(define Walk-exp

  ; an interpreter builder than takes procedures for each
  ;    syntactic category as inputs
  ; all of these procedures take three arguments: 
  ;    the constructed interpreter, 
  ;    the expression to be interpreted (which must be of stated form), and
  ;    the environment

  (lambda (null-proc bool-proc num-proc prim-proc id-proc uop-proc biop-proc
		     map-proc app-proc let-proc if-proc)
    (rec proc (lambda (exp env)
		(cond
		  [(null-exp? exp) (null-proc proc exp env)]
		  [(bool-exp? exp) (bool-proc proc exp env)]
		  [(num-exp? exp)   (num-proc proc exp env)]
		  [(prim-exp? exp) (prim-proc proc exp env)]
		  [(id-exp? exp)     (id-proc proc exp env)]
		  [(uop-exp? exp)   (uop-proc proc exp env)]
		  [(biop-exp? exp) (biop-proc proc exp env)]
		  [(map-exp? exp)   (map-proc proc exp env)]
		  [(app-exp? exp)   (app-proc proc exp env)]
		  [(let-exp? exp)   (let-proc proc exp env)]
		  [(if-exp? exp)     (if-proc proc exp env)]
		  [else (error 'Walk-exp "illegal expression: ~a" exp)])))))