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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: stopping a loop iteration without exiting it
Date: Fri, 5 Jan 2018 15:05:13 +0100
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Date: 2018-01-05T15:05:13+01:00
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On 01/05/2018 02:17 AM, Randy Brukardt wrote:
> 
> There are a number of instances of it in the spam filter. When it is
> determined that a particular pattern cannot possibly match, one does a
> "continue" to move on to the next pattern. This is usually in an inner loop
> (so this is acting as a combination exit and continue). The rules for the
> spam filter turn out to be very complex, so the code implementing them also
> end up complex; simplifying the rules leads to allowing spam through or
> blocking "ham" (good mail).

and also (in another post):

> It's not a frequent need, it's just a non-zero need. I find I end up up 
> writing such a goto (very roughly) once a year. But a good number of them 
> aren't strict continues; there's often something at the bottom of the loop 
> after the label.

I don't know of any language that has a "continue" that will apply to any but 
the innermost containing loop, so such languages would not seem to have any 
advantage in this case anyway. I suppose Ada could be the 1st with such a feature.

I don't know of any language with a "continue-like" feature that includes 
executing some code at the end of the continued loop. I can't imagine what such 
a feature might even look like.

But it appears that even if Ada had a "continue" feature, it might not replace 
the goto of many of your real-world examples.

I've never written a spam filter, but I'd imagine that one would be structured 
something like

All_Msgs : loop
    exit All_Msgs when Msg_Source.Is_Empty;

    Msg := Msg_Source.Get;
    Is_Spam := False;

    All_Patterns : for Pattern of Patterns loop
       Is_Spam := Pattern.Check (Msg);

       if Is_Spam then
          Reject (Message => Msg);

          exit All_Patterns;
       end if;
    end loop All_Patterns;

    if not Is_Spam then
       Allow (Message => Msg);
    end if;
end loop All_Msgs;

and moving on to the next pattern would be "return False;" in Check. Reality is 
probably more complex than this.

-- 
Jeff Carter
"Brave Sir Robin ran away."
Monty Python and the Holy Grail
59