; Simulate a spreadsheet by interpreting commands of the form
;   (set variable constant-value) to assign the value to a variable.
;   (define variable expression) to assign the value of the given
;      expression to the variable.
;   (value-of variable) to print the value of the variable.
; Inputs to the spreadsheet are a list of formulas and a variable-value
; table.  Each formula has the form
;   (variable expression variable-list)
; where variable-list is the list of variables occurring in the
; expression.

(define (spreadsheet formulas var-val-table)
  (display 'command?)
  (let
    ((cmd (read)))
    (cond
      ((equal? (first cmd) 'set)
       (spreadsheet
         formulas
         (new-values 
           (list (second cmd))
           formulas (updated var-val-table (rest cmd))) ) )
      ((equal? (first cmd) 'define)
       (let ((v (eval-expr (third cmd) var-val-table)))
         (if (equal? v 'unknown)
             (spreadsheet
               (cons (new-formula (rest cmd)) formulas)
               var-val-table)
             (spreadsheet
               (cons (new-formula (rest cmd)) formulas)
               (new-values
                 (list (second cmd))
                 formulas
                 (updated var-val-table (list (second cmd) v)) ) ) ) ) )
      ((equal? (first cmd) 'value-of)
       (display (eval-expr (second cmd) var-val-table))
       (newline)
       (spreadsheet formulas var-val-table) ) ) ) )

(define formulas
  '((c (+ a b) (a b))
    (d (* a c) (a c)) ) )