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Date: Mon, 26 Aug 2024 12:32:39 +0200
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Subject: Re: [PHP-DEV] [RFC] Default expression
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From: andreas@heigl.org (Andreas Heigl)

Hey folks.

Am 26.08.24 um 11:26 schrieb Bilge:
> On 26/08/2024 10:03, Andreas Leathley wrote:
>> On 24.08.24 18:49, Bilge wrote:
>>
>> For me there is another question. When using interfaces and classes,
>> default values can be introduced, like this:
>>
>> interface CompressionInterface
>> {
>>     public function compress(string $data, int $level): string;
>> }
>>
>> class GzipCompression implements CompressionInterface
>> {
>>     public function compress(string $data, int $level = 4): string
>>     {
>>         // do something
>>     }
>> }
>>
>> When I have the GzipCompression class, I would know there is a default
>> value for $level, but when using the interface there might or might not
>> be a default value, depending on the implementation. As far as I read
>> the RFC, using "default" when there is no default would lead to a
>> runtime exception, but there is no way of finding out if there is a
>> default if you do not already know. Being able to test that could be
>> useful, although I am not sure about the syntax for that. In the example
>> when getting CompressionInterface, I might test for the existence of a
>> default value of $level and leave it at the default if there is a
>> default (maybe I know that some implementations have a default value,
>> others don't). One could test the specific implementation with
>> instanceof checks, but the advantage of "default" could be that you do
>> not need to know the implementation and could only adapt to possibly
>> defined default values.
> 
> Hi Andreas,
> 
> Thanks for this question; I find this super interesting because it's 
> something we haven't thought about yet. I must admit I completely 
> overlooked that, whilst an interface /can/ require implementers to 
> specify a default, in the case that they do not, it is still valid for 
> implementations to selectively elect to provide one. Therefore I can 
> append to your example the following case (I removed the `string` return 
> type for now):
> 
> class ZipCompression implements CompressionInterface
> {
>      public function compress(string $data, int $level)
>      {
>          var_dump($level);
>      }
> }
> 
> new GzipCompression()->compress('', default ?? 6);
> new ZipCompression()->compress('', default ?? 6);
> 
> In this case, we get the following output:
> 
> int(4)
> Fatal error: Uncaught ValueError: Cannot pass default to required 
> parameter 2 of ZipCompression::compress()
> 
> I would like to fix this if possible, because I think this should be 
> valid, with emphasis on /if possible/, because it may be prohibitively 
> complex. Will update later.
> 
> Cheers,
> Bilge
> 

I think I am missing something here. From my understanding we are 
*either* coding against the interface and then it should not be possible 
to use `default` at all as no default is set in the interface. So the 
fatal error is totally valid for me.

*Or* we are coding against the actual implementations. Then it is 
totally valid IMO that providing `default` when no default is set in the 
concrete implementation of the function signature raises a fatal error.

So in the example above it's either

new GzipCompression()->compress('', default ?? 6);
new ZipCompression()->compress('', default ?? 6);

and it is absolutely valid that the second one triggers a fatal error as 
no default is set in the concrete implementation.

Or it's something like

/** @var CompressionInterface $compression */
foreach ([new GzipCompression, new ZipCompression] as $compression) {
     $compression->compress('', default ?? 6)
}

in which case it should definitely fail as the interface doesn't provide 
a default.

Cheers

Andreas


-- 
                                                               ,,,
                                                              (o o)
+---------------------------------------------------------ooO-(_)-Ooo-+
| Andreas Heigl                                                       |
| mailto:andreas@heigl.org                  N 50°22'59.5" E 08°23'58" |
| https://andreas.heigl.org                                           |
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| https://hei.gl/appointmentwithandreas                               |
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