From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>

> I don't yet understand exactly how private abbreviations are supposed to 
> work.  The currently implemented behaviour doesn't accomplish what I 
> understand to be the intention.  For example, given
> 
>     module Nat :
>     sig
>       type t = private int
>       val z : t
>       val s : t -> t
>     end =
>     struct
>       type t = int
>       let z = 0
>       let s = (+) 1
>     end
> 
> we can write
> 
>     # (((Nat.z : Nat.t :> int) - 1) :> Nat.t);;
>     - : Nat.t = -1

It looks like you've found a serious bug in the current
implementation. Actually you don't even need a coercion:

  # (((Nat.z : Nat.t :> int) - 1) : Nat.t);;
  - : Nat.t = -1

This should certainly not be accepted, as you can see by writing

# (((Nat.z : Nat.t :> int) - 1) : int :> Nat.t);;  (* fails *)

I think I know the reason, which is probably related to the way
abbreviation expansions are cached, but this needs more investigating.

Jacques Garrigue