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Date: Tue, 27 Aug 2024 07:48:01 +0100
To: internals@lists.php.net
Subject: Re: [PHP-DEV] [RFC] Default expression
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")



On 27 August 2024 05:20:32 BST, John Coggeshall <john@coggeshall=2Eorg> wr=
ote:
>> I DO think there are likely whole categories of expressions we can like=
ly say "no" to - anything where the default represents a union type (and _m=
ixed_ is a union type) should likely only allow default by itself or as a b=
are value on the RHS of an expression=2E

I don't think having additional restrictions on union types makes much dif=
ference, because even if a signature doesn't use union types now, it can be=
 extended to do so at any time, either in terms of inheritance or evolution=
 in a new version=2E

It is a well understood principle that widening the type of an input canno=
t break existing uses, and relying on the concrete type of the default will=
 remove that guarantee=2E

While the example of bitwise or in the RFC looks appealing, I am no longer=
 sure I can even support that, because it has the same problem - in a futur=
e version of PHP,  json_encode might change to take int|JsonOptions, with a=
 default value that is an object not an int=2E Code passing an integer won'=
t break, code accepting the default won't break, but code using "default | =
JSON_PRETTY_PRINT" will immediately get a TypeError=2E With the feature in =
place, such an evolution, which was previously safe, would become a hard BC=
 break=2E

Forbidding that evolution, not just to that example, but to every function=
 or method with any default parameter, is too high a price to pay for the b=
enefit we'd get=2E

Regards,
Rowan Tommins
[IMSoP]