Can we have multiple instances of the same Annotation associated to a certain Node?

[ftomassetti]

Let's suppose that I have an Annotation called DocumentationComment. Can I have multiple instances of this Annotation associated to the same Node? Is this true for all the Annotation or should this be something we define for each Annotation?

[dslmeinte]

I think there's not much to be gained by having this constraint. It's a constraint that M2-authors can put in themselves, if they like.

[ftomassetti]

Ok, so in general we should allow for multiple instances of the same Annotation to be associated with a certain Node.

[ftomassetti]

It's a constraint that M2-authors can put in themselves, if they like.

Does this mean that we should have a flag multiple added to the Annotation declaration?
This would allow the M2-authors to specify this constraint

[dslmeinte]

I would leave it unspecified (in the M2), and just allow multiple instances of the same Annotation being applied to a particular Node (in the M1). If constraints are necessary these should be specified next to/in conjunction with the M2 (in a nice constraint language which we need to have anyway).

[enikao]

I think we decided on this in #13: An Annotation has a multiple flag, so we might have multiple instances of the same Annotation on the same Node.

[dslmeinte]

I'm reluctant to say anything more about this, but: Is it even necessary to have a multiple flag? It seems like it's not our business to restrict this: if it makes sense for someone to restrict an Annotation to be used (at most/exactly) once, then that'd be a constraint.

[enikao]

In the end it is only a constraint -- we can still have an (invalid) model that has two instances of the same annotation (with multiple = false) on one node.

I don't recall the discussion around this exactly. One argument was the influence on APIs (getDocu(): Docu vs. getDocu(): sequence<Docu>).

Shall we raise this again next week?

[dslmeinte]

When to call a model "valid" is also still very much up for debate, so we might want to tackle that one first ;)

[enikao]

Latest round of discussion on 2023-10-27

In general: We don't have enough experience to make an informed decision.

Pro (having multiple flag):

• Language designer can ignore the flag (i.e. never set it to false), that effectively works as if the flag would not be there
• Code generators can create nicer APIs (getDocu(): Docu vs. getDocu(): sequence<Docu>).

Con:

• It's easier to add this flag in a later LionWeb version than remove it.

[enikao]

We've decided some time ago not to have Annotation.multiple flag.
This means we don't limit how many instances of the same annotation concept can be attached to one node.