(* Typed call-by-value fix *)

datatype ('a,'b) F = fold of ('a,'b) F -> 'a -> 'b  ;

fun unfold (fold x) = x  ;

val fix 
  = fn f => 
      let 
        val D = fn x => fn y => f ( (unfold x) x ) y 
      in 
        D (fold D)
      end ;