fun factorial 0 = 1
  | factorial n = n * factorial (n-1)

  fun quicksort << xs = let
     fun qs [] = []
       | qs [x] = [x]
       | qs (p::xs) = let
          val (less, more) = List.partition (fn x => << (x, p)) xs
          in
            qs less @ p :: qs more
          end
     in
       qs xs
     end

  exception Err
 
  datatype ty
    = IntTy
    | BoolTy
 
  datatype exp
    = True
    | False
    | Int of int
    | Not of exp
    | Add of exp * exp
    | If of exp * exp * exp
 
  fun typeOf (True) = BoolTy
   | typeOf (False) = BoolTy
   | typeOf (Int _) = IntTy
   | typeOf (Not e) = if typeOf e = BoolTy then BoolTy else raise Err
   | typeOf (Add (e1, e2)) = 
        if (typeOf e1 = IntTy) andalso (typeOf e2 = IntTy) then IntTy else raise Err
   | typeOf (If (e1, e2, e3)) = 
        if typeOf e1 <> BoolTy then raise Err
        else if typeOf e2 <> typeOf e3 then raise Err
        else typeOf e2
  
  fun eval (True) = True
   | eval (False) = False
   | eval (Int n) = Int n
   | eval (Not e) = 
     (case eval e
          of True => False
           | False => True
           | _ => raise Fail "type-checking is broken")
   | eval (Add (e1, e2)) = let
        val (Int n1) = eval e1
        val (Int n2) = eval e2
        in
          Int (n1 + n2)
        end
    | eval (If (e1, e2, e3)) = 
        if eval e1 = True then eval e2 else eval e3
 
  fun chkEval e = (ignore (typeOf e); eval e) ''(* will raise Err on type error *)''