[swift-evolution] [swift-evolution-announce] [Review] SE-0023 API Design Guidelines

Dave Abrahams dabrahams at apple.com
Sun Jan 24 21:17:31 CST 2016

on Sun Jan 24 2016, David Owens II <david-AT-owensd.io> wrote:

>>> I guess my point is this: codifying a convention seems pre-mature as
>>> that convention doesn't bring the safety goals of the language into a
>>> place that's verifiable. All of the other guidelines are simply about
>>> clarity of use, this convention has a far reaching impact.
>> Sorry, could you clarify what you mean by "bring the safety goals of the
>> language into a place that's verifiable" and clarify why having a "far
>> reaching impact" would somehow conflict with being "about clarity of use?"
>> It seems to me that this convention is about how to express whether a
>> method is going to mutate so it's clear at the use-site.  What am I
>> missing?
> The problem is it's unclear to me whether you mean mutate in the true
> sense of the word or only applied to a struct with a function
> annotated with the mutating keyword.
> The naming convention provides no safety when dealing with non-struct
> types as we cannot enforce that a method on a class does not mutate
> it's internal members.
> That's the clarity I'm looking for.

I think I understand what you're going for.  The word "mutating" sounds
like it might literally mean what is expressed by the corresponding
keyword.  I think rephrasing in terms of side-effects, as Erica has
suggested, is one possible fix.  A more limited approach to your
confusion would be to rephrase in terms of "methods that mutate the

> Given this API set:
> protocol InPlaceable {
>     mutating func doInPlace()
> }
> public struct Foo: InPlaceable {
>     mutating func doInPlace() {}
> }
> public class Bar: InPlaceable {
>     func doInPlace() {}
> }
> var lie: InPlaceable = Bar()
> lie.doInPlace()
> let lie2 = Bar()
> lie2.doInPlace()
> The convention will tell us a lie unless we are extremely
> careful. It's this lie that concerns me. We cannot guarantee that the
> "doInPlace" truly matches the definition we are seeking.

Wow, you've completely lost me.  I don't see anything that looks like a
lie, here.  What do you mean?


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