Type issue

[Jonathan T Bryant <jtbryant@valdosta.edu>]

List,

I don't understand the following typing:

# type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a;;
type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a

# let rec f t = match t with
      Cond (c,t,e) -> if f c then f t else f e
    | Value x -> x
  ;;
val f : bool t -> bool = <fun>

I don't understand why f isn't of type 'a t -> 'a.  I understand that f 
is applied to a bool t, but I would think that that would just be a 
particular instantiation of 'a, not enough to fully determine its type.

--Jonathan Bryant