(*  ML does support imperative-style variables (i.e. where the value
    associated with a variable can be changed by assignment) -- they are
    called reference variables.
    The following illustrates how reference variables are created, updated,
    and used.  This also illustrates one of the rare programs where reference
    variables are necessary.  Finally it highlights the fact that "local"
    variables are allocated once (whereas "let" variables are re-allocated
    on every function invocation. *)

(* Linear Congruential Random Number Generator. Returns an integer in [0,728] *)

local val seed = ref 261    (* reference variables are declared by assigning
                               them "reference values" (e.g. ref 261)  *)
      val modulus = 729
      val multiplier = 40
      val increment = 3641
in fun random _ =
   let val _ = seed := (multiplier*(!seed)+increment) mod modulus
   in  !seed           (* ! is the dereference operator.  *)
                       (* := is assignment ... Note that there's no "ref".
                          Assignment is purely a side-effect; the value it
                          returns as a function is () ("unit")  *)
   end
   fun randombool _ =  (random ()) < (modulus div 2)
end;