; Start the program up by printing a welcome and
; returning the result of a visit with the doctor
; starting with an empty history (since nothing has happened yet).

(define (doctor)
    (show "please type all your input within parentheses.")
    (show "")	; print a carriage return
    (show "Type (goodbye) to end the session.")
    (visit-with-doctor '()) )

; Return the result of a visit with the doctor.
; The history variable tells us some information about stuff
; the user has previously typed.
; All we do is print a prompt, read a list from the user
; (she has to parenthesize the input), compute a response, and
; return the result of subsequent interaction with the doctor.
; The response will have two parts:  our actual reply to the
; user is the car, and the updated history is the cdr.
; This version of the program doesnąt collect a history.
(define (visit-with-doctor history)
    (show "")
    (display "** ")
    (let
	((patient-says (read)))	; gets a list typed by the user
	(cond
	    ((equal? (car patient-says) 'goodbye)
	     '(your bill will be in the mail) )
	    (else
		(let
		    ((we-say (response patient-says history)))
		    (show (car we-say))
		    (visit-with-doctor (cdr we-say)) ) ) ) ) )

; Return a response to a sentence the patient has typed.
; The argument history represents a history of the dialog between us
; and the patient. 
; Our response is a pair consisting of what we say to the patient (the car) and 
; the updated history (the cdr).
; Note in this version we don't change the history,
; and we merely parrot back what the user said.

(define (response sentence history)
    (cons sentence history) )