Re: Help Me Please :)

[Tucker Taft <stt@averstar.com>]

Robert A Duff wrote:
> ... 
> (On the other hand, Ada *pretends* that it's OK to raise and handle
> Constraint_Error, but if you read RM-11.6, you'll find that's not quite
> true -- and I doubt if any Ada programmer other than Tucker Taft
> actually *understands* 11.6.  So in practise, 11.6 says, "Do not handle
> predefined exceptions.")

This sounds like a request for a quick tutorial on 11.6 ;-).
Here is my nickel tour:

  RM95 11.6(5) says, effectively, that you can remove dead code,
  even if it might raise an exception due to failing a language-defined check.

  RM95 11.6(6) says that the compiler (or hardware) can do 
  instruction scheduling (i.e. reordering) even 
  if there are language-defined checks among
  the instructions, meaning that if you handle an exception raised
  by the failure of a language-defined check, it might be "imprecise."
  However, it won't be so imprecise as to disrupt some
  separately compiled "abstraction" in the middle of an
  operation, presuming the check failed outside of the abstraction.

Or for the one-cent summary: dead code removal and instruction
reordering can be performed without worrying too much 
about language-defined checks, so long as you don't use
any "undefined" results (i.e., the result of an operation that failed
a language-defined check) and you don't screw up external
abstractions.

> ...
> - Bob

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA