[swift-evolution] Proposal: 0009 Require self for accessing instance members

Daniel Hooper danielchasehooper at gmail.com
Thu Dec 17 13:53:42 CST 2015

I have mixed feelings about this proposal: I see some value in the problems
it is trying to address, though there are other ways to solve them.

The two problems it tried to address that I consider legitimate:
1. bullet proofing against bugs.
The example in the motivation section is a good one. I can imagine this
happening, though I haven't ever personally experienced it, so how often
does it really happen? Is it often enough to warrant this requirement? The
root cause of that bug is that the instance variable was shadowed by a
local one. Requiring self is one way to ensure local variables don't shadow
instance variables, but the compiler could also just disallow shadowing
instance variables. Details aside, the take away here is that there are
other solutions to this problem.

2. Clarity at the call site.
Syntax highlighting solves this with two caveats: color blindness and
non-colored code, such as diffs in the terminal. Color blindness can be
helped by changing the coloring rules to make it easier to differentiate
ivars. I don't have an answer for situations with no syntax coloring,
though in my own workflow that isn't really an issue. For teams that it is,
they could require use of self, or ivar prefixes ("var _ivar")

So while both problems could be legitimate, there are other ways of
addressing them that don't require the verbosity of "self."

Thought experiment:
"self." has a lot of baggage from obj-c, python, etc, so as an experiment
let's extended this proposal to it's logical conclusion and see how we feel
about it: Global variables and functions have this same shadowing situation
that ivars and instance methods do: an identifier without "self." could be
referencing a local or global variable. Here is the same example from the
proposal, but with a global variable instead of the ivar:

let greeting = "Hi There"

class MyViewController : UIViewController {
    @IBOutlet var button: UIButton!

    func updateButton() {
        // var greeting = "Good morning"
        button.setTitle(greeting, forState: .Normal) // forgot to
comment this line but the compiler does not complain and greeting is
now referencing global greeting by mistake

By this proposal's logic we should create a "global." prefix and require it
to access global variables and functions. Only local variables and
functions could be accessed without a prefix. If we approve this proposal
then maybe this is a great idea, or maybe this example shows some flaw in
the proposal. Either way, hopefully it can help us see the proposal in a
new light.


On Thu, Dec 17, 2015 at 1:03 PM Michael Brown via swift-evolution <
swift-evolution at swift.org> wrote:

> On 17 Dec 2015, at 17:54, Dan Loewenherz <dan at lionheartsw.com> wrote:
> On Thu, Dec 17, 2015 at 11:28 AM, Michael Brown via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Tue, Dec 15, 2015 at 21:03 Sune Foldager via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> > “self.” everywhere means you can see what’s an instance member and
>> what’s a local variable. That’s generally a good thing. But it also means a
>> lot of filler text in your code, which makes reading and writing slower.
>> That’s not so good. It’s a balance, and in this case my experience from C#
>> (and, as mentioned, Python) is that I much prefer C#’s non-mandatory use of
>> “this”/“self".
>> >
>> > I see that this proposal is going to be reviewed as SE-0009, and I am a
>> bit concerned that not all arguments are being considered because of the
>> contents of the proposal text: The only counter argument mentioned in the
>> proposal has to do with capturing semantics in closures. This is fine, but
>> why isn’t the counter argument of verbosity being mentioned? This has been
>> brought up on the list as well.
>> >
>> I couldn't agree more on all points. I strongly object to this proposal.
>> Requiring self makes code unnecessarily verbose and is one of the things I
>> least liked about Objective-C compared to other languages (e.g. C#, Python,
>> Java) where the use of self/this is optional. It is only only actually
>> necessary when resolving ambiguity with local variables, in the few
>> situations where such ambiguity exists (initialisers mostly).
> Correction: self is not optional in Python.
> Yes. I realized my mistake soon after I hit send. Apologies.
> Michael.
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