structure Machine =
struct
datatype Action = PushLit of int
                | PushVal of string
                | Mul
                | Add
    
fun execute env code =
let exception Eval

    fun v s = Environment.lookup env s

    fun run(args, PushLit n :: ops) = run(  n :: args, ops)
      | run(args, PushVal e :: ops) = run(v e :: args, ops)
      | run(a::b::args, Mul :: ops) = run(a*b :: args, ops) 
      | run(a::b::args, Add :: ops) = run(a+b :: args, ops) 
      | run([result],           []) = result
      | run _ = raise Eval
in
    run([], code)
end
end