children(type) to allow children only

[TheJJ]

This is a new type that matches to objects that are child of the given type.
This is useful for e.g. describing this case:

DiplomaticAbility(Ability):
    stances : set(children(DiplomaticStance))

The DiplomaticStance is a non-abstract API object, and therefore is valid to be used on its own as a value for stance. As we only want to allow children, the children(DiplomaticStance) matches to that.

This implements preventing "too abstract" objects from being used.

[heinezen]

I would suggest the following syntax

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

We could pass an arbitrary number of additional parameters or flags to complex types this way. It would probably work for nyan object references too:

SomeObject():
    some_member : OtherObject(onlychildren=True)

We don't even have to define children as keyword this way.

[TheJJ]

This was an earlier thought, but for the type checker it's easier to define children(something). We can use the keyword arguments for other things, but this I think can be done better by nesting types.

Your two examples would also need two different implementations - one for the set, one for a regular type.

children is a built-in type, not a keyword. It can be "applied" on non-containers only.

[TheJJ]

Done in #87.