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Date: Fri, 22 Mar 2024 07:47:20 +0000
To: internals@lists.php.net
Subject: Re: [PHP-DEV] Proposal: AS assertions
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")



On 22 March 2024 00:04:27 GMT, Robert Landers <landers=2Erobert@gmail=2Eco=
m> wrote:

>I think that is where we are getting confused: `null` is a value (or
>at least, the absence of a value)=2E The fact that the type system
>allows it to be used as though its a type (along with true and false)
>is interesting, but I think it is confusing the conversation=2E


Every value needs to belong to some type: for instance, true and false bel=
ong to the type "boolean", as returned by the gettype() function=2E There i=
s a value called null, and the type it belongs to is also called "null"=2E=
=20

Unlike some languages, PHP has no concept of a typed null reference - you =
can't have "a null DateTime"; you can only have the one universal null, of =
type null=2E

The existence of "null" in type checks is therefore necessary if you want =
to allow every value to pass some type check=2E There isn't any other type =
that can include the value null because the type of null is always null=2E

That's completely different from true and false, both of which are covered=
 by a type check for "bool"=2E They are special cases, which aren't consist=
ent with anything else in the type system=2E The "false" check was added fi=
rst, as a way to express clearly the common pattern in old standard library=
 functions of returning false on error=2E Then "true" was added later, for =
consistency=2E Both are newer, and far more exotic, than "null"=2E

Disallowing true and false in some type checking contexts would be fine (a=
lthough mostly they're pointless, rather than harmful)=2E Disallowing or re=
purposing null would mean you have an incomplete type system, because there=
 is no other type to match a null value against=2E


Regards,
Rowan Tommins
[IMSoP]