Re: Break statement, again.
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- Subject: Re: Break statement, again.
- From: rw20@...
- Date: Thu, 2 Sep 1999 15:17:20 -0400 (EDT)
Tail-recursion would be cool in general and it would help the break issue
especially when combined with $macros (once they exist).
If macros (with optional '$', since '$' are ugly) could be made to
implement something like the code below (where begin, again, and break are
macros):
begin(loopname)
[code before test]
break(test, loopname)
[code after test]
again
this could be turned into
local loopname = 1
while loopname do
[code before test]
if test then loopname=nil end
if loopname then
[code after test]
end
end
Of course the macros would need to get the text of their arguments
instead of the values in the above code, but that seems like a generally
useful thing.
Russ
On Thu, 2 Sep 1999, Fred Bertsch wrote:
> On Wed, 1 Sep 1999, David Jeske wrote:
>
> > I'm not concerned with whether the solution involves adding a break
> > statement or not. However, I have to second Fred's complaint about
> > "while()" being the only available looping structure.
>
> Here's another possibility that wouldn't require extending the language at
> all. If you add a tail-recursion optimization, you could make a loop look
> like this:
>
> local Loop = function( i )
> [ loop code ]
> if( i == %endvalue ) then
> return
> end
> [ more loop code ]
> Loop( i + %increment )
> end
> Loop( startvalue )
>
> That would give me everything I'd need. It would also make setting the
> initial value for the loop index and incrementing the loop index be in
> standard locations. That might make reading code a bit easier.
>
> I disagree, by the way, that putting a "break" in the middle of a loop
> creates large numbers of infinite loops. In Lua, essentially all of the
> infinite loops that I write are because I forgot to write the loop
> index increment code. (ie: "i = i + 1" or whatever.) Tail recursion
> would make that much less likely, since in order to make it loop at all
> you'd have to write what should be the loop increment code.
>
> I would think that tail recursion is a fairly simple optimization to add,
> too. You just have to check to see if any local variables are used after
> the recursion call and, if not, clean up the stack before making the
> function call.
>
> F
>
>