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Date: Tue, 28 Nov 2023 10:54:58 +0100
In-Reply-To: <AA17AB82-695F-4E4B-ABDA-B8AA085466AC@shymko.net>
Cc: internals <internals@lists.php.net>
To: Sergii Shymko <sergey@shymko.net>
References: <AA17AB82-695F-4E4B-ABDA-B8AA085466AC@shymko.net>
Subject: Re: [PHP-DEV] Callable arguments cannot have default value
From: claude.pache@gmail.com (Claude Pache)

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> Le 28 nov. 2023 =C3=A0 00:59, Sergii Shymko <sergey@shymko.net> a =
=C3=A9crit :
>=20
> Hi,
>=20
> Wanted to bring up an inconsistent behavior of callable arguments =
compared to arguments of other types.
> Callable argument cannot have a default value (tested string or array =
types - both are not permitted).
> The same exact value works perfectly fine when passed dynamically, it =
just cannot be specified as a default.
> The workaround is to remove the type annotation which is obviously =
undesirable.
>=20
> Here=E2=80=99s an example:
> declare(strict_types=3D1);
> function test(callable $idGenerator =3D 'session_create_id') {
>    $id =3D $idGenerator();
>    // ...
> }
>=20
> The function/method declaration above produces the following error on =
all PHP versions:
> Fatal error: Cannot use string as default value for parameter =
$idGenerator of type callable in /tmp/preview on line 4
>=20
> Note that the exact same string argument can be passed without any =
issue:
> function test(callable $idGenerator) {=E2=80=A6}
> test('session_create_id=E2=80=99);
>=20
> Is there a specific architectural limitation causing this that's =
hard/impossible to overcome?
>=20
> I=E2=80=99m aware that class properties cannot be annotated with =
callable - another unfortunate limitation.
> Callable is not a real type like other primitive types which causes =
all these inconsistencies, correct?
> Callable properties (separate topic) may be a challenge, but can at =
least argument defaults be supported?
>=20
> Regards,
> Sergii Shymko


Hi Sergii,

The big problem with the `callable` type, is that it can be check only =
at runtime. For instance:

```php
function foo(callable $x) { }

foo('strlen'); // ok
foo('i_dont_exist'); // throws a TypeError
```

Another complication, is that a value of the form `[ $class, =
$protected_or_private_method ]` may or may not be callable depending on =
whether the method is visible from the current scope.

In other words, contrarily to all other types, `callable` depends both =
on runtime state and on context.

Therefore, an argument of type `callable` cannot have a default value, =
because it is not known in advance whether the default value will be =
valid when used.

For the case of class properties, see  =
https://wiki.php.net/rfc/typed_properties_v2#supported_types

=E2=80=94Claude



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