gistfile1.lisp

(defmacro def-coords-macro (kind)
  (let ((with-coords (intern (format nil "WITH-~a-COORDS" kind)))
        (checking    (intern (format nil "CHECKING-~a-COORDS" kind)))
        (box         (intern (format nil "BOX-~a" kind)))
        (unbox       (intern (format nil "UNBOX-~a" kind))))
    `(progn
       (defmacro ,box (v) `(cons ',',kind ,v))
       (defmacro ,unbox (v)
         (let ((vv (gensym)))
           `(let ((,vv ,v))
              (if (consp ,vv)
                  (if (eq (car ,vv) ',',kind)
                      (cdr ,vv)
                      (error "~a coordinate expected, but got ~s" ',',kind ,vv))
                  ,vv))))
       (defmacro ,with-coords ((x y) &body body)
         `(let ((,x (,',unbox ,x))
                (,y (,',unbox ,y)))
            ,@body))
       (defmacro ,checking ((x y) (def name args &body body))
         `(,def ,name ,args
            (,',with-coords (,x ,y) ,@body))))))