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Date: Fri, 15 Mar 2019 13:27:55 +0000
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To: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [Proposal] Struct Data Types
From: rowan.collins@gmail.com (Rowan Collins)

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On Fri, 15 Mar 2019 at 09:54, Nikita Popov <nikita.ppv@gmail.com> wrote:

> 2. Readonly properties: Without requiring an entire object to be immutable,
> we can add support for readonly properties. This may be either a standalone
> feature, or part of / based on a more general property accessors proposal.
>


This is reasonable, but only if true immutability is the aim, rather than
pass-by-value / copy-on-write.

True immutability is probably fine for small structs, like Point(x, y) or
Money(currency, value), because any "modification" can just construct a new
instance manually. But in some cases, it's more convenient to have the
copying implicit, rather than needing a whole set of withFoo methods.

Note that you could have COW structs with optional immutability, and get
the best of both worlds:

struct Foo {
    readonly int $one;
    int $two;
}
$foo = new Foo(1,2);
$bar = $foo; // implicit copy-on-write clone
$bar->two = 3; // sets the property on $bar, leaving $foo untouched
$bar->one = 42; // error


I'm not sure how to do this with only reference types plus readonly
properties; you'd need some kind of "partial clone":

class Foo {
    readonly int $one;
    readonly int $two;
}
$foo = new Foo(1,2);
$bar = clone $foo { two => 3 }; // need to set the new property value while
constructing, because it's readonly
$bar->two = 3; // error
$bar->one = 42; // error


Regards,
-- 
Rowan Collins
[IMSoP]

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