On 04/02/2019 03:22, Wes wrote:
Recent events convinced me to write this RFC :P
Please have a read here https://wiki.php.net/rfc/allow-void-variance
I was thinking about this this afternoon, and I think the current
behaviour is actually correct, and this constraint should stay invariant.
My reasoning is that ": void" isn't actually a *type constraint* - there
is no "void type" to apply the rules of covariance and contravariance
to. It's tempting to place it on a par with ": null", but a void
function evaluates to null only for the same reason an unset variable
does: because the runtime has to give you *something*.
Instead, ": void" in PHP represents a *behavioural constraint* on the
function - it declares that within this function, anything of the form
"return $value" is to be treated as an error. In other languages, the
same constraint might be enforced by declaring the routine as a
"procedure" rather than a "function".
As such, it is more appropriate to compare "void vs non-void" to "static
vs non-static", which is an invariant constraint - you can't over-ride a
static method with a non-static one, or vice versa.
(Incidentally, the RFC mentions TypeScript as precedent; there, "void"
apparently *is* a type, not just a keyword in function signatures:
https://www.typescriptlang.org/docs/handbook/basic-types.html)
Regards,
--
Rowan Collins
[IMSoP]
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