DIP69 - Implement scope for escape proof references

[Walter Bright via Digitalmars-d]

On 12/14/2014 5:44 PM, Dicebot wrote:
> I am looking for a tool to prevent escaping references to user-defined entities
> / resources from specific scope. RC case is not interesting at all to me though
> I recognize the potential benefits of tweaking the scope system to help with it.
> But there is a big difference between tweak for one specific use case or
> designing feature around it (so it becomes almost useless in other cases).
>
> I have already provided you an example of code I want to be enforced with scope
> and two major issues with existing proposal that make it lacking. This example
> is akin to litmus test for any scope proposal - if it doesn't allow to express
> such design, that proposal is simply not good enough.
>
> In case you have forgotten, I am reminding about two critical points that are
> necessary to make it fly:
>
> 1) inout analog for scope to be able to deduce borrowship of return values based
> on borrowship of input arguments. Current system is conservative and results in
> template bloat (complicated by the fact that it is storage class thus not
> actually part of a type)

1. All inout actually does is reduce code copy/pasta, it is not critical.
2. This is what scope inference is all about.


> 2) at least optional transitivity to be able to express to protect with scope
> data referenced by slice or owned linked list referenced from root node.

1. that won't work unless scope is a type constructor
2. it can be achieved by using wrappers that only allow by-scope references to 
their data (RC is an example of such a wrapper)


> In your tree example I would have never wanted scope protection of one specific
> node of such tree - but a transitive scope protection of whole tree view
> available through on of node pointers/references. It doesn't matter who and how
> owns the data for borrowship implementation - only thing that matters that _it
> is not me_.

As I explained to Manu, transitive scope makes things like doing a symbol table 
lookup, then putting a pointer to the found symbol in an AST, not workable.