[mercury-users] Abstraction barrier

[Michael Day]

> > Perhaps this way, and only provide accessor functions for magic_data that
> > require a magic_token to operate, such that even if the function returns
> > magic_data nothing can be done with it later?

I thought of another possibility:

:- pred foo(magic_data, T, T).
:- mode foo(ui, in, out) is det.

If magic_data must stay unique, then it can only be manipulated with
accessor functions that preserve its uniqueness and cannot be returned or
aliased. Sounds about right, but I expect the limitations of the 
compiler's support for ui modes will be its undoing.

Michael

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