Question

I am looking for a concise way to map a variadic function across multiple lists, but instead of passing the lists as separate arguments as with MAPCAR, I want to pass a single list consisting of any number of lists, and map through those contained lists. I don’t know beforehand how many lists are in the enclosing list, so I can’t destructure it.

I’ve tried combining MAPCAR and APPLY in all sorts of ways, but can’t figure it out. Must I give up on using MAP and just write the iteration explicitly?

Here’s a function that does what I want:

(defun map-within (fn list-of-lists &optional(maptype #'mapcar))
  "Map FN on the lists contained in LIST-OF-LISTS"
  (cond ((null list-of-lists) nil)
    ((null (cdr list-of-lists)) (car list-of-lists))
    (t
     (funcall maptype fn
         (car list-of-lists)
         (map-within fn (cdr list-of-lists) maptype)))))

where

(map-within #'+ '((1 2 3) (10 20 30) (100 200 300))) => (111 222 333)

Is there some magical application of a lambda made from a map that could express this with just one line?

Answer 1

You can use apply like this:

(apply #'mapcar #'+ '((1 2 3) (10 20 30) (100 200 300)))
=> (111 222 333)