Larry Clapp recently responded to my post on making a “mapcar” type routine that works with macros. I managed to make an ugly hack that got the job done, but Larry shows us a much better approach. Instead of using mapcar to force eval to get something done, he uses a macro to expand out a list of expressions in a progn operation. Notice the trick of mapcar’ing a lambda expression that kicks out back-quoted list. That technique will surely be useful in other macros, but is not something I’ve seen highlighted in any tutorials, yet:
CL-USER> (defmacro mapcro (macro &rest args)
`(progn ,@(apply #'mapcar
(lambda (&rest args2)
`(,macro ,@args2))
args)))
MAPCRO
CL-USER> (mapcro defparameter (a b c) (1 2 3))
C
CL-USER> a
1
CL-USER> b
2
CL-USER> c
3
CL-USER> (macroexpand-1 '(mapcro defparameter (x y z) (1 2 3)))
(PROGN (DEFPARAMETER X 1) (DEFPARAMETER Y 2) (DEFPARAMETER Z 3))
T
