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Date: Mon, 26 Aug 2024 11:58:12 +0100
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Subject: Re: [PHP-DEV] [RFC] Default expression
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

On Mon, 26 Aug 2024, at 10:03, Andreas Leathley wrote:
> interface CompressionInterface
> {
>  =C2=A0=C2=A0=C2=A0 public function compress(string $data, int $level)=
: string;
> }
>
> class GzipCompression implements CompressionInterface
> {
>  =C2=A0=C2=A0=C2=A0 public function compress(string $data, int $level =
=3D 4): string
>  =C2=A0=C2=A0=C2=A0 {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 // do something
>  =C2=A0=C2=A0=C2=A0 }
> }
>
> When I have the GzipCompression class, I would know there is a default
> value for $level, but when using the interface there might or might not
> be a default value, depending on the implementation.

This isn't unique to defaults; GzipCompression could also widen the type=
 to int|string, for instance, and there's no syntax for detecting that e=
ither.

If you have access to change class GzipCompression, you can resolve this=
 by creating an additional interface:

interface SimplifiedCompressionInterface extends CompressionInterface
{
 =C2=A0=C2=A0=C2=A0 public function compress(string $data, int $level =3D=
 4): string;
}
class GzipCompression implements SimplifiedCompressionInterface ...

Then, if we can agree an implementation, you could write:

/** @var CompressionInterface $comp */
$comp->compress($myData, $comp instanceof SimplifiedCompressionInterface=
 ? default : MY_DEFAULT_LEVEL);


If you don't have access to change the hierarchy, then what you're proba=
bly looking for is structural typing, or implicit interfaces - i.e. a wa=
y to ask "does this object meet these criteria". For instance, some imag=
inary pattern matching on the signature:

/** @var CompressionInterface $comp */
$comp->compress($myData, $comp is { compress(string, optional int) } ? d=
efault : MY_DEFAULT_LEVEL);


Note how, in both cases, we're not asserting anything about the default =
value itself, only that the signature defines the parameter as optional.=
 It's actually a bit of a quirk that the interface has to specify a valu=
e, rather than just stating this:

interface SimplifiedCompressionInterface extends CompressionInterface
{
 =C2=A0=C2=A0=C2=A0 public function compress(string $data, optional int =
$level): string;
}

Regards,
--=20
Rowan Tommins
[IMSoP]