craps.lisp

(defun throw-die ()
  "Simulate the throw of a regular die; returns a number between 1 and 6, inclusive."
  (1+ (random 6)))

(defun throw-dice ()
  "Simulate the throwing of two dice; returns a list."
  (list (throw-die) (throw-die)))

(defun snake-eyes-p (throw)
  "Is THROW snake-eyes (two ones)?"
  (equal (apply #'+ throw) 2))

(defun boxcars-p (throw)
  "Is THROW boxcars (two sixes)?"
  (equal (apply #'+ throw) 12))

(defun instant-win-p (throw)
  "Is THROW an instant win?"
  (let ((pips (apply #'+ throw)))
    (or (equal pips 7)
        (equal pips 11))))

(defun instant-loss-p (throw)
  "Is THROW an instant loss?"
  (let ((pips (apply #'+ throw)))
    (or (equal pips 2)
        (equal pips 3)
        (equal pips 12))))

(defun say-throw (throw)
  "A representation of THROW."
  (cond ((snake-eyes-p throw) 'snake-eyes)
    ((boxcars-p throw) 'boxcars)
    (t (apply #'+ throw))))

(defun describe-throw (throw win-or-lose)
  "A description of THROW, either a win or a lose."
  `(throw ,(first throw) and ,(second throw) -- ,(say-throw throw) -- you ,win-or-lose))

(defun describe-throw-again (throw)
"A description of THROW, again."
`(throw ,(first throw) and ,(second throw) -- ,(say-throw throw) -- throw again))

(defun describe-point (throw)
"A description of THROW, a point."
`(throw ,(first throw) and ,(second throw) -- your point is ,(say-throw throw)))

(defun craps ()
  "Simulate the first throw in a game of Craps."
  (let ((throw (throw-dice)))
    (cond ((instant-win-p throw) (describe-throw throw 'win))
      ((instant-loss-p throw) (describe-throw throw 'lose))
      (t (describe-point throw)))))

(defun try-for-point (point)
  "Simulate trying for point in a game of Craps."
  (let ((throw (throw-dice)))
    (cond ((equal (apply #'+ throw) point) (describe-throw throw 'win))
      ((instant-loss-p throw) (describe-throw throw 'lose))
      (t (describe-throw-again throw)))))

I followed along with the "Craps" keyboard exercise in Common List: A Gentle Introduction to Symbolic Computing.