🚀 go-pugleaf

In comp.lang.lisp.franz you write:

>In article <1991May15.203735.3850@csusac.csus.edu> cs16011@athena.ecs.csus.edu writes:

>   Hello, I have got an exam question in my Franz Lisp class about whether
>   macros can be recursive or not.  I have check my textbook and Franz Lisp
>   Interpreter's online manual here on campus but found no mentioning about
>   whether this is possible or not.  If macros can be recursive, can anyone
>   please give me an example for supporting the claim?

>A macro is, at some time, expanded.  It's just a template for a function, an
>idea, not a function per se.

No, a macro >is< a function, that happens to be called by the
interpreter, or compiler, on a piece of code.  It returns another
piece of code.  There are no real restrictions on the function
definition of a macro.  That is, macro definition can certainly be
recursive.

But I don't think that's what he's asking.  

(I'm a little confused about the fact that this is an exam question.
Is someone trying to get an answer for a question on a take home
exam?  Net ethics?  I hope this isn't the case.)

Anyway, I assume he's asking whether a macro can be >expanded<
recursively.  Here too the answer is yes.  The result of a macro
expansion is again macroexpanded, so the behavior of a macro is
essentially unlimited.  (As long as the expansion eventually
terminates, that is).

Some mildy bizarre recursive macro examples follow:

(defmacro fibn-inline (n)
  (if (< n 2)
      1
      `(+ (fibn-inline ,(- n 1))
          (fibn-inline ,(- n 2)))))

Or 

(defmacro unroll (repeat-count form &rest forms)
  (if (zerop repeat-count)
      (cons 'progn forms)
      `(unroll ,(1- repeat-count) ,form ,form ,@forms))) 

>Now recursivity demands that one at some time prior to application of the
>function need not know how many times the function will call itself.  A macro
>must know that.  Hence, a macro can only be tail-recursive (i. e. an implied
>for).  Macros in TeX, as an example, are not recursive. 

The first example above isn't tail recursive.

>Now if the macro could calculate its number of expansions during calculation,
>there still would have to be an effective algorithm that could do that _prior_
>to expansion.

>Some of my assumptions here are ...loose... I eagerly await being corrected. 

>Rolf Lindgren		| 	"The opinions expressed above are 
>616 Bjerke Studentheim	|  	 not necessarily those of anyone"	
>N-0589 OSLO 5		|             rolfl@hedda.uio.no