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

Fri Dec 23 13:07:44 CST 2016

On Fri, Dec 23, 2016 at 1:33 PM, Callionica (Swift) <
swift-callionica at callionica.com> wrote:

> I support the principle of progressive disclosure. I still fail to see how
> if class capture were introduced it would prevent people from learning
> about classes, functions, closures or any other existing concept in
> whatever order the student or teacher preferred.
>
> I disagree that there is any significant risk that a novice user that
> doesn't expect to be able to use names that are in scope would be worse off
> with this change. Think about the exact sequence of events that has to
> occur to trigger this user's confusion. OTOH beginners that expect to be
> able to use names that are in scope will be better off.
>
> In general I don't believe learning has the linearity you suggest and I
> don't think there is anything particularly natural about the specific
> sequence of concepts you've listed.
>

I'm making no claim that learning _has_ to be linear or ought to be, nor
that any particular sequence of learning is _particularly_ natural. Rather,
I understand the notion of progressive disclosure to mean that there ought
to be _some_ sequence or sequences of learning where it is possible to
master things in a linear fashion _if_ any particular learner wishes to
proceed linearly, with concepts introduced earlier not making reference to
concepts learned later. It is entirely fair if someone finds it more
intuitive to learn or teach the same material in a different order, or even
to learn or teach multiple concepts simultaneously.

Put another way, the question is not whether it's reasonable to have all
learners progress through Swift in one particular blessed way. The question
is whether, _if_ a learner is not yet ready to learn concept Z, it's
possible to teach unrelated concept Y without reference to Z, for some
reasonable ordering(s) of concepts from A to Z.

In this analysis, I think that classes capturing variables from a
containing function would be detrimental. If one considers classes,
functions, and closures as three topics, it is currently possible (I think)
to master classes and functions without reference to closures. In a
scenario where classes can capture variables in a containing function, one
must understand the concept of escaping closures to master classes and
functions. The argument isn't that one linear sequence of learning is
superior to the others; the argument is that a reasonable linear ordering
of these concepts conducive for at least some learners would become
non-linear by having this new feature.

I think it's worthwhile to analyze features from the point of view of
> beginner users of the language, but I reach the opposite conclusion in this
> case.
>
> -- Callionica
>
> On Fri, Dec 23, 2016 at 9:46 AM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
>> On Fri, Dec 23, 2016 at 03:10 Callionica (Swift) <
>> swift-callionica at callionica.com> 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?).
>>
>>
>> The notion of closing over variables far exceeds what one might
>> reasonably call "a basic understanding of scope." I doubt I'm alone on this
>> opinion, as _Advanced Swift_ (an excellent text written for users already
>> familiar with the basics) reviews closures and closure expressions from the
>> ground up; by contrast, it does not review functions, classes, or even (if
>> I recall correctly) defining classes inside functions.
>>
>> I'm not clear on what you see as the downside of letting classes capture
>> locals for users that are unaware of capturing.
>>
>>
>> It violates the notion of progressive disclosure, which has been an
>> explicit design goal for Swift. In one of the documents on GitHub, the core
>> team writes: "One goal of swift is to provide a very 'progressive
>> disclosure' model of writing code and learning how to write code.
>> Therefore, it is desirable that someone be able to start out with
>> `print("hello world\n")` as their first application." Chris Lattner
>> recently reiterated this idea in a presentation he gave at IBM about the
>> Swift language.
>>
>> The idea of progressive disclosure is that it should be possible to teach
>> concepts a learner must understand first without exposing them to more
>> advanced concepts they must pay attention to but cannot understand. Let's
>> look at some examples:
>>
>> - In Java, one cannot write "Hello World" without being exposed to
>> `public static void main`. However, one cannot understand what it means to
>> be a public or static method until _after_ one has mastered printing "Hello
>> World". This violates progressive disclosure.
>> - In Swift, one's first "Hello World" is a one-liner that introduces the
>> print function and nothing else. This is progressive disclosure, and as
>> quoted above, Swift has been explicitly designed that way.
>>
>> - In Swift, `struct Point { let x, y: Int }` is a fully usable type. With
>> that single line, you can now make a new `Point` by writing `let point =
>> Point(x: 42, y: 42)`.
>> - To do a similar thing in some other languages, you'd have to write a
>> constructor or initializer. However, one cannot really understand what
>> initialization is without first having initialized something, but one
>> cannot initialize something unless there is an initializer. This violates
>> progressive disclosure. In Swift, an initializer is synthesized for you;
>> this promotes progressive disclosure.
>> - There have been proposals to change the default access level to
>> private. If `x` and `y` were private, then `Point` would be unusable
>> without access modifiers for its members. However, one must first
>> understand how structs and other types encapsulate code before one can
>> understand how access modifiers work, but one would not be able to create
>> useful structs or other types without first using access modifiers. This
>> would violate progressive disclosure. In Swift, the default access level is
>> internal; this promotes progressive disclosure.
>>
>> - In Swift, _closure expressions_ don't have to capture anything (i.e.,
>> they don't have to be true _closures_). But they are introduced as closure
>> expressions rather than, say, function expressions, and they are taught in
>> the same breath as the concept of capturing variables. This ensures that
>> every person who reaches for a closure expression knows about its potential
>> for capturing and extending the lifetime of a variable. Closure expression
>> syntax and the concept of closing over its environment are thereby
>> inextricably linked. Thus, one can be well assured that every user who
>> extends the lifetime of a captured variable--unintentionally or no--has
>> already been introduced to the fact that such a thing is a possibility.
>> - If a user who has mastered only functions and classes can silently
>> extend the lifetime of a variable, this would be to that user an
>> inexplicable behavior. One must teach someone either "never" to nest
>> classes inside functions, or to do so only by putting classes before any
>> other code, but one would not be able to explain to the learner _why_ that
>> must be the case. This violates progressive disclosure.
>>
>> Pedagogical points aside, introducing another way to extend the lifetime
>> of a variable with a syntax that currently doesn't do so is, IMO, creating
>> another avenue for footguns. I'm not convinced that this functionality
>> would be used intentionally more often that it would be triggered
>> unintentionally and not discovered.
>>
>>
>> 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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