example of cl-yacc

yacc.lisp

(use :yacc)


(defun integer? (ch)
  (handler-case
    (progn 
      (parse-integer (string ch))
      t)
    (error (c) (declare (ignore c))
           nil)))

(defun kind-of (char)
  (cond 
    ((integer? char)  :integer)
    ((char= char #\+) :plus)
    ((char= char #\-) :minus)))

(defun convert (char)
  (string char))

(defun inner (a b c)
  (list b a c)) 

(defun lexer (str)
  (let ((index 0)
        (end   (1- (length str))))
    (lambda () 
      (if (> index end)
        (values nil nil)
        (let ((next (char str index)))
          (incf index)
          (values (kind-of next) 
                  (convert next)))))))




(define-parser *expr-parser*
   (:start-symbol expr)
   (:terminals (:integer :plus :minus))
   (:precedence ((:left :plus :minus)))

   (expr
     (expr :plus  expr #'inner)
     (expr :minus expr #'inner)
     term
     )

   (term 
     :integer
     )
   
   )


(print (parse-with-lexer (lexer "1+2-3") *expr-parser*))