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Date: Tue, 12 Apr 2022 00:48:20 +0200
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To: Sara Golemon <pollita@php.net>
Cc: =?UTF-8?Q?Tim_D=C3=BCsterhus?= <tim@bastelstu.be>, 
	"G. P. B." <george.banyard@gmail.com>, PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] [RFC] Add true as type
From: andreas@dqxtech.net (Andreas Hennings)

> sketch out a more complete picture for the type
system.  We can implement it in pieces and parts, but we should have a goal
in mind for where we want to land.

I think the type system in psalm is pretty cool. It does have fixed
literal values, but also deeply structured array types.
But:
- I don't think we'll arrive there in one leap
- I am not sure we _want_ to arrive there, ever. Perhaps it is just
too much for native support, especially if we want repeated runtime
checks.
- Runtime checks on array types seem like a nightmare.
- I am pretty confident that adding `true` won't close any doors fur
future type systems, if we already have `false`.

Alternatively we could look at compile-time type systems like in C++
(or Java?), where we don't have to waste time on runtime checks.
The default implicit variable type could be "variant", but variables
with a declared type would behave as in C++, with a faster and slimmer
storage model.
Of course any union type that allows values other than object or null
would have to use the storage model of "variant".
Sounds like a lot of work, but if we want to dream big, it is
something to consider.

(There might be better examples than C++, I only mention it because I
spent some time with it long ago. Perhaps even Java?)

-- Andreas

On Mon, 11 Apr 2022 at 18:18, Sara Golemon <pollita@php.net> wrote:
>
> On Mon, Apr 11, 2022 at 8:59 AM Tim D=C3=BCsterhus <tim@bastelstu.be> wro=
te:
> > For `true` the story is different, as this type is an entirely new type
> > available to userland and by the same argument one could have literal
> > strings or literal integers as distinct types as well. I wouldn't objec=
t
> > to that per se (and there's precedent in TypeScript), but there could b=
e
> > a clearer plan for the type system as a whole, instead of adding types
> > piecemeal.
> >
>
> ^This. The bit about making a plan, I mean.  A slow erosion of what it
> means to be a type might be the easier pill to swallow, but it results in
> more chaos overall in the type system.  I'd like to see someone (literall=
y
> anyone) take the time to sketch out a more complete picture for the type
> system.  We can implement it in pieces and parts, but we should have a go=
al
> in mind for where we want to land.
>
> Using this RFC as an example, it naturally leads into the question of val=
ue
> types (which is what `true` really is, a bool with a value of true), so w=
hy
> can't we say this method must return the number 42 or a string containing
> 'pamplemousse' ?  A roadmap would help us understand what those kinds of
> value types (in an unbounded type like string or a large bounded type lik=
e
> int) compared to the tight bounded type of bool.
>
> And yes.  Generics.   We can't talk about the type system without the
> subject of generics being extremely relevant.  Ignoring them because it's
> complicated is just kicking the can down the road.
>
> All that having been said, sure. I don't mind adding true for completenes=
s,
> because it's fairly obvious and the scope of bool is fairly small, but I
> would really encourage anyone to step up and plan out a wider reaching go=
al
> for the type system.
>
> -Sara