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Date: Thu, 2 Apr 2026 09:23:03 -0500
Message-ID: <CAJp_myWy1PD_cmh0FuM=nGRrzUW7Ot9LhyeDBijX7V7VzwZgFw@mail.gmail.com>
Subject: Re: [PHP-DEV] RFC proposal: Invokable interface
To: Osama Aldemeery <aldemeery@gmail.com>
Cc: internals@lists.php.net
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From: mweierophinney@gmail.com ("Matthew Weier O'Phinney")

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On Sun, Mar 29, 2026 at 4:44=E2=80=AFPM Osama Aldemeery <aldemeery@gmail.co=
m> wrote:

> Hi all,
>
> I'd like to propose an Invokable interface, the Stringable equivalent for
> __invoke().
>
> The idea has come up a few times over the years (most recently in the PR
> #15492 discussion, where Gina suggested this exact approach) but never ha=
d
> a concrete implementation.
>
> I've put one together: https://github.com/php/php-src/pull/21574
>
> It follows the Stringable pattern: auto-implemented for any class definin=
g
> __invoke(), explicitly implementable with enforcement, and covariant to
> callable in return type checks.
>
>
I honestly don't feel there's a need for this, as we already have the
callable type. It shouldn't matter if the callable is an object defining
`__invoke()`, a function, a closure, or a first class callable. If you need
to restrict to an object that's invokable, you can do that already via
(object&callable). Further, PHPStan, Psalm, PHPCS, and php-cs-fixer all
understand annotations around the callable type already to further define
the allowed signature (e.g., number and types of arguments, return value).
This is a solved problem already.

Further, creating an interface for this is hugely difficult, if not
impossible, due to the return value associated with the interface, as
implementations will inevitably conflict with the Liskov Substitution
Principle. If we say "void", you cannot declare an implementation with a
return value. If you define mixed, you cannot have a void or never return.

--=20
Matthew Weier O'Phinney
mweierophinney@gmail.com
https://mwop.net/
he/him

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<div dir=3D"ltr"><div dir=3D"ltr"><br></div><br><div class=3D"gmail_quote g=
mail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Sun, Mar 29,=
 2026 at 4:44=E2=80=AFPM Osama Aldemeery &lt;<a href=3D"mailto:aldemeery@gm=
ail.com">aldemeery@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"g=
mail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204=
,204,204);padding-left:1ex"><div dir=3D"ltr">Hi all,<br><br>I&#39;d like to=
 propose an Invokable interface, the Stringable equivalent for __invoke().<=
br><br>The idea has come up a few times over the years (most recently in th=
e PR #15492 discussion, where Gina suggested this exact approach) but never=
 had a concrete implementation.<br><br>I&#39;ve put one together:=C2=A0<a h=
ref=3D"https://github.com/php/php-src/pull/21574" target=3D"_blank">https:/=
/github.com/php/php-src/pull/21574</a><br><br>It follows the Stringable pat=
tern: auto-implemented for any class defining __invoke(), explicitly implem=
entable with enforcement, and covariant to callable in return type checks.<=
br><br></div>
</blockquote></div><div><br clear=3D"all"></div><div>I honestly don&#39;t f=
eel there&#39;s a need for this, as we already have the callable type. It s=
houldn&#39;t matter if the callable is an object defining `__invoke()`, a f=
unction, a closure, or a first class callable. If you need to restrict to a=
n object that&#39;s invokable, you can do that already via (object&amp;call=
able). Further, PHPStan, Psalm, PHPCS, and php-cs-fixer all understand anno=
tations around the callable type already to further define the allowed sign=
ature (e.g., number and types of arguments, return value). This is a solved=
 problem already.</div><div><br></div><div>Further, creating an interface f=
or this is hugely difficult, if not impossible, due to the return value ass=
ociated with the interface, as implementations will inevitably conflict wit=
h the Liskov Substitution Principle. If we say &quot;void&quot;, you cannot=
 declare an implementation with a return value. If you define mixed, you ca=
nnot have a void or never return.</div><div><br></div><span class=3D"gmail_=
signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature">=
<div dir=3D"ltr"><div>Matthew Weier O&#39;Phinney<br><a href=3D"mailto:mwei=
erophinney@gmail.com" target=3D"_blank">mweierophinney@gmail.com</a><br><a =
href=3D"https://mwop.net/" target=3D"_blank">https://mwop.net/</a></div><di=
v>he/him<br></div></div></div></div>

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