Re: On quitting an iterator prematurely

[louboutn@ccvax.ucd.ie (Sylvain Louboutin, University College Dublin, Ireland)]

In article <1151@enea.se>, sommar@enea.se (Erland Sommarskog) writes:
> Norman H. Cohen (NCOHEN@IBM.COM) writes:
>>Andre' Alguero asks about how to write a procedure to iterate over
>>a set of discrete elements, specifying the actions to be performed
>>at each iteration at the point at which the procedure is invoked.
[...]
> Some years ago I wrote a binary tree package with a similar concept.
> When I was to use the package I discovered one thing I had missed.
> There was no way to interrupt the iteration. One way is of course
> to rewrite the package so that Process_one_element should take a
> second parameter
>    Continue : OUT boolean;
> The problem with this is that if the user in 90% of the time wants to
> iterate through the entire set, this second parameter just clutters
> up his code, and he may even forget to initiate it with interesting
> results, unless the compiler catches the error.

[Erland then proposes to use an exception to interrupt the iteration]

Why don't you write the procedure like this: (note that the 'continue' 
paramter mode is IN OUT instead o OUT)

      generic
         with procedure Process_One_Element (Element: in Element_Type;
                                             Continue : IN OUT boolean);
      procedure Process_Each_Element (Set: in Set_Type);


      procedure Process_Each_Element (Set: in Set_Type) is
        Continue: boolean := TRUE;
        -- other declaration...
      begin
        -- some statements
        loop 
           -- some statements
           Process_One_Element (Current_Element, Continue);
           exit when not Continue;
           -- some statements
        end loop;
        -- some statements...
      end Process_Each_Element;

The user (i.e. the programmer of the procedure 'Process_One_Element')
doesn't need to bother about the parameter continue (except that s/he
has to include it in the header of the procedure, but there is no need
to set a value to it in the procedure Process_Each_Element) if s/he
wants to iterate through all the set... 
-- 
-- Sylvain R.Y. Louboutin                         fax: (+353-1) 69-72-62
-- Dept of Computer Science, University College, Dublin 4, -- Ireland --
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