[swift-evolution] Fwd: Should we relax restriction on closing over outer scope in class declarations?

[Callionica (Swift)]

Missed swift-evol in my reply

---------- Forwarded message ---------
From: Callionica (Swift) <swift-callionica at callionica.com>
Date: Sat, Dec 24, 2016 at 9:44 AM
Subject: Re: [swift-evolution] Should we relax restriction on closing over
outer scope in class declarations?
To: Robert Widmann <devteam.codafi at gmail.com>


These are good questions.

I believe Swift closures currently do implicit self-capture. I wouldn't
expect that behavior to be different when capturing for classes.

If a class needs to refer to an outer self, I would expect the user to have
to give the outer self a new name by assigning to a local. Same deal for
referring to members in an outer class where there are members of the same
name in the inner class: assign outer self to a local and refer to members
through that local.

On Fri, Dec 23, 2016 at 10:16 AM, Robert Widmann <devteam.codafi at gmail.com>
wrote:


On Dec 23, 2016, at 1:10 AM, Callionica (Swift) via swift-evolution <
swift-evolution at swift.org> wrote:

I'm certainly assuming that users have a basic understanding of scope
(without that, what would be the intention behind defining the class within
the function?).


Despite its simplicity to you (given the example you’ve cited) this feature
is quite an advanced extension to the current scoping rules.

I'm not clear on what you see as the downside of letting classes capture
locals for users that are unaware of capturing.


I’ll be blunt: This opens a gaping hole in the semantics of variable
lifetimes and potentially introduces an ambiguity into resolution of the
`self` keyword.  For the former, consider a light extension to your
original example that introduces a nested class:

class Outer {
  let widgetString: String = createWidgetString()

  // Initializer suppressed
  func foo() -> WidgetStringProvider {
    class SimpleProvider: WidgetStringProvider {
      // Initializer suppressed
      func provideWidgetString() -> String {
        return widgetString
      }
    }
    return SimpleProvider()
  }
}

Now, from SIL’s point of view, widgetString  must escape the scope it’s in
and its lifetime should be extended beyond this block.  Fine.  But what
about Outer?  Because we’re returning a reference to SimpleProvider which
escapes the scope of foo(), and instance members of a SimpleProvider
capture outer context, this means we also have to keep Outer  alive and
around for destruction (perhaps at the point where SimpleProvider  is
destroyed?)

For the latter consider trying to explicitly reference widgetString  here.
In provideWidgetString() we  close over self in both foo() and
provideWidgetString(). So, if I wish to make reference to Outer’s self
explicitly,
how would I go about doing it?  Given that today a nested aggregate can
have members with the same identifiers as their parent, how do we know if
we’re capturing them or not when they’re referenced in the inner aggregate?

~Robert Widmann

On Thu, Dec 22, 2016 at 11:26 PM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

Only if you're also assuming that people defining classes within functions
would know about capturing. This violates the principle of progressive
disclosure, since people naturally learn about functions and classes before
they learn about closures and capturing.


On Fri, Dec 23, 2016 at 01:51 Callionica (Swift) <
swift-callionica at callionica.com> wrote:

Assuming capture were allowed, people defining classes within functions who
didn't want them to capture could position the class definition prior to
any other code in the function so that there would be nothing to capture.

On Thu, Dec 22, 2016 at 4:13 PM Xiaodi Wu via swift-evolution <
swift-evolution at swift.org> wrote:

I have to agree with Michael; it seems dangerous to allow implicit capture
by classes. A primary purpose (telos?) of closures is to provide this
functionality, which is actually quite an advanced concept. One knows to
think carefully about this when encountering a closure expression. A
primary purpose of classes is to provide for encapsulation of code.
Accidentally extending the lifetime of a local variable in a containing
scope would be hard to notice and highly unexpected functionality. Better
not to mix these things.

On Thu, Dec 22, 2016 at 17:54 Micah Hainline via swift-evolution <
swift-evolution at swift.org> wrote:

That's exactly what I'm suggesting, the class declaration could work
similarly to a closure.





> On Dec 22, 2016, at 4:15 PM, Michael Ilseman <milseman at apple.com> wrote:


>


> Are you asking for a class declaration to implicitly capture and extend
the lifetime of local variables? That seems like something that’s better
done explicitly. Perhaps it’s better to think about how to reduce the
boiler plate code, e.g. better default initializers.


>


> (this is of course, additive and beyond the current scope of Swift 4
phase 1 planning)


>


>> On Dec 22, 2016, at 2:39 PM, Micah Hainline via swift-evolution <
swift-evolution at swift.org> wrote:


>>


>> Currently we allow declarations of a new class in local scope, but


>> nothing can be referenced from the outer scope. Thus this is illegal:


>>


>> ```


>> func foo() {


>>  let widgetString: String = createWidgetString()


>>  class SimpleProvider: WidgetStringProvider {


>>     func provideWidgetString() -> String {


>>        return widgetString // Illegal, defined in outer scope


>>     }


>>  }


>>  doThingsWithWidget(provider: WidgetStringProvider())


>> }


>> ```


>>


>> I'm trying to feel out the edges of this decision, and figure out why


>> exactly this isn't allowed now, and how we might want to relax this in


>> the future if possible. While not a common construct, it is a useful


>> one.


>>


>> Obviously there are ways around it, very simple to create an init and


>> a private let and do something like this:


>>


>> ```


>> func foo() {


>>  let widgetString: String = createWidgetString()


>>  class SimpleProvider: WidgetStringProvider {


>>     private let widgetString: String


>>


>>     init(widgetString: String) {


>>        self.widgetString = widgetString


>>     }


>>


>>     func provideWidgetString() -> String {


>>        return widgetString // now legal, references


>> SimpleProvider.widgetString


>>     }


>>  }


>>  doThingsWithWidget(provider: WidgetStringProvider(widgetString:


>> widgetString))


>> }


>> ```


>>


>> That's boilerplate I don't want to write though, as it somewhat


>> detracts from the readability of the structure. I'm not super


>> interested in defending the concept of local class definitions itself,


>> it's already allowed in the language, I'm just interested in the


>> syntax limitation and where that line might be able to be redrawn.


>> _______________________________________________


>> swift-evolution mailing list


>> swift-evolution at swift.org


>> https://lists.swift.org/mailman/listinfo/swift-evolution


>


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