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Date: Sun, 01 Sep 2024 20:06:12 +0100
To: internals@lists.php.net
Subject: Re: [PHP-DEV] Re: [RFC] Default expression
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")



On 1 September 2024 17:45:57 BST, Rob Landers <rob@bottled=2Ecodes> wrote:
>Is manually copying the default also not type-safe? Is php a type-safe la=
nguage? I think a lot of the arguments I saw suggested that people don't re=
view libraries and their implementations when upgrading or installing them=
=2E This is just a shorthand for manually copy-pasting the default from oth=
er code, and this argument really only makes sense to me if there are no re=
views before upgrading/using a library=2E

Copying and pasting the default value is no different from providing any o=
ther explicit value; whether you choose 69 because you like the number, or =
because you saw it was the current default, you are passing an integer=2E A=
nd if you write 23*3, you're just writing the same integer a different way=
=2E

PHP guarantees the type safety of this under inheritance by enforcing cont=
ravariance of input: if you try to write a subclass that would not accept a=
n integer, when a parent class would, PHP will refuse to compile your subcl=
ass=2E=20

Similarly, if the class provides a non-final method like `getDefaultFlags(=
): int` then it is type safe to call that and assume the value will always =
be an integer, because PHP enforces covariance of output: a subclass may no=
t return a value that would not have been allowed by the parent class=2E

If you were designing a language where default values were explicitly avai=
lable as outputs, you would need to make them covariant, which is option 2=
=2E

Obviously, the language cannot directly control the compatibility promises=
 of third party libraries, but these principles are well enough known that =
I would expect popular projects to base their versioning / deprecation poli=
cies on them=2E

Regards,
Rowan Tommins
[IMSoP]