(require 2htdp/image)

;; Lecture: Monday, Feb 24th

;; [X Y -> Y] Y [Listof X] -> Y 
(define (process op base lox)
  (cond [(empty? lox) base]
        [(cons? lox) (op (first lox)
                         (process op base (rest lox)))]))


;; all : [X -> Y] [Listof X] -> [Listof Y]
(define (all op lox)
  (cond [(empty? lox) empty]
        [(cons? lox) (cons (op (first lox))
                           (all op (rest lox)))]))

;; all+1 : [Listof Number] -> [Listof Number]
;; add 1 to each number in lon
(define (all+1 lon)
  (all add1 lon))
; using all with X = Number, Y = Number

(check-expect (all+1 '(1 2 3)) '(2 3 4))
(check-expect (all+1 '()) '())

;; all-lengths : [Listof String] -> [Listof Number]
;; determine the length of each string in los
(define (all-lengths los)
  (all string-length los))
;; using all with X = String, Y = Number

(check-expect (all-lengths (list "foo" "a" "bar")) '(3 1 3))
(check-expect (all-lengths '()) '())

;; Notes: 
;; process is built in: it is called foldr
;; all is built in: it is called map