Hi, this language looks very cool!

I am interested in the "No Hidden I/O" section. Can you store, say, the system value (?) to a list? Because I'm afraid this is not enough to properly track side-effects.

Unfortunately I think capabilities/an effect system is required for that (so that any point that actually uses a side effect, taints its whole call tree - the same way async/checked exceptions do, both of which are specific Effects.

Thanks!

Yes, you can store the system value (or indeed any other object capability) in a list.

If a function takes in a List[NodeSystem] parameter, then the function can perform side effects through the methods on NodeSystem.

However, if a function takes in a List[T] where T is a type parameter, then that is not enough to perform side effects, even if that function is later called with a List[NodeSystem] argument. There is no way for the function to access the methods of the concrete type of T.

Hence, you can tell whether or not a function may have side effects from the function signature alone.

Realistically, in a bigger project, you might have a NodeSystem type n layers deep buried.

E.g. you just grabbed a hypothetical canvas dependency, which has a Canvas. Can you tell if it does any form of IO or not?

Only capability type can have fields containing other (concrete) capability types.

If Canvas contains NodeSystem, then Canvas must be a capability type itself.

Interesting. You have a bug in the name of your language. :)