Make illegalMoveFlag a paramter in the function instead of a global variable

I'll give you a simple example with factorials

ie: 0! = 1 n! = n * (n - 1)! when n (1 ... infinity)

lets call this a recursive factorial

(define (fact-r n)
    (if 
       [eq? n 0] 
       1 
       (* n (fact-r (- n 1)))
    )
)

An alternative would be to use a parameter to the function to end the recursion Lets call it iterative factorial

(define (fact-i n total)
  (if 
      (eq? n 0) 
      total
      (fact-i (- n 1) (* n total))
  )
)

total needs to start at 1 so we should make another function to make using it nicer

(define (nice-fact n)
  (fact-i n 1))

You could do something similar with illegalMoveFlag to avoid having a global variable As far as avoiding using set! goes, we'll probably need more information.

In some cases its still rather hard to avoid using it. Scheme is fully turing complete without the use of set! however when it comes to accessing an external source of information such as a database or a robot set! can become the only practical solution...