Re: [BLACKBOX] IN Parameters - Side effects

From: [at]} <CFB>
Date: Thu, 31 Jan 2008 23:33:27 +1030

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Content-type: text/plain

> -----Original Message-----
> From: BlackBox [mailto:BLACKBOX{([at]})nowhere.xy
> Campbell, Robert (SELEX GALILEO, UK)
> Sent: Thursday, 31 January 2008 7:04 PM
> To: BLACKBOX{([at]})nowhere.xy
> Subject: Re: [BLACKBOX] IN Parameters - Making the design more regular
>
> MODULE Horrible;
...
...

Gee, thanks Robert - trust you to put a spanner in the works! Back to the
drawing board ... (or maybe time to throw in the towel?).

You have raised some valid points. I tried your example using records as
parameters instead of INTEGERs with the current BlackBox 1.6 rc5 compiler.
Contrary to what I have been claiming, it demonstrates that the value of an
IN parameter *can* be changed inside the procedure. Does that mean it is not
'read-only' inside the procedure? If so the compiler is not conforming to
the language reference is it?

...
...
PROCEDURE P (IN x : SomeRecord);
  BEGIN
    Q (x)
(* x can now differ from what it was when initially passed to this
procedure *)
  END P;
...
...

Studying how these design issues were tackled in Ada might be instructive. A
quick taster is here:

http://www.cs.arizona.edu/~collberg/Teaching/520/2005/Html/Html-31/index.htm
l

-----------------------------------------------------
* Ada has three modes:

in-parameters pass information from the caller to the callee. The callee
cannot write to them.
out-parameters pass information to the callee from the caller. The callee
can read and write them. They start out being uninitialized.
in out-parameters pass information from the caller to the callee and back.


* For scalars and pointers, all modes should be implemented by copying
values. Thus

in-parameters are passed-by-value.
out-parameters are passed-by-result (the formal is copied into the actual
when the procedure returns).
in out-parameters are passed-by-value/result (On entry, the actual is copied
into the formal. On return, the formal is copied back into the actual).


* For constructed types (records, arrays) an implementation is allowed to
pass either values or addresses.

If an in out parameter is passed by address an assignment to the formal
changes the actual immediately.
If an in out parameter is passed by value an assignment to the formal will
not affect the actual until the procedure returns (and the formal is copied
back into the actual).
Ada disallows programs that can tell which implementation a compiler uses.

(That last one is particularly intriguing!)

Regards,
Chris

Chris Burrows
CFB Software
http://www.cfbsoftware.com/cp

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Received on Thu Jan 31 2008 - 14:03:27 UTC