elisp how to apply a lambda to a list?

Question

What I'm trying to do seems simple enough, and whatever's wrong must be a really dumb mistake, since I couldn't find other people getting the same error. I just want to apply a lambda to a list - the lambda here isn't what I actually want to do, but it gives the same error.

(apply
(lambda (arg)
  (+ 5 arg)
)
(list 2 3 4)
)

When I try to run this, it tells me that I'm passing the lambda an invalid number of arguments. Do you have any advice?

Answer 1

apply calls the function once, passing it the list you've given as the arguments. I think you instead want to use mapcar:

M-: (mapcar (lambda (arg) (+ 5 arg)) (list 2 3 4)) RET

will return the list (7 8 9).

Answer 2

Just to make the problem a bit clearer:

This form

(apply
 (lambda (arg)
   (+ 5 arg))
 (list 2 3 4))

is basically similar to

(funcall
 (lambda (arg)
   (+ 5 arg))
 2
 3
 4)

In above we try to call a function with one parameter arg with three arguments.

Now if you want to pass more than one argument and receive it as a single list you would need a function with a &rest parameter:

(lambda (&rest args) ...)

You say

I just want to apply a lambda

This is not what you want. You want to map the function over a list. Which means calling the function for each element of the list and returning a new list with the results. This operation is called in Lisp mapping. See the answer by Stefan for an example.

Applying a function to a list would be: call the function with the arguments taken from the list.