Using "include" instead of "open" would work, ie. turning your example into:

module Vec_main = struct
   type t = { x: int; y: int }
   let make x y = {x;y}
   let add a b = {x=a.x+b.x; y=a.y+b.y}
end

module Vec = struct
   include Vec_main
   module Type = struct
     include Vec_main
     ...
   end
end

Then:
# let n = Vec.make 2 5;;
val n : Vec.t = {Vec.x = 2; Vec.y = 5}
# open Vec.Type;;
# let m = {x=1;y=2};;
val m : Vec.Type.t = {x = 1; y = 2}
# Vec.add m n;;
- : Vec.t = {Vec.x = 3; Vec.y = 7}

Cheers

-- 
Vincent Aravantinos - Postdoctoral Fellow, Concordia University, Hardware Verification Group


On 11/02/2011 03:41 PM, Anthony Tavener wrote:
> I've been struggling with this occasionally...
>
> I'm using nested modules to "open" access to select features of a 
> module. My problem is I can't find a way to *expose* types in the 
> parent module through such nested modules.
>
> A simplified example of what I'm looking at:
>
>   module Vec = struct
>
>     type t = { x: int; y: int }
>     let make x y = {x;y}
>     let add a b = {x=a.x+b.x; y=a.y+b.y}
>
>     module Type =
>       (* something which has type t = Vec.t,
>        * with exposed structure when "open"ed.
>        * Also note that Vec is not really an
>        * explicit module like this; instead it
>        * is implemented in vec.ml <http://vec.ml> *)
>   end
>
> Example usage...
>
>   let n = Vec.make 2 5
>   open Vec.Type
>   let m = {x=1;y=2}
>   Vec.add m n
>
>
> To date, I've defined the type in the Type submodule, which is then 
> used by the parent module. The unsatisfactory quality of this is that 
> Vec.Type.t is the "true" type. Ideally the concrete type would live at 
> Vec.t, with "open Vec.Type" bringing the fields of the type into scope.
>
> As background, here are examples of opening different features of the 
> Vec module:
>
>   let c = Vec.add a b
>
>   open Vec.Prefixed
>   let c = vadd a b
>
>   open Vec.Ops
>   let c = a +| b
>
>   open Vec.Type
>   let c = Vec.add a {x;y;z=0.}
>
> Apologies if this is really beginner-list material. It's minor, but 
> has been bugging me.
> Thank-you for looking,
>
>  Tony
>