Interesting idea. I tried this in a playground to see what the calling
syntax would be like:
let delegate = Optional(Delegate())
let default = 0
let value: Int = (try? delegate?.returnFive() ?? default) ?? default
Not quite as elegant at the call site, but it works. Sadly the rethrowing
overload of ?? complicates things, I'm not sure of a way to work around
I do like that types could potentially implement the function and do
something, then throw an error rather than returning a value, which would
allow them opt into using the default value. It would complicate matters if
the conforming types throw multiple errors for whatever reason, though.
From James F
Sorry if this has been discussed, but have you considered dropping
optional entirely, making it throw, and a default implementation that
throws a selector not found exception?
This is approximately what you would expect using it in objc. I don't
think it has the complexity discussed in the proposals alternatives for
other call site issues.
If it throws you can call with "try?" to get similar functionality in most
This assumes that respondsToSelector doesn't pick up the Swift default
I had a look at the links in the proposal as you suggested, and I see a
lot of people pointing to protocol extensions as a solution (and
counter-arguments of the inability to optimise code with this method, which
are left unresolved).
To make use of the protocol extensions solution, one would have to define
the protocol, add a protocol extension which implements every function,
then add an empty type which allows access to these implementations.
For a recommended alternative, this seems a lot of work. In a world where
Swift didn't have Objective C compatibility, and this empty-type workaround
to access defaults was the best option available, I'd be inclined to
support a proposal to add optional method requirements. It has the added
optimisation and easy delegate-swapping relative to closure properties, as
you mentioned, and feels less hacky than the closure-function switching
suggested in the proposal.
The protocol extension + default type would provide a direct alternative,
but it leads me to wonder what exactly we're trying to avoid by
discouraging optional methods. The potential for unexpected optimisation,
which seems to be the primary issue, is unsolved, since the type can check
to see if the delegate is its own, default type, and proceed to ignore the
method regardless. In exchange, we make things much harder for types simply
wishing to have a default value when there is no registered delegate.
So what aspect of optional protocol requirements are we actually trying
to discourage, which isn't present in protocol extensions?
PS. If we're concerned about overlap with protocol extensions: it seems a
bit like eliminating functions from the language because they overlap with
the more general concept of closures. It's a fine idea, but it seems more
reasonable to find a solution that handles both cases conveniently before
we start eliminating one of them.
From James F
> On 26 Apr 2016, at 22:56, Douglas Gregor <firstname.lastname@example.org> wrote:
>> On Apr 26, 2016, at 3:33 AM, James Froggatt <email@example.com> >> wrote:
>> Fair enough. Upon reflection, I think my real issue is somewhat
different to what I suggested previously.
>> I wasn't intending to suggest such a thing would be practical, just
that it would be a decent alternative to optional protocol requirements.
The alternative given in the proposal seems to be more of a way to remove
optional protocol requirements on the surface, while actually helping to
make them a native feature, if you see what I mean. It's not a realistic
alternative - it's a worse syntax for the exact same thing, which also
comes with awful side-effects for Swift as a whole. No-one would ever
seriously consider this as an alternative, yet it's listed as under the
heading ‘Alternatives Considered’.
> If you follow the swift-evolution discussion links in the proposal,
you’ll note that a number of people have proposed exactly what is listed in
“Alternatives Considered”. The only truly wacky idea in there is my
caller-side default implementations idea, which I covered simply because it
was my last stab at eliminating optional requirements before giving up and
sequestering them permanently behind “@objc”.
>> You say the arguments given against optional closure properties are
strong, but I don't they would be nearly as relevant to the case I
suggested. By making them properties of the table view, the tableView
parameter would be eliminated, meaning the property names could be unique.
>> var numberOfRows: (inSection: Int) -> Int
>> var cellForRow:: (at: NSIndexPath) -> UITableViewCell
>> var moveRow: (from: NSIndexPath, to: NSIndexPath)
>> This removes the need to add the mentioned workarounds, since a
function could be assigned to the closure property just as easily as an
inline closure. I feel this is much more worthy of being considered as an
alternative. The idea of these proposals is to document why we do things,
so at least for someone wondering why we require all this @objc syntax
rather than support optional protocol requirements natively, this would
actually present them with a viable alternative which could be applied in
> Doing this implies creating a potentially large number of stored
closure properties, which is not as storage-efficient as storing a single
delegate reference. Moreover, it makes it harder to set up your
customization points: instead of implementing one protocol, you’re writing
assignments into some number of stored closure properties. Imaging trying
to change the delegate to some other delegate temporarily: you would have
to manually store each of the closures into some local structure and
introduce your own, except that you can’t get them all because some new
version of the platform would add new stored closure properties. Finally,
Cocoa just doesn’t work like this, so you would require some massive
re-architecture to get there. I don’t see how this is a better design.
> - Doug
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