[swift-evolution] stored properties in extensions (was: associated objects)

[Jay Abbott]

Charlie,

What you suggest defines how you would use it from your code, not how it
would be implemented in the language. If you look at my AO implementation
it does what you say:
https://github.com/j-h-a/AssociatedObjects/blob/develop/AssociatedObjects/AssociatedObjects.swift
i.e. has a protocol called 'Associable' and you opt classes into it to get
the behaviour. This works and is usable, but the implementation leaves a
lot to be desired (it's not optimal and while the interface is clean the
implementation is not). Anyway - I was trying to steer the conversation
AWAY from AOs towards stored properties in extensions, since Robert Widmann
helped me to understand that AO was just a *means*, whereas stored
properties in extensions is the *end*.

In fact we don't need a solution to the problem of "how to define/use
stored properties in extensions" because the existing syntax for extensions
is perfectly fine. Currently you get an error if you try to define a stored
property in an extension, so no new syntax is needed, we just remove that
error and make it work.

Of course a runtime-check may be needed if there is doubt about whether a
dynamically linked module supported this feature - so this might invalidate
what I just said above, or it might still be possible if the runtime does
the check automatically when an extension is linked and puts a different
implementation in place for older modules.

I'm just airing some thoughts at the moment to see what people think and
try to get some technical feedback on viability. So it's not all fully
thought through :D


On Sun, 9 Oct 2016 at 20:54 Charlie Monroe <charlie at charliemonroe.net>
wrote:

There is a 4th way.

Introduce an internal protocol Associatable, which would tell the compiler
to add an additional (hidden) field to the object which would include the
"dictionary" of key -> value associated values. (It would be off-limits to
extensions, of course).

This way:

- it won't be a single dictionary containing all the associated values
- classes can opt-in to this
- the dictionary will be per-instance

This is a midway between the current implementation of ObjC associated
objects and of what someone has suggested to have an extra space for each
object for the AO...


On Oct 9, 2016, at 9:47 PM, Jay Abbott via swift-evolution <
swift-evolution at swift.org> wrote:

I have been thinking further on this and in addition to my previous two
thoughts about implementation, I have had another idea...

3. If there is a bit spare in the object header somewhere (that is
currently always zero), this could be used to signify the presence of an
additional struct that immediately follows after the existing object data.
I *think* that this method would allow binary compatibility with older
modules. Instances that have this bit set would allow stored properties in
extensions. The struct at the end would have one member, a pointer to a
table of additional objects/values, stored properties defined in extensions
could be stored in here, using a hash derived from the
module/protocol/extension/property name (or something better if it exists).

The struct could be very simple as described above or more complex, with
additional features, for example a list of deinit hooks, dynamically added
methods, etc. The struct itself may also be dynamic in size/layout if such
complexity is warranted by size or extensibility concerns. Perhaps it
should start with some flags and its size (size would be fixed and only
increase with revisions so this would double as a 'version' number).

If viable - this would be a much better way to implement this feature than
my previous two ideas. It doesn't require global lookups or additional
levels of indirection beyond accessing the dynamic data/feature itself.


On Mon, 3 Oct 2016 at 04:13 Jay Abbott <jay at abbott.me.uk> wrote:

Are stored properties in extensions already being discussed elsewhere? Is
this one of those deferred-but-not-indexed-anywhere subjects?

I wonder how stored properties could potentially be implemented, I can only
think of two ways:

1. An extra pointer per instance (with resulting ABI compatability
implications) to hold a collection of the stored items.

2. A global lookup for any instance where stored properties have been set.

I'm not a language implementation expert, or familiar with the swift
implementation, so there may be other/better ways - I'd like to know if
there are?

If not, and option 2 was employed, a little foresight might enable the
mechanism to be overloaded in the future for other dynamic features too. A
bit flag (I'm hoping there's a spare bit in an existing flags field
somewhere?) could indicate whether any feature had caused the object to be
added to this lookup and deinit could check this bit and make sure the
object is removed, thus any stored properties are nilled. The lookup value
could be a struct with one member (extensionStoredProperties), and
additional members can be added in future for new features.

I get the impression from the associated objects discussion that perhaps
there are much better, more optimal, more ingenious, more unknown-by-me
ways of doing such things, so apologies if this whole idea is way-off the
mark :D

Jay

P.S. Note that stored properties in extensions could enable developers to
implement their own dynamic features in Swift.. so such desires could be
satisfied in the short term until they could be done properly in the
language.

On Sat, 1 Oct 2016 at 00:49 Chris Lattner via swift-evolution <
swift-evolution at swift.org> wrote:

On Sep 30, 2016, at 2:51 PM, Ted F.A. van Gaalen via swift-evolution <
swift-evolution at swift.org> wrote:
> Is it possible to have best of (these completely different) both worlds?

Yes, of course it is.  Your email spends a lot of words trying to form a
false dichotomy.  Swift can definitely have both awesome dynamic features
while still having performance, predictability and safety.

> Would it be possible in Swift to have facilities to generate objects
> dynamically at runtime? and, if desirable, how can such be implemented?

Here’s an extant implementation that you can use today:
https://github.com/Zewo/Reflection

I’m sure it isn’t ideal, but it proves that it can be done.  When we have
bandwidth to reevaluate this area from first principles, I’m sure we can
make improvements on it.

I will grant you that Smalltalk is a beautiful language in its simplicity,
but for that simplicity it makes many tradeoffs that we’re not willing to
make.  We are willing to make the internal implementation of Swift complex
if that means that we get a beautiful model for programmers - one that
preserves the virtues of safety-by-default, predictability, performance,
and joy-to-develop-in.

The meme of “Swift can never (or will never) support dynamic features” is
tired, and also wildly inaccurate.

-Chris

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