Re: Uninitialized "out" parameters

[dewar@cs.nyu.edu (Robert Dewar)]

Richard said

  True, Ada is so designed that sound and complete compile-time detection
  of using uninitialised variables is impossible.

  But some day Ada will have a successor.  And I can see no reason why that
  successor should not do a better job than Ada in this respect.

I doubt it, you certainly do not suggest what that better job might be. The
rest of your note talks about checking that is not and cannot be completely
statically reliable. As for practical tools, any good Ada compiler should
indeed warn of many common cases, and tools analogous to lclint are
perfectly possible with Ada, and/or should be built into the Ada compiler.

Dynamic bounds of arrays do not help in compile time legality checking, so
I don't see what relevance they have. And yes, container types are certainly
appropriate (after all see Bounded_String, which is exactly what you are
talking about), but again, this has nothing to do with compile time legality
requirements.

Let's try to focus a specific example, the one I gave before, and you tell
me how your improved approach will work at compile time to detect as
illegalities all references to uninitialized elements.

I have an array of 2 gigabytes in an allocate-on-demand environment. I use
this as a sparse hash table, but it is critical that only pages that are
actually used get referenced, so it is out of the question to initialize
the table.

How do I make sure that references to this array correspond to previously
set elements? Even doing this at runtime is awkward, but I see no way of
designing a type system or any other semantic framework to solve this at
compile time as a legality issue.

It is not good enough to just say "we should do better", you have to say
*exactly* how you can do better, or your position is unconvincing. After
all, this is an old problem that has not been solved for 38 years now
(I am counting from Algol-58, it is a bit longer if you count from the
first Fortran), so if you have a solution, it would be nice to present it!