[swift-evolution] [Proposal] Change Void meaning

[Jérémie Girault]

@john, the proposal is the fruit of my imagination and the goal was to
discuss about it.
I’m vastly open to change it with the help of anyone if it can be
implemented in a simple way and leads to better compatibility or
syntactical improvements.

xiaodi told me that he wouldn’t be available to help me but I’m open to any
help and change that would represent an improvement (from the developer or
compiler point of view)

—
very short reply expected - vsre.info
Jérémie Girault

On 13 juin 2017 at 19:15:18, John McCall (rjmccall at apple.com) wrote:


On Jun 13, 2017, at 4:41 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

On Tue, Jun 13, 2017 at 3:06 AM, John McCall <rjmccall at apple.com> wrote:

> On Jun 13, 2017, at 3:30 AM, Jérémie Girault <jeremie.girault at gmail.com>
> wrote:
>
> Exactly,
> The reflexion behind it is:
>
> - Let's understand that 0110 and other tuple SE are important for the
> compiler, we do not want them to rollback
> - However we have number of regressions for generics / functional
> programmers
> - Let’s solve this step by step like a typical problem
>
> - Step 0 is adressing this Void tuple of size zero :
> - Zero is in many problems of CS an edge case, so let’s handle this case
> first
> - The compiler knows what Void is, and its only value (or non-value)
> - It was handled historically by the compiler because of implicit side
> effects
> - Let’s handle it explicitely with rules in current context
> - one effect of the proposal is source compatibility
> - but the goal is to build atop and strengthen 0110, 0066 and other
> tuple-related SE
>
>
> There are four difficulties I see with this proposal.
>
> The first is that it is a first step that quite clearly does not lead to
> anything.  It resolves a difficulty with exactly one case of function
> composition, but we would need completely different solutions to handle any
> of the other compositional regressions of SE-0110.
>
> The second is that it's a huge source of complexity for the type system.
> The type checker would not be able to do even rudimentary type matching,
> e.g. when checking a call, without having first resolved all of the
> argument and parameter types to prove that they are not Void.  This would
> probably render it impossible to type-check many programs without some
> ad-hoc rule of inferring that certain types are not Void.  It would
> certainly make type-checking vastly more expensive.
>
> The third is that it is not possible to prevent values of Void from
> existing, because (unlike Never, which cannot be constructed) they are
> always created by returning from a Void-returning function, and a generic
> function can do anything it likes with that value — turn it into an Any,
> store it in an Array, whatever.  The proposal seems to only consider using
> the value as a parameter.
>

Hang on, though. If Jérémie is interested only in addressing the issue of
Void as a parameter and his idea can be adequately carried out by inferring
a default value of Void for every parameter of type Void, this should be a
fairly self-contained change, should it not? And would the impact on the
cost of type checking really be vastly greater in that case?


If the proposal was phrased in terms of defaults, e.g. "trailing parameters
do not require a matching argument if they have Void type", then yes, that
would be implementable because it still admits a "local" reduction on call
constraints, one which does not need to immediately reason about the actual
types of arguments.  It is not clear that this rule allows function
compositions of the sort that Jérémie is looking for, though.

Anyway, that is not the proposal; the proposal is that parameters — in any
position — are simply removed from the parameter sequence if they have Void
type.  In order to allow composition (i.e. f(g(x)), where g: X -> Void),
you then need a matching rule that arguments are dropped from the argument
sequence (for purposes of type-checking) if they have Void type.  Either of
these rules is sufficient to turn the reduction of function-type matches
into an extremely messy combinatoric matching problem where e.g. (τ0, Int)
can be passed to a function taking (Int, τ1) if we can decide that τ0 == τ1
== Void.

This idea is now rather intriguing to me because it extends beyond just
addressing one symptom of SE-0110. Swift allows us to omit the spelling out
of return types that are Void, it allows warning-free discarding of return
values that are Void, etc. This could add a nice consistency and
rationalize some of the weirdness of passing a value that is stipulated by
the parameter type.

Finally, it would allow a lot of inadvertent errors with the use of generic
> functions, because any argument of unconstrained type could be accidentally
> specialized with Void.  For example, if you forgot to pass an argument to
> this function, it would simply infer T=Void:
>   func append<T>(value: T)
> It seems more likely that this would lead to unexpected, frustrating bugs
> than that this would actually be desired by the programmer.  You really
> just want this to kick in in more generic situations.
>

Hmm, at first glance, that seemed like it could be bad. But if, say, a
particular collection can store an element of type Void, is it so
undesirable to allow `append()` to append Void?


append on a Collection is not an unconstrained generic; its parameter type
is determined by the type of the collection.  Perhaps this was a
poorly-chosen example.

John.



> John.
>
>
> —
> very short reply expected - vsre.info
> Jérémie Girault
>
> On 13 juin 2017 at 00:44:52, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>
> On Mon, Jun 12, 2017 at 5:38 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
>> On Mon, Jun 12, 2017 at 5:25 PM, Jérémie Girault <jeremie.girault@
>> gmail.com> wrote:
>>
>>>
>>>
>>> —
>>> very short reply expected - vsre.info
>>> Jérémie Girault
>>>
>>> On 12 juin 2017 at 23:56:37, Xiaodi Wu (xiaodi.wu at gmail.com) wrote:
>>>
>>> On Mon, Jun 12, 2017 at 4:47 PM, Jérémie Girault <jeremie.girault at gmail
>>> .com> wrote:
>>>
>>>> - Void as arguments is pretty common when using generics, that’s a core
>>>> point of this proposal. An maybe that’s why we misunderstood ourselves
>>>> (around 0110 / 0066). This proposal addresses arguments.
>>>> - maybe it should be revised around this ? Simple example :
>>>>
>>>> `typealias Callback<T> = (T) -> Void` -> `Callback<Void>` will give
>>>> `(Void) => Void`.
>>>>
>>>> It was acceptable before swift4 but no more. However nobody cares about
>>>> this `Void` argument and actually we know it’s value. So why let the
>>>> developer type it ?
>>>>
>>>
>>> Ah, I see. The purpose of SE-0029...SE-0110 was to make it possible to
>>> distinguish an argument list `(Void)` from an argument list `()`. This does
>>> cause some verbosity where previously users relied on implicit tuple
>>> splatting. Ideally, we would bring back some syntactic sugar to make this
>>> more ergonomic. But, whether or not the spelling is made more
>>> user-friendly, the point here is that _everybody_ should care about this
>>> `Void` argument.
>>>
>>> It is still be typechecked and appropriate errors should be reported to
>>> the user so _nobody_ will ignore it.
>>>
>>> But with the proposal the code will be striped out of Void arguments at
>>> compile-time. I think it's a win for the developer on a lot of grounds. The
>>> fact that this proposal integrates with the type-system is also important.
>>>
>>> If you are not comfortable about Void being stripped, we can also
>>> discuss alternatives: someone was suggesting me that it would be possible
>>> to replace :
>>>
>>> ```
>>>
>>> func foo<T, U, V>(t: T, u: U) -> V {
>>>
>>>   // do something with t and u
>>>
>>>   // return some V
>>>
>>> }
>>>
>>> ```
>>>
>>> with
>>>
>>> ```
>>>
>>> func foo<Void, Int, String>(u: Int) -> String { let t = ()
>>>
>>>   // do something with t and u
>>>
>>>   // return some V
>>>
>>> }
>>>
>>> ```
>>>
>>> or
>>>
>>> ```
>>>
>>> func foo<Void, Int, String>(t: Void = (), u: Int) -> String {
>>>
>>>   // do something with t and u
>>>
>>>   // return some V
>>>
>>> }
>>>
>>> ```
>>>
>>> I don’t know what you would consider more effective or elegant (at an
>>> implementation level) but it’s the same result for the developper.
>>>
>>
> Ah, but I think I catch your drift with the last example. Is this a more
> general point that the compiler should treat every parameter of type Void
> as having an implied default value of Void? That would be an interesting
> idea.
>
> What is the goal of such changes? Is it to allow you to write `foo()`
>> instead of `foo(())` for a function `foo` of type `(T) -> Void`?
>>
>> If so, then I think what you're seeking to do is reverse SE-0066 (as
>> Vladimir points out), which explicits details how it's is an intentional
>> change to require such a spelling. I think you're starting from the premise
>> that this is unintended or undesirable, when in fact it is deliberate and
>> approved.
>>
>> It is also, unless I'm mistaken, not the issue that was raised initially
>> with respect to SE-0110, which had to do with the extra boilerplate of
>> destructuring a tuple inside a closure, something that was not so obvious
>> before implementation.
>>
>> My point here is that `Void` should be “striped” by “reducing” argument
>>>> list signatures.
>>>>
>>>> —
>>>> very short reply expected - vsre.info
>>>> Jérémie Girault
>>>>
>>>> On 12 juin 2017 at 19:15:18, John McCall (rjmccall at apple.com) wrote:
>>>>
>>>>
>>>> On Jun 12, 2017, at 4:48 AM, Jérémie Girault via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>> Hi here,
>>>>
>>>> As I tested swift4 in xcode9b1 I noticed a lot of regressions about
>>>> tuples usage.
>>>>
>>>> After documenting myself about the changes which happened, I thought
>>>> that they could be improved. Instead of fighting these propositions (which
>>>> make sense), I wanted create a few proposal which would improve these
>>>> recent changes with a few simple rules.
>>>>
>>>> My propositions are based on the recent decisions and in the
>>>> continuation of SE-0110. The first one is about Void.
>>>> Void is historically defined as the type of the empty tuple. The reason
>>>> of this is that arguments were initially considered as tuple.
>>>>
>>>>
>>>> The dominant consideration here was always return types, not
>>>> parameters.  I'm not sure there was ever much point in writing Void in a
>>>> parameter list, but whatever reasons there were surely vanished with
>>>> SE-0066.
>>>>
>>>> Note that 'void' in C was originally exclusively a return type.  ANSI
>>>> gave it a new purpose it with void*, but the meaning is totally unrelated.
>>>>
>>>> John.
>>>>
>>>>
>
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