Re: Access to lexical environment?

[Cris Kobryn <esosun!balder!kobryn@sdcsvax.ucsd.edu>]

            I am not sure how to properly develop simple extensions to Lisp
            syntax (using CL).  For example, suppose that I'd like to set up a
            way to create new things that has the following (familiar) syntax:
  
            (def-newthing fred (other-thing) ((n :initform (+ a b))
				              (ls :initform (rest ls1)))
				              ...etc.)
  
     The mechanism in Lisp for capturing the lexical environment is LAMBDA.
     Try this

     (defmacro def-newthing (name superthings specs)
       `(progn (setf (getf ',name 'superthings) ',superthings)
	       (setf (getf ',name 'slots)
		    (list ,@(map 'list
			      (lambda (spec)
				(destructuring-bind (name &key initform) spec
				  `(list ',name (lambda () ,initform))))
			      specs)))))

     (def-newthing fred (other-thing) ((n :initform (+ a b))
				  (ls :initform (rest ls1))))
      ==> (PROGN (SETF (GETF 'FRED 'SUPERTHINGS) '(OTHER-THING))
	         (SETF (GETF 'FRED 'SLOTS)
		       (LIST (LIST 'N (LAMBDA () (+ A B)))
			     (LIST 'LS (LAMBDA () (REST LS1))))))

     I hope this clarifies the situation for you.

The following version evaluates better and conforms to CLtL:

(defmacro def-newthing (name superthings specs)
  `(progn (setf (get ',name 'superthings) ',superthings)
	  (setf (get ',name 'slots)
		(list ,@(map 'list
			     #'(lambda (spec-list)
				  `(list ',(car spec-list)
					 #'(lambda () ,(caddr spec-list))))
			      specs)))))

def-newthing 
[2] What?.. (pprint (macroexpand '(def-newthing fred (other-thing) 
				    ((n :initform (+ a b))
				     (ls :initform (rest ls1))))))

(progn
  (setf (get 'fred 'superthings) '(other-thing))
  (setf (get 'fred 'slots)
        (list (list 'n #'(lambda ()
                           (+ a b)))
              (list 'ls #'(lambda ()
                            (rest ls1))))))

-- ck

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|   Cris Kobryn                         UUCP:  {sdcsvax|seismo}!esosun!kobryn |
|   Geophysics Division, MS/22          ARPA:  esosun!kobryn@sdcsvax.ucsd.edu |
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