[swift-evolution] [Pitch] Add the DefaultConstructible protocol to the standard library

[Tony Allevato]

On Mon, Dec 26, 2016 at 8:31 PM Daniel Leping via swift-evolution <
swift-evolution at swift.org> wrote:

> Braeden, a good point as for inheritance. Totally agree here.
>
> Though the generic factory problem remains. Maybe it could be solved
> differently? Any ideas?
>

As a matter of fact, I've used a different approach in some of my own
projects that has ended up working out well.



> The only thing that pops up in mind right now is to have some "compiler
> magic" that deals with the constraints. Maybe a concrete class can fall
> into the category (be DefaultConstructable).
>
> Anyways, my point is that compile time constraints for a type that can be
> created with a default constructor are important for certain patterns. I'm
> not saying the protocol is the right or the only way, but I want to find a
> solution.
>
> On Tue, 27 Dec 2016 at 5:22 Braeden Profile via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> I’m gonna do my best to explain my thoughts on this, as I just spent an
> hour reading the whole thread…………
>
> I’m -1 on adding the protocol DefaultConstructible to the standard library
> (especially under that name).  It doesn’t explain its semantics
> effectively.  I agree that a protocol should have definite semantics that
> are hopefully explained by the name.  This protocol fails that test—a
> default instance of value types is completely context-specific, and default
> class instances are just iffy to me.
>
> I’m firmly in the camp that you could create a protocol like this for your
> own project and apply it to the types you think semantically fit the
> purpose…
> protocol ZeroConstructible { init() }
> extension Int: ZeroConstructible {  }
> …but I wouldn’t do this myself, as there are too many use-cases with too
> many definitions of “default”.  What if I wanted Int to conform to
> multiple?  It only can have one init().  I’d do something like this…
> protocol ZeroConstructible { static func constructZero() }
> protocol UnsafeConstructible { static func constructUnsafe() }
> protocol FactoryConstructible { static func constructDefault() } // I’ve
> never needed to use a factory, myself...
> …and create those new functions when I conform my types to it.  It’s
> cumbersome, but correct.  As of yet, I’ve never needed to do such a thing,
> and nearly all the use-cases brought up in the thread can be solved with
> something of the like.
>
> Every “default" is context-dependant.
>
>
> Addressing other parts of the thread:
>
>
>    - I read a new name suggested for the protocol:  “Identity”.
>    Unfortunately, I associate that with the proposed protocol HasIdentity {
>    func === }, not a mathematical identity.
>    - DefaultConstructible could never be a normal protocol that magically
>    gets applied where init() exists.  protocol required inits are just
>    that—`required`.  If a superclass conforms to DefaultConstructible, every
>    subclass must, too!  This would give most every class tree the infinite
>    chain of `init()` that NSObject suffers from.
>    - AnyObject was used to justify compiler magic that could be applied
>    for DefaultConstructible.  I disagree that this is appropriate, as
>    AnyObject most certainly implies semantics.  Every AnyObject is a class,
>    with reference semantics, unsafe-weak-strong references, and more.  I could
>    not see definite semantics evolve for DefaultConstructible throughout the
>    whole discussion.
>
>
> That’s my two cents.  Granted, no one would be hurt by its addition except
> those who try to understand this protocol, but I want to avoid that chaos.
>
>
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