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Wojtek, the idea is interesting but seems to require a lot of run-time overhead:
When a request to unload a module comes from somewhere, how the run-time would know all the procedure variables that point to the module's procedures? A list has to be maintained with back references. This contradicts the Oberon philosophy: complex things should be made manifest to the programmer.
Procedure variables are a low-level tool, and should perhaps be kept that way -- and you were perfectly right with your suggestion about SYSTEM.
(Sorry, I did not acknowledge your priority with the idea -- as I did not with Stan's proposal to pre-package standard function objects; plunged into discussion without studying the accumulated mail...)
Function types provide a convenient mechanism, additional "tagged procedure variables" may be an overkill. I don't know -- it's a matter of balance and compromise.
Cheers
Fyodor
> On Fri, 19 Nov 2004, Fyodor Tkachov wrote:
>
> > A procedure variable is just an address of the entry point
> > into the procedure: there is no auxiliary information
> > to help the run-time system.
>
> Is this a statement of current implementation or a statement of necessity?
> Does it have to be so?
>
> This may be similar to safe versus unsafe arrays.
>
> In Fortran and C, an array is just an address of the 1st element and there
> is no auxillary info such as index bounds. As a consequence, arrays in
> Fortran or C are unsafe and lots of fun can happen using them.
> IN CP arrays have been made safe and the fun is long gone, thank goodness.
>
> How about the procedure variables? Can they be made similarily safe?
>
> How about differentiating between safe high-level procedure variables
> and low-level [untagged] procedure variables that you would need to
> interface with Windows and/or Linux?
> The latter would only be available under SYSTEM,
> same as [untagged] records and [untagged] arrays.
> Similar problem, similar solution.
>
> Note, I am not a compiler writer, so I am only posing this is a question.
> I do not know whether safe procedure variables can be implemented.
> I also do not know whether or not the current BB implementation is safe.
>
> W.
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Received on Sat Nov 20 2004 - 06:56:57 UTC