[swift-evolution] My personal beef with leading-dot syntax

Joe Groff jgroff at apple.com
Mon Apr 4 13:48:37 CDT 2016

> On Apr 4, 2016, at 11:44 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> on Mon Apr 04 2016, Joe Groff <jgroff-AT-apple.com> wrote:
>>> On Apr 4, 2016, at 11:05 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> on Mon Apr 04 2016, Erica Sadun <swift-evolution at swift.org> asked:
>>>> Can you ping me off-list or in another thread and explain what the
>>>> issues are?
>>> All of the following make me uncomfortable with our leading-dot thang:
>>> * The rules for lookup don't seem obvious to me.  I admit this is very
>>> personal/subjective.
>>> * There is some evidence that people think it means something it doesn't
>>> (“enum case”), as mentioned in SE-0036.  That suggests it is a
>>> confusing feature.  That confusion may be fairly harmless so far, but
>>> still.
>>> * The dot doesn't seem to buy enough to be worth the complexity it adds
>>> to the language; why not just let those names be looked up without the
>>> dot?  You can always disambiguate with full qualification if you have
>>> to.
>> Making every unqualified reference context-dependent would be
>> impractical. `foo.bar(bas)` would become an exponential search to find
>> a contextual type containing a `foo` which has a `bar` member that can
>> accept an type containing a `bas` member.
> I don't think I'm talking about making every unqualified reference
> context-dependent.  When I have a context that demands an instance of a
> particular enum type, I think it's reasonable to look up the names in
> the enum without qualification, and I strongly question the value of
> leading-dot syntax for general static member lookup.

Therein lies the rub—*any* context an unqualified name can appear in could potentially demand an instance of a particular enum type, until the type checker rules that out. Limiting the behavior to enums doesn't simplify the implementation.

> I would normally
> never think of using it that way—because, guess what? I think of
> leading-dot as a notation for enums like everybody else does—and I don't
> think there are any important idioms it supports, that couldn't be
> equally well handled by something like `Self.x` if we were allowed to
> use it.

Enums are only the most common place where static members of Self type appear in numbers, but option sets also do. While this may be a matter of taste, being able to refer to `.max`, `.infinity`, `.nan`, or other abstract constants of numeric types in a concrete-type-agnostic way also seems like a win to me.


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