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Date: Tue, 20 Jul 2021 13:07:21 +0200
Message-ID: <CAF+90c-dnOm2vj2YSFqqC-e_w5rny09D+emE3paHRidrZ=Y7Jw@mail.gmail.com>
To: "G. P. B." <george.banyard@gmail.com>
Cc: Guilliam Xavier <guilliam.xavier@gmail.com>, Dan Ackroyd <Danack@basereality.com>, 
	Nicolas Grekas <nicolas.grekas@gmail.com>, PHP Internals List <internals@lists.php.net>
Content-Type: multipart/alternative; boundary="000000000000c7c63505c78c0f31"
Subject: Re: [PHP-DEV] intersection types and null for defaults, properties
 and return types
From: nikita.ppv@gmail.com (Nikita Popov)

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On Mon, Jul 19, 2021 at 8:16 PM G. P. B. <george.banyard@gmail.com> wrote:

> On Mon, 19 Jul 2021 at 18:26, Guilliam Xavier <guilliam.xavier@gmail.com>
> wrote:
>
> > On Mon, Jul 19, 2021 at 4:26 PM Nicolas Grekas <nicolas.grekas@gmail.com
> >
> > wrote:
> >
> > >
> > > https://github.com/php/php-src/pull/7259
> > >
> >
> > Great! Thanks! Interesting how it works out-of-the-box with just this
> > addition in Zend/zend_language_parser.y:
> >
> > ```diff
> >  type_expr:
> >   type { $$ = $1; }
> >   | '?' type { $$ = $2; $$->attr |= ZEND_TYPE_NULLABLE; }
> >   | union_type { $$ = $1; }
> >   | intersection_type { $$ = $1; }
> > + | '?' intersection_type { $$ = $2; $$->attr |= ZEND_TYPE_NULLABLE; }
> >  ;
> > ```
> >
> > On Mon, Jul 19, 2021 at 5:09 PM Dan Ackroyd <Danack@basereality.com>
> > wrote:
> >
> > > nicolas-grekas wrote on the PR:
> > > > ?X&Y cannot be confused with
> > >
> > > It confused me. A compiler might understand it, but as a human I have
> > > trouble understanding it.
> > >
> > > Trowski wrote:
> > > > The syntax should be either ?(X&Y) or (X&Y)|null
> > >
> > > Non-ambiguous syntax is much better than ambiguous syntax.
> > >
> >
> > Maybe it's just a matter of habit?
> > For instance I got used to seeing things like `!$x = f()` (e.g. `if (!$x
> =
> > f()) { throw /*...*/; } /* use $x */`) because some CS consider explicit
> > parentheses in `!($x = f())` redundant (as PHP has a special case that
> > "overrides" the normal precedence `(!$x) = f()` which would be an error).
> > If you first consider `X&Y` as a type "unit", then it makes sense to make
> > it "nullable" by prefixing it with `?`, I think?
> >
> >
> > >
> > > But this discussion is moot for 8.1.
> > >
> > > This limitation might make intersection types not be considered usable
> > > by some projects, but the feature freeze is today.
> > >
> >
> > Which can also be reversed: "The feature freeze is today, but this
> > limitation might make intersection types not be considered usable by some
> > projects"? (playing devil's advocate, I don't master the process)
> >
> > Regards,
> >
> > --
> > Guilliam Xavier
> >
>
> Since when is usability for a specific project a consideration an RFC needs
> to have?
> If Symfony can't use it in its current state tough luck,
> I'm sure plenty of other projects can, especially now that using 'new
> Class' is possible as a default object value.
>
> I frankly don't care about being able to have some sort of partial union
> possible with the usage of intersection types,
> because it seems the machinery in the engine which makes null work, should
> also allow any standard PHP types as those are part of a bitflag and
> handling variance with them seems to work just fine...
>
> But for the love of god, this proposed syntax is horrendous, saying ?X&Y is
> unambiguous is disingenuous.
> It can either mean (?X)&Y = (X|null)&Y or ?(X&Y) = (X&Y)|null, the former
> one being bogus as null&Y is an impossible type, something which should
> error out to point out a potential bug in the same way we do for redundant
> types that we know at compile time.
> And if we allow this syntax then we really should be allowing ?A|B which is
> dumb.
> (and no ?X&Y is NOT something I consider forward compatible when it is just
> going to become one more edge case we need to maintain because people have
> no patience).
>
> If ?(X&Y) is allowed then ?(A|B) should also be allowed, and that needs an
> RFC for sure due to the controversy it had in the union type RFC.
>
> The only remaining sensible choice is (X&Y)|null / null|(X&Y), but as I
> said above if the machinery for null is there it must mean the machinery
> for int/string/array/float/bool is also there, and frankly being able to do
> something like (Traversable&Countable)|array is also extremely valuable,
> maybe even more than nullability, but in any case this is going to be
> confusing for end-users why only null (or standard PHP types) can be
> combined in a union with intersection types.
>
> That's one reason why it's only pure intersection types, if I had more time
> (or for crying out loud somebody would have paid me) to work on this I
> would have loved to get full composite types working.
>
> And I find it frankly insulting that in the four month this RFC has been
> published for discussion, with multiple push backs for voting due to bugs
> and me wanting that people know what implementation is - for the most part
> - going to land in php-src, this specific point has not been raised.
> It just feels like you are pissing on 6+ months of *unpaid* work I did
> because it doesn't suit your needs, and you just realised that and so
> decided to throw a wrench into a carefully crafted RFC to "fix" it by using
> a Twitter mob to put pressure on us/me.
>
> Maybe this topic didn't come up because for nearly everyone else "Pure
> Intersection Types" means what it says on the can, moreso that in the RFC
> the following line:
> > This means it would *not* be possible to mix intersection and union types
> together such as A&B|C, this is left as a future scope
> makes it clear, and most voters also understood that '?' is not a type
> "flag" but is syntactic sugar for 'null|'.
>
> There are plenty of issues with the implementation, from the whack parser
> hack, the non support for static/parent/self, to the complexity of the
> variance code, but I made all of those clear because they made *me*
> uncomfortable with the implementation.
> So if you are going to force this crappy syntax, then I'd rather axe this
> feature completely (and nobody can use/play with it) then have something I
> do not accept, nor are forced to accept because of a vote, land in core
> making PHP even more of an inconsistent joke.
>

Hey George,

I don't think there's been any malicious intent here -- it was obvious to
you and I that not allowing unions implies not allowing nullability, but I
can see how people less familiar with our type system implementation would
not make that connection. After all, we do provide the separate ?T syntax,
even if it is an internal alias for T|null.

It's an unfortunate fact of the RFC process that concerns are sometimes
only raised when voting starts and people start looking at the
implementation -- or in this case, when they test the implementation after
it has landed...

In any case, I think it's pretty clear by now that there are some fairly
strong opinions both on what the syntax for this should be (between ?X&Y,
?(X&Y), (X&Y)|null and X&Y|null, all of which have reasonable arguments for
and against) and whether we should have this special case at all. I'm not
sure we will be able to reach a sufficient consensus in this discussion.

For that reason, I think this change should go through the RFC process. We
can grant this an exemption from the feature freeze (as a clarification of
an accepted proposal), but it should still go through the process.

Regards,
Nikita

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