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Date: Sat, 27 Jul 2024 00:42:47 +0200
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Subject: Re: [PHP-DEV] Explicit callee defaults
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To: Bilge <bilge@scriptfusion.com>, internals@lists.php.net
References: <ab9ded6e-05be-43e7-bafb-f8a81eeba7e9@scriptfusion.com>
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From: cmbecker69@gmx.de ("Christoph M. Becker")

On 26.07.2024 at 23:54, Bilge wrote:

> New RFC idea just dropped. When writing a function, we can specify
> defaults for its parameters, and when calling a function we can leverage
> those defaults /implicitly/ by not specifying those arguments or by
> "jumping over" some of them using named parameters. However, we cannot
> /explicitly/ use the defaults. But why would we want to?
>
> Sometimes we want to effectively /inherit/ the defaults of a function
> we're essentially just proxying. One way to do that is copy and paste
> the entire method signature, but if the defaults of the proxied method
> change, we're now overriding them with our own, which is not what we
> wanted to do. It is possible, in a roundabout way, to avoid specifying
> the optional parameters by filtering them out (as shown in the next
> example). The final possibility is to use reflection and literally query
> the default value for each optional parameter, which is the most awkward
> and verbose way to inherit defaults.
>
> Let's use a concrete example for clarity.
>
> function query(string $sql, int $limit =3D PHP_INT_MAX, int $offset =3D =
0);
>
> function myQuery(string $sql, ?int $limit =3D null, ?int $offset =3D nul=
l) {
> =C2=A0=C2=A0=C2=A0 query(...array_filter(func_get_args(), fn ($arg) =3D>=
 $arg !=3D=3D null));
> }
>
> In this way we are able to filter out the null arguments to inherit the
> callee defaults, but this code is quite ugly. Moreover, it makes the
> (sometimes invalid) assumption that we're able to use `null` for all the
> optional arguments.
>
> In my new proposal for /explicit /callee defaults, it would be possible
> to use the `default` keyword to expressly use the default value of the
> callee in that argument position. For example, the above implementation
> for myQuery() could be simplified to the following.
>
> function myQuery(string $sql, ?int $limit =3D null, ?int $offset =3D nul=
l) {
> =C2=A0=C2=A0=C2=A0 query($sql, $limit ?? default, $offset ?? default);
> }
>
> Furthermore, it would also be possible to "jump over" optional
> parameters /without/ using named parameters.
>
> json_decode($json, true, default, JSON_THROW_ON_ERROR);
>
> This proposal is built on the assumption that it is possible to specify
> that PHP should only accept the `default` expression in method and
> function call contexts. For example, it would not be valid to return
> `default` from a function and substitute it that way; my proposal is to
> only permit `default` in literal function calling contexts. My knowledge
> of internals is insufficient (read: non-existent) to know whether or not
> this restriction is possible to implement, but if it is, I think this is
> a good idea. What do you think?

I have only skimmed your suggestion, but it sounds quite similar to
<https://wiki.php.net/rfc/skipparams>.

Cheers,
Christoph