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Subject: Re: [PHP-DEV] [RFC] Immutable variables and objects
From: rowan.collins@gmail.com (Rowan Collins)

On 01/02/2015 01:01, Andrea Faulds wrote:
> Given that PHP is dynamic, not compiled, and function calls can have side effects, though, this would be difficult to enforce. You’d need to check that all calls made within the function do not have any side effects on that value…
>
> I’m not sure how workable this is.

If you can implement copy-on-write, you can implement immutability, and 
vice versa. However, any context where its possible to mix mutable and 
immutable (or mutable-reference and copy-on-write) would get confusing 
very quickly, I think.

Consider some possible behaviours of this set of objects:

$a->b = $b;
$a2 = $a;
$a2->b->value = 42;

If every object is immutable or copy-on-write, this is easy to reason 
about: $b and $a->b remain the same, while $a2->b is created as a new 
clone; in doing so, $a is separated from $a2 to reference this new copy.

But what if $a/$a2 are immutable, or copy-on-write, but $b is not - do 
$b, $a->b and $a2->b all reflect the change, with no error? That would 
certainly make no sense for const parameters, and would be a massive 
gotcha for "value objects" defined as such at the class (rather than 
instance) level.

Regards,

-- 
Rowan Collins
[IMSoP]