Bill Clementson's Blog

Funcallable Macros

Thursday, May 27, 2004

Ever want to funcall or map using a macro? Although there are some good reasons for not doing this (see this thread for some of them), Lars Brinkhoff posted some code (actually 2 different versions) that shows how one could. Cute.

;;; Lisp apprentice's wet dream: funcallable macros.
;;; Probable newbie usage example: (funcallable-macro 'and)
;;;                                (reduce #`and '(t t t nil t t))
;;; Two implementations are provided: one trivial using eval, and one
;;; that memoizes compiled functions.

;;; #1 Trivial version using eval.
(defun funcallable-macro (name)
  (lambda (&rest args)
    (eval (cons name args))))

;;; #2 Version that memoizes compiled functions.
(defun funcallable-macro (name)
  (let ((table (make-hash-table :test #'eql)))
    (lambda (&rest args)
      (let ((n (length args)))
	(apply (or (gethash n table)
		   (setf (gethash n table)
			 (make-funcallable-macro-function name n)))
	       args)))))

(defun make-funcallable-macro-function (name n)
  (let ((args nil))
    (dotimes (i n)
      (push (gensym) args))
    (compile nil `(lambda ,args (,name ,@args)))))

;;; Do this for either version that is used.
(set-dispatch-macro-character ## #`
			      (lambda (s c n)
				`(funcallable-macro ',(read s t nil t))))