eval-gadt.lisp

(defclass (term A))

(defclass literal-term ((term number))
  ((value number))
  (:constructor ((value number))))

(defclass is-zero-term ((term boolean))
  ((subterm (term number)))
  (:constructor ((subterm (term number)))))

(defclass (if-term A) ((term A))
  ((test-term (term boolean))
   (then-term (term A))
   (else-term (term A)))
  (:constructor ((test-term (term boolean)) (then-term (term A)) (else-term (term A)))))

(defgeneric eval ((term (term A)) -> A))

(defmethod eval ((term literal-term) -> number)
  (.value term))

(defmethod eval ((term is-zero-term) -> boolean)
  (= 0 (.value (.subterm term))))

(defmethod eval ((term (if-term A) -> A))
  (if (eval (.test-term term))
      (eval (.then-term term))
      (eval (.else-term term))))