When I started designing this language, I had a lot of fundamental decisions to make. Among those, one of the easiest was whether or not to allow virtual functions in scrml structs.
If I had chosen to include virtual functions; I would have just called these data strctures objects.
If you were paying attention, you might see why.
A struct is a data structure. Plain and simple. But, you see, it is within that simplicity, the power is made plain.
Lets define a unit of data as a value over over a type ( x/T ). Some would claim that it is state over type. Philistines!
State is data that is presently being witnessed.
While this might seem like a trivial distinction. The real differentiation comes from the realization that state is not witnessed in piecemeal. it is accepted as a whole, or thrown out as invalid state.
State is, therefore, immutable by nature.
So then, what of structs? I'll cover that in the next article "State. Why its not data."
Just kidding. I hope this next part is becoming more obvious.
So are struct mutable? ... Depends. But, let the default be: the node-tree structure should be immutable. Hence, a user definable structure. the values should mostly bi mutable, with each node mutation re-validated against it's type (or predicate if run-time validation is reqd)
Simple. And because it's simple, we can analyze it, exhaustively.
Enter enums, engine, and other super-powers that "objects" just can't match .)