Re: For loops
CChris wrote:
>
> John E wrote:
> >
> > Andy Drummond wrote:
> > >
> > > CChris wrote:
> > > >
> > > > Since there hasn't been much activity on this after some ideas wwere
> > > > tossed,
> > > > here would be my proposal for changes/extensions:
> > > >
> >
> > Maybe there has been little response because FOR is OK as it is and
> > doesn't need changing ;)
> >
>
> If you read the forum for the past 7-8 days, you'll see that asimple bug
> report
> about trace(1) triggered some activity indeed, and some requests for
> additions/changes.
> The problem was that some of the changes were mutually incompatible, so there
> was something to sort out.
>
> >
I had indeed read most, but not all. I took your post to be a what
you considered to be a summary, my mistake. I also misunderstood
you when you said 'not much activity since...'. I'm not about to
declare war over it :)
But you know you are not just proposing this as a bug fix, but in fact
you are proposing an innovation.
> > > > 1/ Change to the for loop semantics
<snip>
> > > > So this would be like a for loop, but using an existing variable.
> > > >
> >
> > Does this offer anything WHILE can't do?
>
> 1/ avoiding to forget to increment counter at the end;
> 2/ protecting loop index from unwanted interference (by internally setting its
> status to constant);
> 3/ protecting limit and increment, just like in a for loop, if they are
> (subscripted)
> variables.
Righto, some worthwhile protections against programmer error, but no
enhanced ability.
>
> This is actually a variant of a for loop, rather than a while loop. Or a cross
> breed between the two. The variant syntax "for with ..." was suggested instead
> of "for_existing". It avoids a new keyword, but perhaps isn't as obvious for
> the reader of the code. I certainly could live with it.
>
An underscore in keywords seems... wrong :)
> >
> >
> > > > 3/ Change in ex.err dumps
> > > >
> > > > If a variable is defined several times in a scope, like for loop
> > > > indexes, only
> > > > the one defined last at the time of the dump will be displayed. This is
> > > > a
> change.</font></i>
> > > >
> > > >
> > > > Expected code breakage:
> > > > * none for 1/, since the change in 1/ will make legal code that
> > > > currently
> isn't.</font></i>
> > > > * some (or perhaps none) with 2/: programs which define variable(s)
> > > > named
> > "for_existing"</font></i>
> > > > will require this name be changed.
> > > > * none for 3/, since code isn't involved.
> > > > No performance hit is expected.
> > > >
> > > >
> > > > Any comments?
> > > > Since I expect to move this week, the changes, if agreed upon, won't be
> > > > committed
> > > > before next month.
> > > >
> > > > CChris
> > >
> > > For those who have followed my meanderings it will no surprise if I accept
> > > this proposal. I just wondered why it would not be possible to write the
> > > for_existing iterator within the loop. This agrees with current for-next
> > > loop
> > > coding, but I don't see why it needs be preserved. Otherwise, I concur
> > > with
> > > CChris's proposition.
> > >
> > > AndyD
> >
>
> It is possible. But then what would be the difference with the modified for
> loop?
> The idea is that a for/for_existing loop uses a fixed increment to traverse
> some fixed data structure or perform some operation. When it gets more dynamic
> than that, use the while loop. That's my understanding at least. Currently,
> you must use a while loop only beause you need some interaction with the
> counter.
> This doesn't enhance code readability.
>
> >
> > The changes proposed, IMHO, amount to a work-around for the absence of
> > local variables. Maybe add those?
>
> If by "local" you mean with nested, coder defined scope like in C, then yes,
> the issue could be handled this way. This is in Advanced Euphoria already (see
> below), as a new scope ... end scope block, at the top of which you can define
> variables (local or private) that go out of scope on reaching the end scope
> statement.
> There hasn't been much demand for this feature, so I didn't plan to introduce
> it in standard Eu. But I'd consider it a good thing. Didn't you already see
> a flurry of private vars most of which are used only in some branch of an if
> statement? Or reused accidentally a var that should have been scoped out long
> ago?
>
> >
> > If you want to go to the trouble, I suggest that adding a missing
> > control feature such as a do..while would be a more appreciated use
> > of your time. This might prove to be simpler to do, too. (Only
> > guessing there).
> >
>
> This was also suggested, but not much debated, as a do ... while loop is
> strictly
> equivalent to:
> }}}
<eucode>
> while 1 do
> -- something
> if not <loop_again_condition> then exit end if
> end while
> </eucode>
{{{
>
> Equivalent, but less verbose and making the intent clearer. I have it
> implemented
> in my projet of modified interpreter - it may surface some day as Advanced
> Euphoria,
> but is low priority at the moment.
>
> The implementation of either is pretty simple actually, as it adapts the for
> loop/while loop handling respectively. Scoped variables require adding an
> extra
> field to the IL - and preventing ? to display an out of scope for loop var
> when
> tracing could require it as well. This is a little harder, but can be done.
>
> CChris
> >
> > Regards, John
I think I'll stick with V2.5. I think RDS got the language more or less
right, that's why I bought a licence.
All the best with your version anyway, CChris, you are obviously keen to
get on and develop the language. No more posts from me.
John
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