I see that you solved your problem in a way you find satisfying, but I
would like to point out that the reason why your original code didn't
work isn't exactly what you seem to think.
When you define a submodule, the types defined before in the parent
modules are perfectly accessible and can be referred, just as you
would do when referring to types defined at the toplevel. You need not
qualify the type with the outer module name (Vec.t in your example),
as you are still *inside* this parent module.
module Vec = struct
type t = int
module Type = struct
type u = t
end
end
(1 : Vec.Type.u);;
The problem in your case is that you wish to give the same name to the
type in Vec and in Vec.Type. This would lead to the following:
... module Type = struct type t = t end ...
But this is ill-defined : it is a recursive type defined as being
itself. The problem is that the OCaml syntax for type declarations
always consider them recursive (for values you have "let" and "let
rec", for types you have "type" which behaves like "type rec" with no
opt-out way possible). This is a flaw of the OCaml syntax which is
relatively well-known, see eg.
http://ocaml.janestreet.com/?q=node/25A workaround is to define your inner type "t" in two steps, using an
different intermediate name to break the cycle:
module Vec = struct
type t = int
module Type = struct
type u = t
type t = u
end
end
(1 : Vec.Type.t);;
On Wed, Nov 2, 2011 at 10:14 PM, Anthony Tavener
<
anthony.tavener@gmail.com> wrote:
Oops, I didn't do a group-reply... so in case anyone is interested in what I
ended up with:
---------- Forwarded message ----------
From: Anthony Tavener <anthony.tavener@gmail.com>
Date: Wed, Nov 2, 2011 at 2:50 PM
Subject: Re: [Caml-list] Nested module exposing type from parent?
To: Vincent Aravantinos <vincent.aravantinos@gmail.com>
Actually, better than I initially thought...
I keep this as I have them defined already, except as you said: include
instead of open.
module Vec = struct
module Type = struct
type t = { x: int; y: int }
end
include Type
let make x y = {x;y}
let add a b = {x=a.x+b.x; y=a.y+b.y}
end
Before, I had instead of the include:
type t = Type.t
open Type
Which worked, but then the type used everywhere was Vec.Type.t
Thanks again! Simple and effective, and I was looking in all the wrong
places. :)
On Wed, Nov 2, 2011 at 2:36 PM, Anthony Tavener <anthony.tavener@gmail.com>
wrote:
Thank-you Vincent!
Though this requires a home for the "source type" module, at least the
types come out right in the end. Thanks!
And this led me to read specifically about include to understand what it
really does. :)
On Wed, Nov 2, 2011 at 2:19 PM, Vincent Aravantinos
<vincent.aravantinos@gmail.com> wrote:
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 *)
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
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