Non generalizable type of constants?

[Thorsten Ohl <ohl@hep.tu-darmstadt.de>]

OK, maybe I'm really too dumb to use O'Caml, but can some kind soul
explain why in

    module type Ring = sig type t val unit : t end
    
    module Group (R : Ring) =
      struct
	type 'a t = Unit | Prod of (R.t * 'a)
	let unit = Unit
	let atom a = Prod (R.unit, a)
      end
    
    module FreeRing (R : Ring) =
      struct
	module M = Group(R)
	module A = Group(R)
	type 'a t = 'a M.t A.t
	let unit_good = A.Prod (R.unit, M.unit)
	let unit_bad = A.atom (M.unit)
      end

O'Caml infers the types

      val unit_good : 'a M.t A.t
      val unit_bad : '_a M.t A.t

respectively?  As one might guess, I want to make [['a Group.t]]
abstract (because I want to hide a more complicated structure), in
which case the definition of [[unit_good]] will not work any more.  At
the same time, I need [[unit]] to have type [['a FreeRing.t]] and not
[['_a FreeRing.t]].

The usual tricks for functions with non generalizable argument types
don't work.  What can one do for constants?  Or am I overlooking
something obvious?

Merci,
-Thorsten
-- 
Thorsten Ohl, Physics Department, TU Darmstadt -- ohl@hep.tu-darmstadt.de
http://heplix.ikp.physik.tu-darmstadt.de/~ohl/ [<=== PGP public key here]