Idea: break/continue and scope guards

[Derek Parnell]

On Fri, 06 Jul 2007 11:03:04 -0700, BCS wrote:

> mike wrote:
>> Hi!
>> 
>> Don't know if that makes any sense or if it would be of any use, but I had an idea lately and wanted to share it here, maybe it's some little thing that could be nice in D.
>> 
>> So here we go:
>> 
>> ' foreach (customer; customerList)
>> ' {
>> '     foo(customer);
>> '     scope(failure) foo_rollback(customer);
>> '
>> '     bool ok = check_valid_after_foo(customer);
>> '     if (!ok) continue(failure);
>> '
>> '     commit(customer);
>> ' }
>> 
>> Basically the idea is to trigger the appropriate scope guards when a break or continue occurs. Another thing where this could be handy is in situations where you loop over an array to search for a specific item and do something when it is found, like:
>> 
>> ' foreach (customer; customerList)
>> ' {
>> '     scope(success) result = customer;
>> '     if (customer.ID == wantedID) break(success);
>> ' }
>> 
>> What do you think?
>> 
>> -mike
> 
> [posted in D and D.announce NG, please reply in D ng]
> 
> This is much like an idea I had, but backwards from the way I thought of 
> it. Add more options to scope, not break/continue.
> 
> the second code example would work like this the way I thought of.
> 
> ' foreach (customer; customerList)
> ' {
> '     scope(break) result = customer;
> '     if (customer.ID == wantedID) break;
> ' }
> 
> other scope "types":
> goto            // exit by goto
> goto : label    // exit by goto to a given label
> continue        // duh
> enter           // execute on entrance by any means (goto, switch, etc.)
> out             // used in a loop same as exit but for scope including
>                  // loop. Note: scope(exit) in a loop runs on each cycle,
>                  // 'out' would only run at the end of the last cycle.

I like this too. I think scope(next) rather than scope(exit) though.

I vaguely remember that this idea was first documented back in the 1960s by
Donald Knuth or Edsger Dijkstra, but I can't locate the reference just now.

-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell