Let objects be declared as abstract even if they contain no members

[heinezen]

According to the specification, nyan objects "remain abstract until all members have values" ("A nyan::Object is "abstract" iff it contains at least one undefined member"). Some of the openage API objects would require objects to be abstract that contain no members. These objects are often used as generalization objects that should be inherited from, but not be used on their own.

Examples : GameEntityType, DiplomaticStance, TradeRoute, DropoffType

Proposed solution: Use the same idea as in #58 and introduce parameters for sets. In this case the definition could look like this: set(typename, only_children=True)

For single references, use a new child container type: child(typename)

[TheJJ]

If the generalization objects don't add any member, why are they needed then?

[heinezen]

Mostly, modders are expected to inherit from the generalization object for a specialized case and not use the generalization object itself. For example, in AoE2 we derive the stances Enemy, Neutral, Ally from DiplomaticStance which is used in DiplomaticAbility:

DiplomaticAbility(Ability):
    diplomatic_stances : set(DiplomaticStance)

It should only be possible to add Enemy, Neutral, Ally and not engine.DiplomaticStance.

[TheJJ]

And DiplomaticStance doesn't add any new (undefined) members? If it does, I think it would be better to allow any other child object there, e.g. if a mod adds another stance like TradePartner which is still an enemy but you can trade with. If it doesn't, what there's surely a common parent object that can be used instead?

[heinezen]

We agreed to introduce a new only_children parameter for sets, similar to #58. In the set declaration this parameter is optional and can be either true or false. See comment below

Example:

DiplomaticAbility(Ability):
    stances : set(DiplomaticStance, only_children=True)

[heinezen]

Turns out this only solves the problem for sets. We have situations in the API where a member has the nyan::Object type, but only its children should be referenced.

Example:

FlatAttributeChange(ContinuousEffect):
    type              : AttributeChangeType
    min_change_rate   : optional(AttributeRate)
    max_change_rate   : optional(AttributeRate)
    change_rate       : AttributeRate
    ignore_protection : set(ProtectingAttribute)

The type member should only be allowed to reference children of AttributeChangeType.

[TheJJ]

Possible solution: Another container type like childs, which behaves similar to optional, but enforces that only child objects are assigned:

type : childs(AttributeChangeType)

[TheJJ]

I've created #65 to track this feature.