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Comment by mitchellh | Hacker News Reader

For a good example of this sort of pattern in the real world, take a look at the Zig compiler source code. I'm sure others might do it but Zig definitely does. I have a now very outdated series on some of the Zig internals: https://mitchellh.com/zig/parser And Andrew's old DoD talk is very good and relevant to this: https://vimeo.com/649009599

More generally, I believe its fair to call this a form of handle-based designs: https://en.wikipedia.org/wiki/Handle_(computing) Which are EXTREMELY useful for a variety of reasons and imo woefully underused above the lowest system level.

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My hypothesis is that handles are underused because programming languages make it very easy to dereference a pointer (you just need the pointer) whereas "dereferencing" a handle requires also having the lookup table in hand at the same time, and that little bit of extra friction is too much for most people. It's not that pointers don't require extra machinery to be dereferenced, it's just that that machinery (virtual memory) is managed by the operating system, and so it's invisible in the language.

My current research is about how to make handles just as convenient to use as pointers are, via a form of context: like a souped-up version of context in Odin or Jai if one is familiar with those, or like a souped-up version of coeffects if one has a more academic background.

Great summary and I think your argument is sound.

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.

> 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.

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