fexpr kinda definitional scoping

;; http://blog.hostilefork.com/rebol-vs-lisp-macros/

(ql:quickload '( :alexandria :closer-mop))
(in-package :closer-mop)
(use-package :alexandria)

(defvar *env* ())
(defclass fexpr (funcallable-standard-object)
  ((env :accessor env :initform nil :initarg :env)
   (src :accessor src :initform nil :initarg :src)
   (effective-fn :accessor fn :initform nil))
  (:metaclass funcallable-standard-class))

(defun make-fexpr (src &optional (env *dynamic-env*))
  (let ((fexpr
	 (make-instance 'fexpr :env env :src src)))
    (setf (fn fexpr)
	  `(let (,@env)
	     ,(src fexpr)))
    (set-funcallable-instance-function fexpr (lambda () (eval (fn fexpr))))
    fexpr))

(defmethod (setf env) :after (nenv fexpr)
  (setf (fn fexpr)
	`(let (,@nenv)
	     ,(src fexpr))))
(defmethod (setf src) :after (nsrc fexpr)
  (setf (fn fexpr)
	`(let (,@(env fexpr))
	   ,nsrc)))

(defmacro use ((&rest vars) &body body)
  `(let ((*env* ',vars)
	 (vars ',vars))
     ,@body))

(set-macro-character #\[
		     (lambda (s c)
		       (declare (ignore c))
		       `(make-fexpr ',(read-delimited-list #\] s))))
(set-macro-character #\] (get-macro-character #\) nil))

;; example:
;; (use ((a 10) (b 5)) (let (( fn [+ a b])) (funcall fn)))
;; => 15