Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!daemon
From: miller@ACORN.CS.ROCHESTER.EDU (Brad Miller)
Newsgroups: comp.lang.lisp
Subject: Re: CL question - conditional list elements
Message-ID: <6371@sol.ARPA>
Date: 30 Jan 88 02:34:42 GMT
Sender: daemon@cs.rochester.edu
Lines: 54


    Date: 28 Jan 88 11:13:45 GMT
    From: ok@quintus.UUCP (Richard A. O'Keefe)

    Here's the maybe-list macro I suggested as an answer.
    It has the same properties as the Pop version.  (Note that as in my
    maybe-cons, I assume that the thing is defined in my: package and
    that my:R is not exported.)  This has been tested (somewhat).

    (defmacro maybe-list (&rest L)
	`(let ((my:R '()))
	    ,@(expand-maybe-list L)))

    (defun expand-maybe-list (L)
	(cond
	    ((endp L)                       ; L = ()
		nil)
	    ((endp (cdr L))                 ; L = (x)
		`((setq my:R (list ,(car L)))))
	    ((eq (cadr L) ':if)             ; L = (x :if y . z)
		`(,@(expand-maybe-list (cdddr L))
		    (if ,(caddr L) (setq my:R (cons ,(car L) my:R)))))
	    (t                              ; L = (x y . z)
		`(,@(expand-maybe-list (cdr L))
		    (setq my:R (cons ,(car L) my:R))))))

    ;;; Example (it prints (A B E F)):
    (print (maybe-list
	    'a
	    'b :if T
	    'c :if NIL
	    'd :if NIL
	    'e :if T
	    'f
    ))

possibly more true to the original syntax would be:

(let (foo)
	(push 'a foo)
	(if condition (push 'b foo))
	foo)

and similarly could be marcoized if desired... if the original order is
needed, return (nreverse foo) instead of foo...

Besides, who cares about efficiency? THe compiler is supposed to worry about
that not the programmer... (only 1/4 :-))

------
miller@cs.rochester.edu {...allegra!rochester!miller}
Brad Miller
University of Rochester Computer Science Department
'If anything is True, this is.'