[swift-evolution] Proposal: Universal dynamic dispatch for method calls

Gwendal Roué gwendal.roue at gmail.com
Wed Dec 9 15:28:05 CST 2015

I quite knew about the caveat you’re talking about. This is what had prevented me from shipping this technique in the public API yet. I still need thinking.

And my current thinking is that this case is unlikely to happen (in my particular case, at least). Further, I’d rather document against this usage or even cutting off this feature, rather than adding this extra _f function. Talk about a simple API, when the library users needs a PhD in Swift dispatch subtleties to properly validate a poor struct before storing it in the database. That’s not my ambition when I write a library.

I hope, nevertheless, that I had shown:

1. that protocol default implementation can actually be "overriden", even if it is dangerous. If this danger can not be alleviated, then this is a hole in the language, and this hole may well need to be fixed because the inadequate usage I’ve show, should it reveal actually improper, will be discovered by others developers.

2. that there is a quite valid use case for letting adopting types "override" the default implementation of their protocol. The _f extra method is a lacking workaround.


> Le 9 déc. 2015 à 22:13, Kevin Ballard <kevin at sb.org> a écrit :
> That is nice, but if someone writes a method that's generic over <T: P> then your "override" won't get called. Seems like it's better to structure it like
> protocol P {
>     func f()
> }
> extension P {
>     /// Default implementation for `f`. Calls through to `_f()`.
>     func f() { _f() }
>     /// Helper that provides the base functionality for `f`.
>     func _f() { ... }
> }
> struct S {
>     func f() {
>         ...
>         _f()
>     }
> }
> This way you can write code that's generic over <T: P> (or that takes a P object directly) and it will still call the overrides.
> -Kevin Ballard
> On Wed, Dec 9, 2015, at 11:01 AM, Gwendal Roué wrote:
>>> Le 9 déc. 2015 à 19:52, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>> b) methods defined in protocol extensions by definition can't be overridden already,
>> Methods defined in protocol extension actually can, sort of, be overridden, and this is very useful:
>>     protocol P { }
>>     extension P {
>>         func f() { … }
>>     }
>>     struct S {
>>         func f() {
>>             ...
>>             (self as P).f()
>>>>         }
>>     }
>> I know only one use case for this technique, in the groue/GRDB.swift SQLite wrapper:
>> In this library, a DatabasePersistable protocol provides basic CRUD operations, and a Record class adopts this protocol and "overrides" with the technique above the protocol methods with extra features provided by the class (especially change tracking).
>> The benefits of this architecture are:
>> - You can subclass use the full-featured Record base class, and get CRUD + change tracking for free.
>> - The Record subclasses can override the CRUD methods, and add custom code (validation, for example).
>> - You can have a custom struct adopt DatabasePersistable, and get CRUD for free.
>> - The custom structs that can also "override" the CRUD methods, and add custom code (validation, for example).
>> This is, in my opinion, a very valid use case for this "overriding".
>> Gwendal Roué

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