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Date: Mon, 12 Sep 2016 19:07:51 -0500
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Subject: Re: [PHP-DEV] RFC - Immutable classes
From: larry@garfieldtech.com (Larry Garfield)

On 09/12/2016 06:47 PM, Stephen Reay wrote:
>> Ah, I did see that one, but there was a lot of discussion after it so I thought the idea evolved.  Response below based on skimming the responses after that as well...
>>
>> It sounds like there's more that needs to go on, though.  It sounds like that thread is suggesting that $this in a method of an immutable object is always cloned, which seems excessive to me.
>>
>> The point about identity is well-taken.  However, it's more complex on objects because equality is not as simple as it is on strings. Two objects can be logically identical but physically different. For instance:
>>
>> class HttpHeaders {
>>   public $attributes = [];
>> }
>>
>> $a = new HttpHeaders();
>> $a->attributes['foo'] = 'bar';
>> $a->attributes['baz'] = 'buzz';
>>
>> $b = new HttpHeaders();
>> $b->attributes['baz'] = 'buzz';
>> $b->attributes['foo'] = 'bar';
>>
>> $a and $b are now not physically identical,  since their attributes array is in a different order.  However, header order doesn't matter in HTTP, or rather isn't supposed to.  (Modulo buggy implementations, of course.)  So are $a and $b identical?  I could very easily argue both directions on that.  Physical identity would be easier to automate checking in the engine; logical identity would require user-space code to make such decisions.
>>
>>  From the discussion of "transformer" methods, I'd propose a slightly different keyword.  To wit:
>>
>> 1) A class marked as "immutable" may not have any of its properties altered, EXCEPT in certain unlocked scopes.  The constructor is an unlocked scope.
>>
>> 2) A method may be marked "clone": public clone function foo() {}. A clone method is identical to any other method except that A) $this in its scope is not the original object, but the result of clone($this); B) A clone method is an unlocked scope, so modifying $this (the clone) is legal.  That is more self-descriptive than "transformer", and also doesn't require a new keyword.  (By implication, clone and static are mutually exclusive since clone requires an object to clone.)
>>
>> I don't know that there are any other unlocked scopes to consider...
>>
>> #2 does leave us with the identity question that Richard raises, in that returning an unmodified $this from a clone method still creates a new object identity.  However, I posit that is to be expected.  In Python, eg, while some built-in objects are immutable they are not, necessarily, always the same physical object in memory.  (999+1 is not the same object as 1000, but 1+2 and 2 likely will be due to internal engine optimizations.)  You need to do a value comparison of them.
>>
>> I don't see the identity case being resolved without an __identity() method, or similar.  Which could be useful in its own right so I'm not necessarily against it, but it's an extra, and I'd argue optional, piece of the puzzle.
>>
>> Related to whether or not the properties of an object may be mutable (arrays, other objects, etc.), they would in practice probably need to blacklist resources.  We ran into that issue in PSR-7, where the body stream has to be mutable, because streams.  Since PSR-7 is based on PHP 5.3 that doesn't cause any syntactic issues, just logical issues.  If the classes were marked immutable explicitly, it likely would be a problem since the streams cannot be immutable, thus they can't be used on an immutable object.  Which is... very sad making. :-(
>>
>> --Larry Garfield
>>
>> -- 
>> PHP Internals - PHP Runtime Development Mailing List
>> To unsubscribe, visit: http://www.php.net/unsub.php
>>
> I like your suggestion with the ‘clone’ keyword on methods.
>
> As I said before, I think it’s a mis-step to prevent manual cloning at the engine level (even if the result is that you return the same instance) - there is *bound* to be user land code that clones objects. Without this, any use of clone in a distributed library/framework will have to do a check to see if an object (except those accepting final classes of known definition) is immutable before attempting to clone it.

I think most people in the thread have agreed that blocking clone() is 
unnecessary, the RFC just hasn't been updated yet.  With a clone method, 
an external clone becomes basically a pointless but harmless operation, 
I'd think.  A clone within the class would be fairly nonsensical, except 
for the clone methods.

> Regarding identity, I’m going to refer back to the DateTimeImmutable class. I know its not the same implementation, but honestly I don’t think that matters. Developers use PHP because they generally *don’t* have to worry about the internal details of the engine.
>
> e.g.:
> $d = new DateTimeImmutable();
> $e = $d->add(new DateInterval('PT0S'));
>
> var_dump($d === $e);  // bool(false)
>
>
> Personally I think if we want to worry about whether two objects represent the same value, wouldn’t that be better handled (and with much greater positive effect for developers) by finalising & passing the Comparable RFC (https://wiki.php.net/rfc/comparable)?
>
> Cheers
>
> Stephen

That does seem like the more targeted solution, yes.

I forgot to mention before, there was discussion of marking interfaces 
as immutable.  I would argue yes, they should, because you'd want to 
mark methods within the class as clone methods.  While I suppose that 
would technically not require an immutable class/interface, it's the 
obvious place to use it.

(Which is another interesting question: is there a reason to not allow 
clone methods on non-immutable classes?  That seems... I suppose 
unnecessary but harmless to allow, so may as well?)

--Larry Garfield