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

---
This message is a formal comment which was submitted to formal-comment_at_r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
---
The specification of for-each (in R5RS as well as R6RS) has a weird
asymmetry in that the result value is the result of calling the function
on the last elements.  Furthermore,, it is required to tail-call the
function on the last element.
The problem is that this specification makes it more difficult to
optimize for-each.  If we didn't have this requirement a compiler
could transform (for-each proc list) into:
   (do ((p list (cdr p)))
     (if (not (null? p))
        (let ((x (car p)))
           (proc x))))
(Notice the unspecified else clause.)
The key point is that this transformation further allows the
compiler to inline proc.  If the last tail call to proc must
be a tail-call, then the inlining gets ugly.  I think you'd
need two copies of the call to proc: one for each iteration
except the last, and then a final tail-call.  The former can
be inlined, but the latter cannot be,  Duplicate copies is
bad: it leads to code bloat, more difficulties for a debugger
in setting breakpoints, etc.
Now presumably an even-more-sophisticated compiler would
figure out when it is safe to use a non-tail-call for the
final element.  But that requires global analysis.  If we
get rid of the weird last-element semantics even a fairly
simple compiler can do this optimization.
In addition to hurting optimization the asymmetry in the
specification is inelegant, in my opinion.
I propose that the return value of for-each be "the unspecified
value" [pending resolution of this issue], and the requirement
for special handling of the last element be removed.
-- 
	--Per Bothner
per_at_bothner.com   http://per.bothner.com/