;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-beginner-abbr-reader.ss" "lang")((modname data_abstraction_1) (read-case-sensitive #t) (teachpacks ((lib "image.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "image.ss" "teachpack" "htdp")))))
; a person is:
;  - number ssn
;  - symbol name
(define-struct person (ssn name))

(define Adam (make-person 1 'adam))
(define George (make-person 2 'george))
(define Jon (make-person 3 'jon))

; lookup : number database -> person or false
; returns person if ssn in database, false otherwise
(define (lookup n db) 
  (cond [(empty? db) false]
       [(cons? db) 
        (cond [(person-matches? (first db) n) (first db)]
              [else (lookup n (rest db)) ])])
  )

(check-expect (lookup 2 (list Adam George Jon)) George)
(check-expect (lookup 2 empty) false)

;person-matches? : person ssn
; returns if person has ssn
(define (person-matches? person ssn) 
  (= (person-ssn person) ssn))

; add : person database -> database
; adds person to database
(define (add per db) (cons per db))

; start : database
; an empty database
(define start empty)
 
; a database is:
; - empty
; - (cons person database)
; template:
;  (cond [(empty? db) ...]
 ;       [(cons? db) ... (first db) ... 
  ;                  (fun-for-db (rest db)) ...])