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Date: Tue, 24 Sep 2019 19:24:26 +0200
Cc: Nikita Popov <nikita.ppv@gmail.com>,
 PHP internals <internals@lists.php.net>
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To: Benjamin Morel <benjamin.morel@gmail.com>
Subject: Re: [PHP-DEV] [RFC] Union Types v2
From: claude.pache@gmail.com (Claude Pache)



> Le 23 sept. 2019 =C3=A0 22:14, Benjamin Morel =
<benjamin.morel@gmail.com> a =C3=A9crit :
>=20
>=20
> So although true as a type does not have the same historical =
background as
> false, it does seem that it's being used by enough packages to be =
worth
> considering; what do you think?
>=20

Considering to support `true` will raise several questions. They can be =
resolved, of course, but that will put more design decisions over the =
shoulders of the Union Types RFC:

* The first one, of course, is why the list of possible literal values =
is *still* restricted. Because now you=E2=80=99ll want to use other =
literal values, e.g., `0` or `"some-string"`.

* Another issue is the relation between `Foo | false | true` and `Foo | =
bool`: whether the first form is allowed; whether they are equivalent =
w.r.t. variance rules; how casting to `true | false` works (or doesn=E2=80=
=99t work) under `declare(strict_types=3D0)`.

* Also, if we support more than two literal values as types (and =
especially when not all of them indicate failure), we=E2=80=99ll want =
more strongly to have them to appear as standalone types, e.g.,  `true | =
null`; and then, naturally, just `true` and just `null`. At this point, =
the question will be raised whether it is desirable to have both `null` =
and `void` as return types.

-------------

The choice of supporting precisely the two literal values `null` and =
`false` is not arbitrary: They are the two values that are the most =
often used as sentinel values (for indicating failure or absence). It is =
true that `true` is also sometimes used as sentinel value (more rarely =
and not among the internal functions), but the same can be said of other =
literal values (one of your examples includes `0`).

=E2=80=94Claude