Re: new revision ada

[Brian May <bam@snoopy.apana.org.au>]

>>>>> "tmoran" == tmoran  <tmoran@acm.org> writes:

    >> ... and can pass more information in the
    >> exception instead of just a string).
    tmoran> I don't understand why this is considered a problem.  Why not make:
    tmoran> type data_to_pass is record ...
    tmoran> function enstring(x : data_to_pass) return string;
    tmoran> function destring(s : string) return data_to_pass;

That seems like a very ugly hack.

Isn't the philosophy behind Ada that you specify what you want to
happen, not how it happens? If you have to mangulate exception
information into a string (whatever encoding you choose to use, you
could use XML or you could use something else) in order to pass it to
an exception, I think it has hardly succeeded. I have always thought
of the text string to be a string to log/show the user, not one for
processing internally by the application.

Thats also a lot of overhead just to pass variables from one part of
the program to another part of the same program, too.

If you want to try to recover from an error condition (as opposed to
simply displaying/logging an error and failing) you often need to have
detailed information on what was going on when the error occurred.

eg. ENGINE_FAILED(Number=>1, RPM=>xxxx,
Occurred=>After_Switching_Fuel_Tanks) is a lot more informative then
ENGINE_FAILURE and allows some sort of recovery process (eg. switch
back to the previous fuel tank) instead of trying to guess information
that was already known. (disclaimer: I haven't written such an
application...)

The solution I have heard of for Ada is to use global variables, but
this is not a thread safe solution, you risk one thread overwriting
the exception information of another thread. eg. in the above example,
multiple threads might detect that multiple engines have failed
simultaneously.
-- 
Brian May <bam@snoopy.apana.org.au>