Question

Just to understand how the Scheme macros work i’m trying to define a new command, sum that works exactly like the common operator + (i.e. also undefined number of parameters).

I wroter this code:

(define-syntax sum
  (syntax-rules ()
    ((_ arg1 arg2 args...)
     (sum (+ arg1 arg2) args...))
    ((_ arg1 arg2)
     (+ arg1 arg2))
    ((_ arg1)
     arg1)))

It works if i pass it 1, 2 or 3 arguments. But with 4 arguments i get this error:

sum: bad syntax in: (sum 1 2 3 4)

I tried to expand the macro step-by-step with DrRacket but it stops immediately.

Can someone explain me the cause of this error?

Answer 1

In Scheme, ... is just another identifier, so you need a space between args and the ellipses (...) in both places, like this:

(define-syntax sum
  (syntax-rules ()
    ((_ arg1 arg2 args ...)
     (sum (+ arg1 arg2) args ...))
     ___etc___))

BTW, you don’t need to create macros in order to accept arbitrary numbers of arguments. You can also use “rest args”:

(define (sum . args)
  ;; args is bound to a list of all the arguments
  (foldl + 0 args))