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Date: Sat, 6 Dec 2014 22:38:30 +0000
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To: Yasuo Ohgaki <yohgaki@ohgaki.net>
Subject: Re: [PHP-DEV] Re: Only variables can be passed by reference
From: ajf@ajf.me (Andrea Faulds)

Hi!

> On 6 Dec 2014, at 22:25, Yasuo Ohgaki <yohgaki@ohgaki.net> wrote:
>=20
> Value is vague term. I would like to use "expression". Expression is
> anything that has value.
> Variables/constants are values that have actual storage.
> Literals are values that may not have actual storage.

The C world already has terminology for this:

* lvalues - values on the left-hand side of an assignment, they can be =
assigned to

  Examples in PHP include:

  * $foo
  * $foo->bar
  * $foo->bar[0]

* rvalues - a superset of lvalues, these are values that can be on the =
right-hand side of an assignment

  Examples include:

  * All the lvalues above
  * 3
  * 2 + 3
  * foobar()
  * $foo + $bar
  * (int)$foo

In C, you (usually) can only get pointers to lvalues, as only lvalues =
can be written to. In PHP, it would make sense only to be able to =
reference lvalues, as only lvalues can be written to. In fact, we =
(almost?) always treat references as if they were assignments, hence why =
you can reference non-existent variables, because you can assign to =
non-existent variables. AFAIK this may be because referencing a value =
inherently modifies it (in PHP 5 and earlier, it sets the if_ref flag, =
while in PHP 7 it changes it to an IS_REFERENCE type). Even if =
referencing a value didn=E2=80=99t modify it, it allows you to do so =
later.

Doing something like &(2+3) makes no sense as (2 + 3) isn=E2=80=99t a =
variable, it=E2=80=99s a temporary rvalue that is only used once.

As others have said, there is no use-case for allowing assignments to or =
references to rvalues, it=E2=80=99s completely nonsensical. What would =
make sense is adding things like array_top() or array_bottom(), and =
perhaps allowing NULL rvalues to be passed to by-reference parameters =
with defaults.

Thanks!
--
Andrea Faulds
http://ajf.me/