;; An entry holds a key/value pair
;; for use in Dictionaries.
;;
(define-struct entry (key value))

;; have-key? key entry --> boolean
;; Returns whether the key is the same as that of the entry.
(define (have-key? key entry)
  (equal? key (entry-key entry)))


;; A Dictionary is a structure with two "methods"
;;   (fields which are functions):
;; add-fn!  : ANY ANY --> list-of-entries
;;   SIDE-EFFECT: change the contents.
;; lookup-fn: key --> entry or false
;;   Does the key occur inside "contents"?
;;   If so, return that entry, else return the sentinel false.
;;
(define-struct dict (add-fn! lookup-fn))

  ; add!: ANY ANY --> ??

  ; lookup: key --> entry or false

;;;;;;;;;;;;;;;;;;

(define (dict-Factory)
  (local [;; a local list-of-entries,
          ;; used only by the dictionary we're creating now:
          ;;
          (define contents empty)

          
          ; Return the list-of-entries, for debugging as much as anything.
          ;
          (define (add! k val)
            (begin (set! contents (cons (make-entry k val) contents))
                   contents))


          (define (lookup key)
            (local [;; helper: list-of-entries --> entry or false
                    ;; Does the key occur inside "entries"?
                    ;; If so, return that entry, else return false.
                    ;;
                    (define (helper entries)
                      (cond [(empty? entries) false]
                            [(cons?  entries)
                             (cond [(have-key? key (first entries))
                                    (first entries)]
                                   [else (helper (rest entries))])]))]
             (helper contents)))
          ]
    (make-dict add! lookup)))

;;;;;; Tests

(define english->german (dict-Factory))
((dict-lookup-fn english->german) 'health)
((dict-add-fn!   english->german) 'health 'gesundheit)
((dict-lookup-fn english->german) 'health)
((dict-add-fn!   english->german) 'science 'wissenschaft)
((dict-lookup-fn english->german) 'health)

(define english->french (dict-Factory))
((dict-lookup-fn english->french) 'health)
((dict-add-fn!   english->french) 'health 'sante)
((dict-add-fn!   english->french) 'computer 'ordinateur)
((dict-add-fn!   english->french) 'weekend  'weekend)
((dict-lookup-fn english->french) 'health)


((dict-add-fn!   english->german) 'computer-science 'informatik)
((dict-lookup-fn english->german) 'science)
((dict-lookup-fn english->german) 'health)
((dict-lookup-fn english->german) 'gymnasium)