Re: Strategies with SPARK which does not support exceptions

[Martin <martin.dowie@btopenworld.com>]

On Jun 18, 9:06 am, Phil Thornley <phil.jpthorn...@gmail.com> wrote:
> 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

No there's a fancy 'goto'! :-)

-- Martin