Re: why tail recursion matters and why Java isn't it, was Re: lispperformance was Re: problems with lisp

[Pascal Costanza <address@hidden>]

Guy Steele - Sun Microsystems Labs wrote:

> Now try writing down solutions where the accumulating
> operation is vector-add, where
> 
>  (defun vector-add (x y) (mapcar #'+ x y))
> 
> So I write
> 
>  (loop for v in list vector-sum v)
> 
> right?  Um, well, no, here the paradigm breaks down a bit,
> and things get more complicated after all.

(loop for i in x
       for j in y
       collect (+ i j))

...doesn't look too bad yet. ;)

But the following would become really unhandy:

(defmacro vector-add (&rest vectors)
   (when vectors
     `(mapcar #'+ ,@vectors)))

or:

(defun vector-add (&rest args)
   (reduce (lambda (&optional v1 v2)
             (mapcar #'+ v1 v2))
           args))

or:

(defun vector-add (&rest args)
   (when (some #'identity args)
     (cons (apply #'+ (mapcar #'car args))
           (apply #'vector-add (mapcar #'cdr args)))))


I don't know how to do that with LOOP. So what? I am not purist. ;)


Pascal

-- 
Pascal Costanza               University of Bonn
mailto:costanza@web.de        Institute of Computer Science III
http://www.pascalcostanza.de  Römerstr. 164, D-53117 Bonn (Germany)