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In-Reply-To: <bcfa8d1f-8a2f-4c39-bf5d-a85f0ba4e20e@app.fastmail.com>
Date: Mon, 26 Aug 2024 17:28:40 +0200
Message-ID: <CA+hqhpdWF+_vSopLs4_zEddc6gGQ=qSphT6FF3sgW6eoca=ifw@mail.gmail.com>
Subject: Re: [PHP-DEV] [RFC] Default expression
Cc: internals@lists.php.net
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From: weirdan@gmail.com (Bruce Weirdan)

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On Mon, Aug 26, 2024 at 12:47=E2=80=AFPM Rowan Tommins [IMSoP] <imsop.php@r=
wec.co.uk>
wrote:

> Another approach that occurred to me was in the executor: rather than
> evaluating to the default value immediately, "default" could resolve to a
> special value, essentially wrapping the reflection parameter info. Then
> when the function is actually called, it would be "unboxed" and the actua=
l
> value fetched, but use in any other context would be a type error.
>

I guess what the opponents to this RFC are zeroing on in this thread is the
conclusion that `default` (as proposed) is a form of contravariant return,
and thus breaks Liskov Substitution Principle. Your suggestion of making it
an opaque value that cannot be read outside of the called function is a
nice (and maybe the only) way to resolve this problem.


--=20
  Best regards,
      Bruce Weirdan                                     mailto:
weirdan@gmail.com

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<div dir=3D"ltr"><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"g=
mail_attr">On Mon, Aug 26, 2024 at 12:47=E2=80=AFPM Rowan Tommins [IMSoP] &=
lt;<a href=3D"mailto:imsop.php@rwec.co.uk">imsop.php@rwec.co.uk</a>&gt; wro=
te:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px =
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Another appr=
oach that occurred to me was in the executor: rather than evaluating to the=
 default value immediately, &quot;default&quot; could resolve to a special =
value, essentially wrapping the reflection parameter info. Then when the fu=
nction is actually called, it would be &quot;unboxed&quot; and the actual v=
alue fetched, but use in any other context would be a type error.<br></bloc=
kquote><div><br></div><div>I guess what the opponents to this RFC are zeroi=
ng on in this thread is the conclusion that `default` (as proposed) is a fo=
rm of contravariant return, and thus breaks Liskov Substitution Principle. =
Your suggestion of making it an opaque value that cannot be read outside of=
 the called function is a nice (and maybe the only) way to resolve this pro=
blem.<br></div><div>=C2=A0<br clear=3D"all"></div></div><br><span class=3D"=
gmail_signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signa=
ture">=C2=A0 Best regards,<br>=C2=A0 =C2=A0 =C2=A0 Bruce Weirdan=C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=C2=A0 mailto:<a href=3D"mailto:we=
irdan@gmail.com" target=3D"_blank">weirdan@gmail.com</a></div></div>

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