Scheme - n-ary subtraction function issues

I have the following n-ary function I defined:

(define (- . l)
(cond   ((null? l) 0)
        (else (b- (car l) (apply - (cdr l))))))

It works fine for two arguments, but anymore and it starts to add numbers in a strange way and I don't understand why.

Alternatively, I have a check implemented in a different version of this function in case there is only one argument:

(define (- . l)
(cond   ((null? (cdr l)) (b- (b* l 2) l))
        (else (b- (car l) (apply - (cdr l))))))

This second one does not work at all when I change the first condition. Input should be something like (- 10 6 1)

Solution

I assume that b- is a binary subtraction, and that you want to mimic the usual subtraction of Scheme, which is a function such that:

with no arguments, gives an error,
with an argument, changes the sign of the argument,
with more than one argument, substracts from the first one all the others.

Here is a possibile solution (note that I've called the function n-):

(define (n- . l)
  (define (aux l dec)
    (if (null? l)
        dec
        (aux (cdr l) (b- dec (car l)))))
  (cond ((null? l) (println "Error"))
        ((null? (cdr l)) (b- 0 (car l)))
        (else (aux (cdr l) (car l)))))

(n-)            ; => Error
(n- 3)          ; => -3
(n- 10 6 1)     ; =>  3
(n- 11 4 8 2)   ; => -3

The auxiliary function subtracts all the numbers from the list first argument to its second argument, and it is defined as a tail recursive function, so that it can be implemented in an iterative way.