jcp@arizona.edu (John Peterson) (10/27/88)
The discussion comparing different ways to sum a list has missed an
important point. Although the solution
(apply '+ list)
is very elegent, it ignores a basic limitation of Common Lisp:
call-arguments-limit. This will not work (nor will eval) if the
list is longer than this value. CLtL only states this limit must be at
least 50, not much of a list. To be safe (or at least postable), either the
iterative solution or reduce should be used. Also, note that this
brings up the issue of whether &rest should provide a freshly consed
list or not. If it is, &rest would incur the additional overhead of a
list copy. This issue is not addressed in CLtL.
(There are some very good implementation reasons for this limit. Using
registers for parameter passing yields dramatic speedup, but only a
finite number of registers or psuedo registers are available.)
John Peterson
University of Arizona, Dept of Computer Science
jcp@megaron.arizona.edu
Think Granitesfk@otter.hple.hp.com (Stephen Knight) (11/01/88)
jcp@arizona.edu (John Peterson) writes: > The discussion [comparing different ways to sum a list] > ignores a basic limitation of Common Lisp: call-arguments-limit. A very good point! I would be interested in the typical values of this system parameter. Poplog CL has a default call-arguments-limit of 536870912. Is this unusual? > (There are some very good implementation reasons for this limit. Using > registers for parameter passing yields dramatic speedup, but only a > finite number of registers or psuedo registers are available.) Although this is an explanation, I don't think it is an acceptable justification for strange limits like 50. Sure, there are only a finite number of registers. When they are all used up another parameter passing mechanism is required. This does not seem to be especially impractical or even difficult. I am reminded of a similar case in Pascal, where the guaranteed maximum size of a set is something like 64 -- which is a crippling limitation in terms of guaranteed portability. In these cases one has to make a choice based on the actual limits of popular compilers. Steve Knight
hall@nvuxh.UUCP (Michael R Hall) (11/04/88)
In article <1350018@otter.hple.hp.com> sfk@otter.hple.hp.com (Stephen Knight) writes: =jcp@arizona.edu (John Peterson) writes: => The discussion [comparing different ways to sum a list] => ignores a basic limitation of Common Lisp: call-arguments-limit. = =A very good point! I would be interested in the typical values of this =system parameter. Poplog CL has a default call-arguments-limit of =536870912. Is this unusual? Sun Lucid Common Lisp Development Environment 2.0.3 has 512 for the call-arguments-limit value; I would guess that 536870912 is quite unusual. [How about trying an experiment - call a function in Poplog CL with 536870912 arguments and then call it with 536870913 arguments and see what happens...] -- Michael R. Hall | Bell Communications Research "I'm just a symptom of the moral decay that's | nvuxh!hall@bellcore.COM gnawing at the heart of the country" -The The | bellcore!nvuxh!hall