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Date: Thu, 20 Jun 2024 20:32:43 +0000
To: "php internals" <internals@lists.php.net>
Subject: Re: [PHP-DEV] [Early Feedback] Pattern matching
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From: larry@garfieldtech.com ("Larry Garfield")

On Thu, Jun 20, 2024, at 8:22 PM, Lynn wrote:
> On Thu, Jun 20, 2024 at 7:41=E2=80=AFPM Larry Garfield <larry@garfield=
tech.com> wrote:

>> https://wiki.php.net/rfc/pattern-matching

> I have been looking forward to this RFC, it's such a quality of life t=
o=20
> be able to do all this! In terms of things to focus on, I'd personally=20
> be very happy with the property/param guards, "as" and "is <regex>",=20
> but I won't say no to anything we can get extra here because it's all=20
> really nice to have.
>
> I noticed that with array structure patterns the count is omitted when=20
> using ...
> ```php
> if ($list is [1, 3, ...]) {
>   print "Yes";
> }
> // True.  Equivalent to:
> if (is_array($list)=20
>     && array_key_exists(0, $list) && $list[0] =3D=3D=3D 1=20
>     && array_key_exists(1, $list) && $list[1] =3D=3D=3D 3
>     ) {
>     print "Yes";
> }
> ```
> Wouldn't this need a `count($list) >=3D 2`? I'm not sure if the=20
> underlying mechanism does the count check as well, but it seems to me=20
> like a guard clause for performance reasons in the PHP variant.=20

At the moment, the implementation doesn't actually compile down into tho=
se primitives; it has its own all-C implementation.  Having it instead c=
ompile down to those operations is something Ilija is exploring to see h=
ow feasible it would be.  (The main advantage being that the optimizer, =
JIT, etc. wouldn't have to do anything new to support optimizing pattern=
s.)  The examples shown in the RFC for now are just for logical equivale=
ncy to explain the functionality.  In this case, the array_key_exists() =
checks are sufficient for what is actually being specified, so the count=
() is redundant.  The final implementation will almost certainly be more=
 performant than my example equivalencies. :-)

> Maybe a tangent, what about iterators?

Not supported, as you cannot examine them "all at once", by definition. =
 I don't even know what an iterator-targeted pattern would look like, th=
ough if someone figured that out in the future there's no intrinsic reas=
on such a pattern couldn't be added at that time.

> "Limited expression pattern"
> I think this would be very valuable to have, though in the proposal it=20
> seems cumbersome to use regardless of syntax. It feels like I'm going=20
> to be using the variable binding less often than matching against othe=
r=20
> variables, what about an "out" equivalent?
>
> ```php
> $result =3D match ($p) is {
>   Point{x: 3, y: 9, $z} =3D> "x is 3, y is 9, z is $z",
>   Point{$x, $y, $z} =3D> "x is $x, y is $y, z is $z",
> };
> // vs
> $x =3D 3;
> $y =3D 9;
> $result =3D match ($p) is {
>   Point{x: 3, y: 9, out $z} =3D> "x is 3, y is 9, z is $z",
>   Point{$x, $y, out $z} =3D> "x is 3, y is 9, z is $z",
> };
> ```
> To me this makes it much more readable, just not sure if this is even=20
> feasible. This is not meant as bikeshedding the syntax, more of an=20
> alternative approach to when to use which.

A couple of people have noted that.  Assuming at least one of those two =
synaxes makes it into the initial RFC (I think variable binding has to f=
or it to be really useful), we'll have a whole separate sub-discussion o=
n that, I'm sure.

Though, I would expect variable binding to be used more than expressions=
, not less, which would make the marker make more sense on the expressio=
n.  But that's something to bikeshed later.

--Larry Garfield