ;-)

TSPHP would substitute object casts for you (without the performance penalty
of an additional function call and the use of is_a) but you could emulate it
with a function this way:

function cast($object, $type){
    if($object == null || \is_a($object,$type)){
        return $object;
    }else{
        trigger_error('cast error, $object was not of type '.$type);
    }
}

I have chosen trigger_error to be consistent with (int), (string) etc. which
trigger an error as well if something goes wrong. Replace it with a throw if
you like.

And use it like this - a one liner as well ;-)
class A{}
class B extends A{}
$a = new B();
$a = cast($a, "A");

Of course, $a will not change its type but is guaranteed to be of type A. 
There was a discussion about down casting in another email. That's not
possible with this function since it is a cast and not a conversion. Btw the
Liskov Substitution Principle tells that you should be able to replace your
class with your sub-class without breaking things but not that a class
magically can do the same thing as a sub-class. But you were probably
thinking about a conversion/mapping and fair enough, that might be possible
but there are a lot of cases where this is not as straight forward as you
may think.

Cheers,
Robert