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Date: Sun, 23 Jun 2024 13:16:21 +0100
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Subject: Re: [PHP-DEV] [Early Feedback] Pattern matching
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")



On 22 June 2024 22:57:24 BST, Robert Landers <landers=2Erobert@gmail=2Ecom=
> wrote:
>In general, you assign the result of the operation so that the output
>is useful=2E Here's how that might look in PHP with the C# rules:
>
>function foo(BarInterface $bar) {
>  $baz =3D $bar as Baz;
>  $baz?->thing();
>  $bar->otherThing();
>}


The difference is that in C#, there is an actual difference between the me=
thods available on $bar and $baz=2E The "as" is performing a conversion, so=
 the English word makes sense=2E

In PHP, all we could do is have a one-off runtime check, then return the v=
alue unchanged=2E So written out in full, that would be:

function foo(BarInterface $bar) {
  $baz =3D null;
  if ( $bar is Baz ) $baz =3D $bar;
  $baz?->thing();
  $bar->otherThing();
}

Or:

function foo(BarInterface $bar) {
  $baz =3D $bar;=20
  if ( ! $bar is Baz ) $baz =3D null;
  $baz?->thing();
  $bar->otherThing();
}


I can see some use in a shorthand for that, but it doesn't have much in co=
mmon with casts, or any meaning of "as" that I can think of, so I don't thi=
nk that would be the right name for it=2E



>With fibers/async, "as" is actually more important than "is" (at least
>as far as crashing goes):
>
>class Foo {
>  public BarInterface $bar;
>
>  public function doStuff() {
>    $baz =3D $this->bar as Baz;
>    // some stuff with $baz
>    callComplexThing(); // suspends current fiber,
>    // $this->bar is no longer the same object
>    // or concrete type when we return
>    $baz->something();
>  }
>}


The thing that's stopping this crashing is not the "as", it's that you're =
backing up the old value of $this->bar into a local variable=2E Nothing abo=
ut $baz will "remember" that you made some type or pattern check against it=
 just before/after you assigned it=2E



>However, it becomes far more useful to dealing with
>scalar casts:
>
>function foo(int $type) {}
>
>foo(123=2E456 as int); // crashes
>foo(null as int); // crashes


Yes, those are actually casts/conversions in PHP, and I've repeatedly said=
 I'd like to have those, on top of the "never fail" casts we have now=2E Wh=
ether that's with an "as" operator, a "cast" function, or some extension of=
 the existing cast syntax, is an open question=2E=20

Crucially, though, I don't think most *patterns* would be relevant in that=
 context, as I pointed out in a previous message=2E


Regards,
Rowan Tommins
[IMSoP]