[swift-evolution] Proposal: Re-instate mandatory self for accessing instance properties and functions (David Hart)

Dennis Lysenko dennis.s.lysenko at gmail.com
Mon Dec 14 09:14:45 CST 2015 

To expand, even on its own this could be beneficial in reminding
programmers of capture semantics in cases where they might forget, even
when they are experienced and aware of those semantics.

On Mon, Dec 14, 2015, 10:13 AM Dennis Lysenko <dennis.s.lysenko at gmail.com>
wrote:

> Nick, thanks for clarifying. Perhaps we could require an explicit [strong
> self] in the cases where it is implicit, and Xcode autocomplete would
> insert that any time it inserted a closure?
>
> On Mon, Dec 14, 2015, 12:30 AM Nick Shelley <nickmshelley at gmail.com>
> wrote:
>
>> @Andrew: I agree that the discussion got a little sidetracked with people
>>> debating whether self should be required at all in closures. I personally
>>> think that your proposed solution may lead to needless ambiguity and
>>> confusion with regards to self-capture, and I would be careful with
>>> bringing it up as it could be a point of contention that may derail the
>>> discussion further. I personally don't agree with it but will reserve my
>>> judgment in the interest of staying on the topic of mandatory self.
>>
>>
>> I think you misunderstood Andrew's point.
>>
>> From what I understand, the main reason that self is required in closures
>> and not elsewhere is to serve as a compiler-enforced reminder about the
>> potential pitfalls of using self within closures (which I happen to think
>> it does well). Unless and until a different way of calling out those
>> pitfalls is proposed and accepted (as Andrew was suggesting), that point is
>> an essential part of the discussion IMO.
>>
>> I don't think Andrew was suggesting that the discussion got sidetracked,
>> but realized that these two points can't be separated as the language
>> stands today, and was proposing a way to make them separate by
>> discontinuing the use of self as the way to call out capture semantics in
>> closures. At least that's what I understood when he said (regarding the use
>> of self in closures) "Perhaps if this could be modified *first* it would
>> help." (emphasis mine)
>>
>>
>>> And on the con side, I have heard --
>>> - Annoying to do
>>> - Makes it harder to move from local to instance context
>>> - Makes code ostensibly less readable through "self" proliferation
>>>
>>
>> You forgot "Makes capture semantics of using self inside of closures less
>> apparent." I consider this the main con of the current proposal, and it
>> seems others do as well.
>>
>>
>>
>> On Sun, Dec 13, 2015 at 8:55 PM, Dennis Lysenko via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> @Andrew: I agree that the discussion got a little sidetracked with
>>> people debating whether self should be required at all in closures. I
>>> personally think that your proposed solution may lead to needless ambiguity
>>> and confusion with regards to self-capture, and I would be careful with
>>> bringing it up as it could be a point of contention that may derail the
>>> discussion further. I personally don't agree with it but will reserve my
>>> judgment in the interest of staying on the topic of mandatory self.
>>>
>>> Could anyone provide a reasonably unbiased round-up of the pros/cons so
>>> far presented regarding required self capture?
>>>
>>> To start, on the pro side I have heard --
>>> - Differentiates locals and ivars, making code ostensibly more readable
>>> - Makes code more refactorable (proposed by me, counterargument: it may
>>> not be good to be able to move code around willy-nilly to and from closures
>>> when considering capture semantics)
>>>
>>> And on the con side, I have heard --
>>> - Annoying to do
>>> - Makes it harder to move from local to instance context
>>> - Makes code ostensibly less readable through "self" proliferation
>>>
>>> Unless I missed something (very possible seeing as I believe I've
>>> entered this discussion somewhat late), the rest seems to be anecdotal data
>>> and there does seem to be a solid split between the two schools.
>>>
>>> On Sun, Dec 13, 2015 at 5:14 PM Andrew Brown via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>>> I think the requirement to use self in closures complicates the
>>>> discussion.  Perhaps if this could be modified first it would help.
>>>> Perhaps instead of self to indicate capture, we should use
>>>> weak/strong/unowned etc as keywords to be explicit about the type of
>>>> capture.
>>>>
>>>> then the discussion on self wouldn't be side tracked with this?
>>>>
>>>>
>>>>    1. class HTMLElement {
>>>>    2.
>>>>    3.
>>>>    4.   let name: String
>>>>    5.   let text: String?
>>>>    6.
>>>>    7.
>>>>    8.   lazy var asHTML: Void -> String = {
>>>>    9.     if let text = unowned.text {
>>>>    10.       return "<\(weak.name)>\(text)</\(weak.name)>"
>>>>    11.     } else {
>>>>    12.       return "<\(unowned.name) />"
>>>>    13.     }
>>>>    14.   }
>>>>    15.
>>>>    16. }
>>>>
>>>>
>>>> ABR.
>>>>
>>>> On 7 Dec 2015, at 01:10, David Hart via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>> Hi Nick,
>>>>
>>>> I understand the quote "This makes the capturing semantics of self
>>>> stand out more in closures”, but this is a very weak statement in Swift for
>>>> me. Let me try to explain.
>>>>
>>>> If we use the try keyword as an example:
>>>>
>>>> try foobar()
>>>> barfoo()
>>>>
>>>> If the previous lines of code compile without error, we know without a
>>>> shadow of a doubt that foobar is a throwing function and that barfoo
>>>> does not throw. The compiler will not compile the first line without the
>>>> keyword and would not allow it in on the second line.
>>>>
>>>> Now if we go back to the example of self in closures:
>>>>
>>>> foobar({
>>>> print(self.description)
>>>> })
>>>>
>>>> The self keyword in the previous lines of code does not tell us
>>>> anything at all:
>>>>
>>>>
>>>>    - self might have been forced by the compiler to warn us.
>>>>    - self might have been a programmer choice if the closure was
>>>>    non-escaping.
>>>>
>>>>
>>>> And the reverse:
>>>>
>>>> barfoo({
>>>> print(description)
>>>> })
>>>>
>>>> This also does not tell us much:
>>>>
>>>>
>>>>    - The closure might be non-escaping.
>>>>    - description might be referring to a local variable (which we
>>>>    missed the declaration) shadowing the instance property in an escaping
>>>>    closure.
>>>>
>>>>
>>>> In both of these last examples, we can’t tell by having a quick look at
>>>> the code at the point of call if we should really be careful about memory
>>>> or not.
>>>>
>>>> With the proposition, self gets some meaning back: it indicates which
>>>> are local and which are instance properties.
>>>>
>>>> David.
>>>>
>>>>
>>>> On 06 Dec 2015, at 23:55, Nick Shelley via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>> I like that self is only required in closures because it serves as a
>>>> good reminder that there are memory and safety implications with using self
>>>> in a closure, such as creating retain cycles or having the closure run
>>>> after self has been deallocated.
>>>>
>>>> I can't seem to find an official Apple Swift style guide, but github's (
>>>> https://github.com/github/swift-style-guide) suggests only using self
>>>> in closures with the rationale: "This makes the capturing semantics of self
>>>> stand out more in closures, and avoids verbosity elsewhere."
>>>>
>>>> On Sat, Dec 5, 2015 at 3:16 AM, Yichen Cao <ycao at me.com> wrote:
>>>>
>>>>> Teaching wise, its much less confusing for self to be required so
>>>>> students don't mix up instance properties and local vars. Especially when
>>>>> self is required in closures, it confuses students. If self is mandatory
>>>>> for all instance properties, it would be so much clearer and much easier to
>>>>> read.
>>>>>
>>>>> Yichen
>>>>>
>>>>> On Dec 5, 2015, at 18:11, swift-evolution-request at swift.org wrote:
>>>>>
>>>>> Re: Proposal: Re-instate mandatory self for accessing
>>>>>      instance properties and functions (David Hart)
>>>>>
>>>>>
>>>>>
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>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>
>>>>>
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>>
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