[swift-users] Are value semantics really appropriate in a diagramming app?

Rick Mann rmann at latencyzero.com
Mon Aug 1 21:24:17 CDT 2016

> On Aug 1, 2016, at 19:18 , Jack Lawrence <jackl at apple.com> wrote:
> Jens: Why? There are significant benefits to value semantics for this type of problem, for the reasons laid out in the WWDC videos. It would be helpful to know why you disagree in this case—maybe there are solutions to the issues you’re thinking of.
> Rick: I’d think that value semantics would be the right choice here. When you do a mutation, you would copy the state of the entire diagram. It should be efficient via COW, but if not you can implement you own more fine-grained COW types with isUniquelyReferenced(). This would allow you to easily support things like undo.

The more I consider this, the more I think value semantics won't work for me. I think, to take advantage of the easy undo feature, my entire model *must* be implemented with value semantics.

But my model has implicit reference semantics: multiple instances of a part can share a PartDefinition; it is intended that if the PartDefinition changes, all the referencing instances get the change. There are additional situations in which reference semantics are at play, as well: a PartDefinition can have one or more labels, but each instance can specify the relative location of the label for that instance. So, there is struct that contains a position and a reference to the label in the PartDefinition. But if the contents of the label changes, all the instances need to see that change.

I don't think I get to take advantage of value semantics, and it makes me wonder if any typical, non-trivial model's object graph really has no reference semantics.

> Jack
>> On Aug 1, 2016, at 4:32 PM, Jens Alfke via swift-users <swift-users at swift.org> wrote:
>>> On Aug 1, 2016, at 1:19 AM, Rick Mann via swift-users <swift-users at swift.org> wrote:
>>> It seems like reference semantics are more appropriate here.
>> Yes, they are. (Just because structs exist doesn’t mean you have to use them everywhere.)
>> —Jens
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Rick Mann
rmann at latencyzero.com

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