[swift-evolution] [swift-evolution-announce] [Review #3] SE-0117: Allow distinguishing between public access and public overridability

[Dave Abrahams]

on Thu Jul 21 2016, John McCall <swift-evolution at swift.org> wrote:

>> On Jul 21, 2016, at 10:47 AM, Dave Abrahams via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> on Thu Jul 21 2016, Matthew Johnson
>> <swift-evolution at swift.org
>> <mailto:swift-evolution at swift.org>>
>
>> wrote:
>> 
>>>> 	* What is your evaluation of the proposal?
>>> 
>>> +1 to the first design.  I think this is a great solution that
>>> balances the many considerations that have been raised on all sides of
>>> this issue.  `open` is 2 characters shorter than `public` so
>>> complaints about boilerplate are no longer valid.  `internal` is the
>>> “default” - neither `public` nor `open` are privileged as a “default”
>>> for publishing API outside of a module.
>>> 
>>> I am interested in language enhancements such as exhaustive pattern
>>> matching on classes and protocols which rely on knowledge of the full
>>> class hierarchy.  Such enhancements will be far more useful if the
>>> language supports non-open, non-final classes.
>>> 
>>> There are design techniques that would require additional boilerplate
>>> if we cannot have non-open, non-final classes.
>>> 
>>> Most importantly, requiring library authors to choose `public` or
>>> `open` provides important documentation value.  Users of the library
>>> will know whether the author intends to support subclasses or not.
>> 
>> I think this reasoning is flawed.
>> 
>> If you make any methods overridable outside your module (“open”),
>> obviously you mean to allow subclassing outside the module.  If you have
>> no open methods, there's absolutely nothing you need to do to “support
>> subclasses,” and from a design point-of-view, there's no reason to
>> restrict people from subclassing.
>
> Superclasses can have superclasses, which can themselves have open methods.
> This is, in fact, quite common for Cocoa programmers.

Okay, good point.

Making a class non-subclassable seems like a pretty indirect way to say
“not even inherited methods should be overridden outside the defining
module,” though.  

Wouldn't we prefer to have a way to hide the inheritance relationship
(and thus prevent overriding of inherited methods) outside the module?
Or are these independently useful axes?

>
>
> John.
>
>> 
>> The only reasons I can see for allowing people to prevent non-final
>> classes from being subclassed outside the module in which they are
>> defined are:
>> 
>> 1. It feels like a nice point of control to have.
>> 
>> 2. Marginal performance gains as noted in the proposal
>> 
>> I personally don't find these to be convincing.  #1 in particular seems
>> like a poor way to make language design decisions.  If we decide to add
>> this point of control, I'll justify it to myself in terms of #2.
>> 
>> P.S., I can live with either alternative; it's just important to me that
>> we understand the situation clearly when evaluating them.
>> 
>> HTH,
>> 
>> -- 
>> Dave
>> 
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-- 
Dave