Mapping a linked list in Scheme (Lisp)

linked_list_2.scm

; Mapping a singly linked list in Scheme.

; map-1 - the name of our mapping function
; proc - the tranformation
; items - the list

(define (map-1 proc items)
  (if (null? items)
    ()
    (cons (proc (car items))
          (map-1 proc (cdr items))
          )))

; Psuedo-logic below
; Luckily Scheme is clear to read!

; The expression is (map-1 proc items)
; If items is empty, return empty
; else, cons the following pair:

; A: (process (car items))
; (car items) is the first item in the list
; (proc (car items)) is the process being applied to the first item in the list

; B: (map-1 proc (cdr items))
; This is telling the function to run map-1 on (cdr-items)
; (cdr-items) represents the rest of the list. Running map-1 on (cdr items):
; this invokes running proc on (car (cdr items)), the second element

; What is proc? It can be any process that can be applied to a datum
; in our case, we work with integers, so a valid proc might be

(define (abs-1 x)
  (if (< x 0) (- x) x))

; So as an example..
(define sample-list (list -1 -2 -3 -4))