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From: Jenny.Rowland@levels.sait.edu.au
Newsgroups: comp.lang.lisp
Subject: macro misbehaviour
Message-ID: <16456.285261a2@levels.sait.edu.au>
Date: 9 Jun 91 07:43:06 GMT
Organization: University of South Australia
Lines: 50

Please help..... I use VAX Lisp, and have come across some annoying behaviour.
I would like to know:
1. why it occurs;        (I suspect that it is some kind of optimization)
2. how to get around it.

The Problem:
Given the following macros and function:

    (DEFMACRO TRY (X) (LET ((A (EVAL X))) `(TT ,(MAPCAR #'ADD-1 A))))

    (DEFUN ADD-1 (X) (1+ X))

    (DEFMACRO TT (X) `(FORMAT T "~a ~%" ',X))

and the following code:

   (dotimes (n 3)
     (let ((a (list n n n)))
       (try a)))

I get the following results:

   (1 1 1)
   (1 1 1)
   (1 1 1)

when I would expect:

   (1 1 1)
   (2 2 2)
   (3 3 3)

It appears that the first call is evaluated, but further calls are 'recognised'
as being the same as previous calls (despite the fact that the value of
n and therefore a has changed), bypasses evaluation of the mapcar form, and
sends the previous result to macro tt.

Has anyone and explanation? and HOW CAN I GET AROUND IT???


Regards, and thanks in advance,
Jenny Rowland

--------

Jenny.rowland@sait.edu.au
University of South Australia

.....You play, you win, you play, you lose, you play...
                                                      J Winterson