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Subject: Re: [PHP-DEV] [RFC] Default expression
In-Reply-To: <bf9b4a5b-108c-4092-8751-569c1374b054@rwec.co.uk>
Date: Sun, 25 Aug 2024 13:44:19 -0400
Cc: internals@lists.php.net
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Message-ID: <F773FEAA-0717-43ED-A6D5-98C348298DD7@zort.net>
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To: "Rowan Tommins [IMSoP]" <imsop.php@rwec.co.uk>
From: jbafford@zort.net (John Bafford)

Hi Rowan,

> On Aug 25, 2024, at 11:31, Rowan Tommins [IMSoP] =
<imsop.php@rwec.co.uk> wrote:
>=20
> 3) The expression should be passing additional information into the =
function, not pulling information out of it. The syntax shouldn't be a =
way to write obfuscated reflection, or invert data flow from callee to =
caller.
> - No assignments.
> - No ternaries with "default" on the left-hand side - "$foo ? $bar : =
default" is acting on local knowledge, but "default ? $foo : $bar" is =
acting on information the caller shouldn't know
> - Same for "?:" and "??"
> - No "match" with "default" as the condition or branch, for the same =
reason. "match($foo) { $bar =3D> default }" is fine, match(default) { =
... }" or "match($foo) { default =3D> ... }" are not.


I think this brings up a good question on what exactly should be =
intended to be public API. Currently, there's two main ways to =
effectively write a default parameter:

	function foo(int $param =3D 42) {}
	function bar(?int $param) { $param ??=3D 42; }

In the former, the default value is listed in the function declaration, =
along with the function name, and parameter type and name, which are =
already part of the public interface.
In the latter, the default value is an implementation detail of the =
function, and is not part of the function declaration.

(You could also ?int $param =3D 42, but I'd argue that at that point, if =
you really need to distinguish among the set of (unspecified, null, =
value), you're better off with an ADT, which we don't have yet. And you =
can also explicitly expose a default value as a static value on a type, =
when a function itself doesn't want to/shouldn't make the policy =
decision of what the default is.)


Although I'm not sold on the idea of using default as part of an =
expression, I would argue that a default function parameter value is =
fair game to be read and manipulated by callers. If the default value =
was intended to be private, it shouldn't be in the function declaration.

One important case where reading the default value could be important is =
in interoperability with different library versions. For example, a =
library might change a default parameter value between versions. If =
you're using the library, and want to support both versions, you might =
both not want to set the value, and yet also care what the default value =
is from the standpoint of knowing what to expect out of the function.

-John=