resource factorial
   op fact(k: int) returns f: int
body factorial()
   proc fact(k) returns f
      if k < 0 -> f := -1
      [] k = 0 or k = 1 -> f := 1
      [] k > 1 -> f := k * fact(k-1)
      fi
   end fact
end factorial

resource driver()
   import factorial
   var n, stat : int
   var fac_cap : cap factorial
   fac_cap := create factorial()

   do true ->
      writes("What factorial to compute? ")
      stat := read(n)
      if stat = EOF ->
         write("no more input")
         stop
      [] stat = 0 ->
         write("illegal input -- try again")
      [] else ->
         write(n, "factorial is", fac_cap.fact(n))
      fi
   od
end driver

/* ............... Example compile and run(s)

% sr -o factorial factorial.sr
% ./factorial
What factorial to compute? 5
5 factorial is 120
What factorial to compute? abc
illegal input -- try again
What factorial to compute? 3
3 factorial is 6
What factorial to compute? ^D
no more input
                                              */