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Date: Sun, 1 Feb 2015 02:56:22 +0000
Cc: Larry Garfield <larry@garfieldtech.com>,
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To: Stanislav Malyshev <smalyshev@gmail.com>
Subject: Re: [PHP-DEV] [RFC] Immutable variables and objects
From: ajf@ajf.me (Andrea Faulds)

Hi,

> On 1 Feb 2015, at 02:09, Stanislav Malyshev <smalyshev@gmail.com> =
wrote:
>=20
>> Here, there=E2=80=99s no redundant objects made, but if you pass $a =
on, it=E2=80=99d
>> be automatically copied by PHP, so you don=E2=80=99t need to worry =
about it
>> being modified.
>=20
> I don't think it's a particularly good solution in this case, as in =
many
> cases (especially DI setups, many design patterns, etc.) the whole =
point
> of creating the object is to pass it around. Just pointlessly copying =
it
> out of fear somebody somewhere could modify it doesn't sound the best
> way.

Well, that=E2=80=99s the advantage of copy-on-write: you can avoid =
needless manual copies, and instead have automatic copying done for you =
by the language, where needed.

Although implementing copy-on-write for object methods might be a =
challenge, give PHP doesn=E2=80=99t track which functions have side =
effects.

> I'd rather just have a clear separation between mutating and
> non-mutating APIs, and instruct people to use the right ones in right
> situation - i.e. if you created the object or own it, use mutating =
ones,
> if you got object from outside and do not have full ownership of it, =
use
> non-mutating ones.
>=20

This isn=E2=80=99t very nice in practice, though. Mutating APIs are easy =
to use and performant, non-mutating APIs are neither of these things. =
What you want is the benefits of the first without the disadvantages of =
the second.

>> Would that make sense? It=E2=80=99s no different than how our =
existing value
>> types like scalars and arrays work.
>=20
> Scalars don't have this problem as, except for string offsets (IMHO =
not
> the best idea to have mutable strings too) scalars can not really be
> changed, just replaced with other scalars. But implementing value
> objects in PHP is not hard right now - if you don't provide any =
methods
> that allow changing state, you've got an immutable object. It's just =
not
> always what people using it would want, especially with something as
> complex as HTTP message.

You=E2=80=99re ignoring arrays, which *also* have our copy-on-write =
behaviour. Also, the bigint RFC (if passed) would be another kind of =
mutable scalar. It=E2=80=99s not mutable in very many cases, but it is =
in some places as a performance optimisation.

We have value objects, sure, but they=E2=80=99re not efficient. Every =
mutation requires the creation of a new object, because you can=E2=80=99t =
do copy-on-write. Compare that to arrays: mutations there only create =
new arrays if the refcount is > 1.

The following code using immutable value objects requires the creation =
of five new objects:

$a =3D $somefoo
     ->withBar(=E2=80=A6)
     ->withBaz(=E2=80=A6)
     ->withQux(=E2=80=A6)
     ->withoutFooBar();

Yet the following code using arrays, which are passed by value in PHP, =
requires the creation of only one new array, and modifies in-place:

$a =3D $somefoo;
$a[=E2=80=98bar=E2=80=99] =3D =E2=80=A6;
$a[=E2=80=98baz=E2=80=99] =3D =E2=80=A6;
$a[=E2=80=98qux=E2=80=99] =3D =E2=80=A6;
unset($a[=E2=80=98foobar=E2=80=99]);

Is that not superior?

--
Andrea Faulds
http://ajf.me/