Abhorrent Interface implementation

gistfile1.lisp

(defun make-interface (&rest args &key &allow-other-keys)
  (let ((methods (make-hash-table)))

    (loop for (name function . rest) on args by #'cddr
       do
         (setf (gethash name methods) function))
    
    (lambda (action name &optional value)
      (ecase action
        (:get (gethash name methods))
        (:set (setf (gethash name methods) value))))))

(let ((my-num 
       (make-interface :add #'+ :sub #'- :identity (lambda (x) x))))
  (funcall (funcall my-num :get :add) 1 3))
;; => 4