Passing in a binary function into a Scheme function

Apologies if this is a common question. Searching up "Racket passing in lambda as an argument" doesn’t return much. I’m trying to write a Racket function lsf that takes in an optional lambda function, and applies it to corresponding elements in two same-sized lists. This lambda defaults to addition.

For example, > (lsf '(2 4 5) '(6 7 8) (lambda (a b) (- b a))) should return a list '(4 3 3)

> (lsf '(2 5) '(3 6)) returns '(5 11) (defaults to addition).

Here’s what I have so far:

( define ( lsf list1 list2 )
    ( if ( null? list1 )
        '()
        ( cons( + ( car list1 list2 )) 
            ( lsf ( cdr list1 ) ( cdr list2 )))
    )
)

How do I add this default addition function as a parameter (I think it goes after list2?), and how would I use it in place of the "+" that I have after cons?

>Solution :

Use . in the parameter list to define a rest-arg, which gets a list of all the additional arguments. If the optional argument is supplied, this will be non-null and you can get the argument as its first element.

(define (lsf list1 list2 . rest)
  (if (null? list1)
      '()
      (let ((func (if (null? rest) + (first rest)))) ; default func to +
        (cons (func (car list1) (car list2))
              (lsf (cdr list1) (cdr list2) func)))))