How to pass functions as parameters in lisp

functional-lisp.el

;; definitions
(defun simple-handler (f args)
  (if (and (functionp f) (listp args))
      (apply f args)))

(defun apply-handler (X)
  (if (and (symbolp X) (fboundp X) (boundp X))
      (apply (symbol-function X) (symbol-value X))))

(defun apply-cons-handler (X)
  (if (and (consp X) (fboundp (car X)))
      (apply (symbol-function (car X)) (cdr X))))

;; applications
(simple-handler (symbol-function '+) '(7 9))

(let ((a '(2 3)))
  (fset 'a (symbol-function '+))
  (apply-handler 'a))

(let ((b '(t)))
  (setcar b '+)
  (setcdr b '(5 7))
  (apply-cons-handler b))