Guided Practice 2.2: The Prisoner's Dilemma

Here's one solution. We use the template for Move on move1, and then we call one of two help functions. Each help function uses the template for Move on move2.

;; STRATEGY: Use template for Move on move1

(define (outcome move1 move2)
  (cond
    [(string=? move1 "betray")
     (outcome-after-betray move2)]
    [(string=? move1 "don't betray")
     (outcome-after-dont-betray move2)]))

;; outcome-after-betray : Move -> Number
;; GIVEN: player2's move
;; RETURNS: the outcome if player1 moved "betray"
;; STRATEGY: Use template for Move on move2

(define (outcome-after-betray move2)
  (cond
    [(string=? move2 "betray") -3]
    [(string=? move2 "don't betray") 0]))

;; outcome-after-dont-betray : Move -> Number
;; GIVEN: player2's move
;; RETURNS: the outcome if player1 moved "don't betray"
;; STRATEGY: Use template for Move on move2

(define (outcome-after-dont-betray move2)
  (cond
    [(string=? move2 "betray") -12]
    [(string=? move2 "don't betray") -1]))

Notice how the information we have about the situation in which we call the function is shown twice:

• once in the function name
• and again in purpose statement

We'll see much more about expressing context information in later modules.