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Date: Sun, 23 Jun 2024 11:23:05 -0500
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 Sat, Jun 22, 2024, at 12:14 PM, Brandon Jackson wrote:

>> To clarify here, these all come as a set.  Array shapes aren't their own "thing", they just fall out naturally from array patterns.  So it's not possible for associative patterns to conflict with array shapes, as they are literally the same thing. :-)  I'd have to check with Ilija but I don't believe there's much internal difference between list and associative patterns.  This one isn't on the ADT hot path, so it could be postponed
>>
>> I see no way for associative array patterns/shapes to conflict with generics at all.
>
> Thanks for the clarification. I was looking at the example given in
> the overview trying to figure out the boundaries between array
> structure patterns, array shape patterns, and nested patterns. It
> sounds like it can be summarized to nested patterns with the option to
> add or omit `...` determining if unexpected key/values are allowed.
> Also again, just throwing out hypothetical curve balls that could
> delay or halt other good parts. At this point I don't actually expect
> generics to ever be a thing.

To clarify a bit more: There's really only two slight variations on the same pattern for arrays, list and associative.  In both cases, each specified element of the array is matched against its own sub-pattern, recursively.  That sub-pattern may be (almost) any other pattern.

So this:

$arr is ['a' => 'A', 'b' => 'B', 'c' => [int, int, int], 'd' => string]

Is entirely valid, and decomposes to:

$arr['a'] is 'A'
&& $arr['b'] is 'B'
&& $arr['c'][0] is int
&& $arr['c'][1] is int
&& $arr['c'][1] is int
&& $arr['d'] is string

(This is why literal patterns are basically non-negotiable.  On their own, they're kinda pointless.  As a sub-pattern of another pattern, they're mandatory.)  Array shapes aren't really a feature per se; they're just a natural fall-out of the recursive syntax.

--Larry Garfield