[r6rs-discuss] [Formal] last-element behavior of for-each

[Per Bothner]

AndrevanTonder wrote:
> The fact that, in the proposed modification, the last call to f is not a 
> tail call, will adversely affect the space behavious of useful 
> programs.  Here is a natural example:
> 
>   (define (print-list list)
>     (for-each (lambda (node)
>                 (if (not (pair? node))
>                     (display node)
>                     (print-list (car node))))
>               list)
> 
> It is easy to exhibit an s-expression for which the r6rs semantics will 
> have constant space behaviour, whereas the proposed modification would 
> need space
> proportional to the size of the s-expression to run.

I'm having problems seeing the purpose of the function, and also
which lists would be affected by tail-call semantics.  (I'm not
not very good with complex recursion, and this is effectively
two nested cursive functions.)  Could you explain?

> I would therefore vote for the current tail-recurive specification.

I won't say that the tail-recursive specification is never useful.
However, I think in terms of general performance it is better to
remove the requirement, because it makes generating good code for
for-each much harder. (See my original posting.)  People who write
the kind of tricky code where a tail-recursive for-each would be
helpful are capable of writing the needed recursion by hand.  (It's
not as if for-each is a very complicated function.)  On the other
hand, requiring tail-recursion hurts performance in the normal case,
if you use a compiling that can do inlining without global analysis.
-- 
	--Per Bothner
per at bothner.com   http://per.bothner.com/