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Date: Fri, 21 Jun 2024 17:42:11 +0200
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Subject: Re: [PHP-DEV] [Early Feedback] Pattern matching
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From: dossche.niels@gmail.com (Niels Dossche)

On 21/06/2024 14:43, Robert Landers wrote:
> On Fri, Jun 21, 2024 at 5:08 AM Andreas Hennings <andreas@dqxtech.net> wrote:
>>
>>> Ilija and I have been working on and off on an RFC for pattern matching since the early work on Enumerations.
>>
>> I like what I see, a lot!
>> One quick thought that came to my mind, regarding objects:
>> Could we check method return values?
>>
>> if ($x is Countable { count(): 0 }) ...
>> if ($p is Point { getX(): 3 }) ...
>> if ($x is Stringable { __toString(): 'hello' }|'hello') ...
>> while ($it is Iterator { valid(): true, current(): $value, next(): null }) ...
>>
>> Maybe it goes too far.
>>
>> For the variable binding, I noticed that we can overwrite the original variable:
>> $x is SomethingWrapper { something: $x }
>> In this case the bool return is not really needed.
>> For now this usage looks a bit unintuitive to me, but I might change
>> my mind and grow to like it, not sure.
>>
>>
>> For "weak mode" ~int, and also some other concepts, I notice that this
>> RFC is ahead of the type system.
>>
>> E.g. should something like array<int> be added to the type system in
>> the future, or do we leave the type system behind, and rely on the new
>> "guards"?
>> public array $values is array<int>
>> OR
>> public array<int> $values
>>
>> The concern here would be if in the future we plan to extend the type
>> system in a way that is inconsistent or incompatible with the pattern
>> matching system.
>>
>> --- Andreas
> 
> I'm always surprised why arrays can't keep track of their internal
> types. Every time an item is added to the map, just chuck in the type
> and a count, then if it is removed, decrement the counter, and if
> zero, remove the type. Thus checking if an array is `array<int>`
> should be a near O(1) operation. Memory usage might be an issue (a
> couple bytes per type in the array), but not terrible.... but then
> again, I've been digging into the type system quite a bit over the
> last few months.

And every time a modification happens, directly or indirectly, you'll
have to modify the counts too. Given how much arrays / hash tables are
used within the PHP codebase, this will eventually add up to a lot of
overhead. A lot of internal functions that work with arrays will need
to be audited and updated too. Lots of potential for introducing bugs.
It's (unfortunately) not a matter of "just" adding some counts.