Arbitrary stack access. What to do with tail calls?

scmvm/vm.scm

@@ -58,6 +58,9 @@
       [(->list)
        (lambda ()
 	 (reverse-vector->list the-stack 0 top))]
+      [(set!)
+       (lambda (k obj)
+         (vector-set! the-stack k obj))]
       [else (error "stack dispatch unknown value")])))
 
 (define (push stack v)
@@ -81,6 +84,9 @@
 (define (stack->list stack)
   ((stack '->list)))
 
+(define (stack-set! stack k obj)
+  ((stack 'set!) k obj))
+
 
 ;;; IO
 (define (read-word)
@@ -116,6 +122,7 @@
     (rot    #x18)
     (over   #x19)
     (not    #x1a)
+    (set!    #x1b)
     (bye    #xff)))
 
 (define instruction-name car)
@@ -243,6 +250,11 @@
        (push data-stack b)
        (push data-stack a)
        (push data-stack b))]
+    [(set!)
+     ;; use let* to induce an order of evaluation
+     (let* ([idx (pop data-stack)]
+            [obj (pop data-stack)])
+       (stack-set! data-stack idx obj))]
     [(bye) (set! exit? #t)])
   (when (not exit?)
     (run-vm vm)))