Re: Strategies with SPARK which does not support exceptions

[Phil Thornley <phil.jpthornley@gmail.com>]

On 17 June, 16:33, Yannick Duchêne (Hibou57)
<yannick_duch...@yahoo.fr> wrote:
> Hello,
>
> (This topic will probably not be the most exiting topic to some people).
>
> When I use SPARK, or even when I don't use SPARK while I still have SPARK  
> design style in mind (even with Pascal which I still use), I have like any  
> one else, to forget about exceptions.
>
> The only one thing which seems simple and clean enough I could figure, is  
> to use a Boolean variable, named “OK”, and a sequence of “if OK then”  
> statements. That is, execution of one sequence of statements, which set OK  
> to False is something goes wrong, and execute clean-up statements by the  
> way. Then a next sequence of statements wrapped in a “if OK then” which do  
> the same, and so on with next statements groups which are also wrapped in  
> a “if OK then” (as many as needed).
>
> Although this seems clean and maintainable enough to me so far, I was  
> still wondering how other people deal with this (and may be by the way, I  
> may have comment about why my way could be bad in some way).
>
> Note: a similar strategy may also obviously applies to fulfill the “only  
> one single exit point” requirement.
>
> So, let us talk and comment about design strategies in this area (if  
> someones wish to).

(Perhaps not an entirely serious suggestion, but...)
The only way of interrupting a code sequence in SPARK is the exit
statement for a loop, so one possibility might be to create a 'single
pass' loop with an unconditional exit at the end.

Unfortunately such an unconditional exit is illegal, so you have to
use 'exit when True;'

Also there may be flow errors for some or all of the 'exit when'
statements (if the OK values only depend on the program inputs) - but
here we can put an accept statement - which will usefully document
that the loop is a not a real loop.

--# accept F, 40, "This is a single pass loop.";
OK := True;
loop
   Some complicated code that might set OK;
   exit when not OK;
   Some more complicated code that might set OK;
   exit when not OK;
   Some more code;
   exit when True;
end loop;

:-)

Cheers,

Phil