I think that it's a generic programming problem: pointers are easier because the type of the pointee is easy to get (a deref) and also its location (memory) but with index-based handles into containers you can no longer say that given a handle `H` (type H = u32) I can use it to get a type `T` and not only that, you've also introduced the notion of "where", that even if for each type `T` there exists a unique handle type `H` you don't know into which container instance does that handle belong. What you need is a unique handle type per container instance. So "Handle of Pool<T>" != "Handle of Pool<T>" unless the Pool is bound to the same variable.
As far as I know no language allows expressing that kind of thing.
But from what I understand (being a nonexpert on Scala), this scheme actually causes a lot of problems. I think I've even heard that it adds more undecidability to the type system? So I'm exploring ways of managing context that don't depend on inferring backward from the type.
What you need is a unique handle type per container instance.
You can do this with path-dependent types in Scala, or more verbosely with modules in OCaml. The hard part is keeping the container name in scope wherever these handle types are used: many type definitions will need to reference the container handle types. I'm currently trying to structure code this way in my pet compiler written in OCaml.
Comments
I think that it's a generic programming problem: pointers are easier because the type of the pointee is easy to get (a deref) and also its location (memory) but with index-based handles into containers you can no longer say that given a handle `H` (type H = u32) I can use it to get a type `T` and not only that, you've also introduced the notion of "where", that even if for each type `T` there exists a unique handle type `H` you don't know into which container instance does that handle belong. What you need is a unique handle type per container instance. So "Handle of Pool<T>" != "Handle of Pool<T>" unless the Pool is bound to the same variable.
As far as I know no language allows expressing that kind of thing.
I think actually Scala does exactly this style of inferring the container instance from its type: https://docs.scala-lang.org/scala3/book/ca-context-parameter...
But from what I understand (being a nonexpert on Scala), this scheme actually causes a lot of problems. I think I've even heard that it adds more undecidability to the type system? So I'm exploring ways of managing context that don't depend on inferring backward from the type.
You can do this with path-dependent types in Scala, or more verbosely with modules in OCaml. The hard part is keeping the container name in scope wherever these handle types are used: many type definitions will need to reference the container handle types. I'm currently trying to structure code this way in my pet compiler written in OCaml.