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Date: Sun, 25 Aug 2024 16:32:04 -0400
To: Bilge <bilge@scriptfusion.com>
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Subject: Re: [PHP-DEV] [RFC] Default expression
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From: john@coggeshall.org (John Coggeshall)

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> public function suspend(float|int|null $delay = null) {
> parent::suspend((int)(($delay ?? 0) * 1000) ?: default);
> }
> }
>
> new MySuspension()->suspend(2.2345); // int(2234)
> Not only have I demonstrated the need to use multiplication or division
> to change the scale, but also the need to cast.
>

I appreciate what you're saying here.
I've been struggling a little bit to really nail my language here on what I think should and shouldn't be allowed. Essentially I'm trying to say (and I think others are too) is this:
The engine should not allow the use of default in an expression that doesn't ultimately evaluate to default *.
In the above example the left - hand of the ?: operator doesn't use default , so it's evaluation is whatever it's evaluation is. The right-hand of the ?: operator DOES use default , and thus it must evaluate ultimately to default or that would be an error. Another example:
parent::foo((default >= 10) ? default : 10)
Would be permitted because the left-hand uses default , but the evaluation if the conditional where default was used for the true case true is default . Likewise the right-hand is just 10 and irrelevant
This would not be permitted
parent::foo((default >= 10) ? (default + 1) : 10)
Because now the ultimate evaluation of default is default + 1 -- not default
*The exception to the rule I've described above would be IFF the expression default is in only uses specific allowed operators like a subset of the bitwise operators.
I very much appreciate that what is being described here is a significant effort to achieve, I'm not even sure it's reasonably possible.. but I just can't get behind the idea that (default)->foobar() is a valid expression in this context or a good idea for the language. The use of this proposed default keyword must have guardrails IMO. I think my definition above is a pretty reasonable attempt at capturing where I think the line is here and hopefully that helps guide this discussion.
John


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<br><br><blockquote><div><div>&nbsp;&nbsp;&nbsp; public function suspend(=
float=7Cint=7Cnull =24delay =3D null) =7B</div><div>&nbsp;&nbsp;&nbsp;&nb=
sp;&nbsp;&nbsp;&nbsp; parent::suspend((int)((=24delay =3F=3F 0) * 1000) =3F=
: default);</div><div>&nbsp;&nbsp;&nbsp; =7D</div><div>=7D</div><br><div>=
new MySuspension()-&gt;suspend(2.2345); // int(2234)</div><br><div>Not on=
ly have I demonstrated the need to use multiplication or division</div><d=
iv>to change the scale, but also the need to cast.</div><br></div></block=
quote><div><br><div>I appreciate what you're saying here.</div><br><div>I=
've been struggling a little bit to really nail my language here on what =
I think should and shouldn't be allowed. Essentially I'm trying to say (a=
nd I think others are too) is this<span data-emoji-typing=3D=22true=22>:<=
/span></div><br><div>The engine should not allow the use of <code>default=
</code>&nbsp; in an expression that doesn't ultimately evaluate to <code>=
default</code>&nbsp;*.</div><br><div>In the above example the left - hand=
 of the =3F: operator doesn't use <code>default</code>&nbsp;, so it's eva=
luation is whatever it's evaluation is. The right-hand of the =3F: operat=
or DOES use <code>default</code>&nbsp;, and thus it must evaluate ultimat=
ely to <code>default</code>&nbsp; or that would be an error. Another exam=
ple:</div><br><div><code>parent::foo((default &gt;=3D 10) =3F default : 1=
0)</code>&nbsp;</div><br><div>Would be permitted because the left-hand us=
es <code>default</code>&nbsp;, but the evaluation if the conditional wher=
e <code>default</code>&nbsp; was used&nbsp; for the true case <code>true<=
/code>&nbsp; is <code>default</code>&nbsp;. Likewise the right-hand is ju=
st 10 and irrelevant</div><br><div>This would not be permitted</div><br><=
div><code>parent::foo((default &gt;=3D 10) =3F (default + 1) : 10)</code>=
</div><br><div>Because now the ultimate evaluation of <code>default</code=
>&nbsp; is <code>default + 1</code>&nbsp; -- not <code>default</code>&nbs=
p;</div><br><div>*The exception to the rule I've described above would be=
 I=46=46 the expression <code>default</code>&nbsp; is in only uses specif=
ic allowed operators like a subset of the bitwise operators.</div><br><di=
v>I very much appreciate that what is being described here is a significa=
nt effort to achieve, I'm not even sure it's reasonably possible.. but I =
just can't get behind the idea that <code>(default)-&gt;foobar()</code>&n=
bsp; is a valid expression in this context or a good idea for the languag=
e. The use of this proposed <code>default</code>&nbsp; keyword must have =
guardrails IMO. I think my definition above is a pretty reasonable attemp=
t at capturing where I think the line is here and hopefully that helps gu=
ide this discussion.</div><br><div>John</div><br><br><br></div>
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