Re: [Caml-list] polymorphic variant question

[Andreas Rossberg <rossberg@ps.uni-sb.de>]

Charles Martin wrote:
> 
> I have a number of modules A, B, C that share a single data structure
> between them, all under the control of a central module.  Each wants to
> store different kinds of data in the data structure.  I can create a
> variant type to classify the data by module:
> 
> data.mli:
> type ('a, 'b, 'c) t = A of 'a | B of 'b | C of 'c
> 
> So each of the individual modules then has code that looks like this:
> 
> a.ml:
> type ('b, 'c) t = {
>   data : (int, 'b, 'c) Data.t list ref;
> }
> 
> b.ml:
> type ('a, 'c) t = {
>   data : ('a, string, 'c) Data.t list ref;
> }
> 
> Then the central module can have code like this:
> 
>   let data = ref [] in
>   let a = { A.data = data } in
>   let b = { B.data = data } in
>   let c = { C.data = data } in
> 
> The trouble with this approach is that every time I add a new module, I
> must add a new tag to the classify data type, and update all of my
> signatures and type declarations in modules A, B, C, etc.  This seems wrong
> since the whole point is to separate these abstractions.
> 
> I am hoping I can use polymorphic variants to escape this trap.  Thus, the
> code for an individual module would look like this:
> 
> a.ml:
> type t = {
>   data : [`A of int] list ref;
> }
> 
> Of course, then in the central module the type [`A of a_data] list ref
> conflicts with the type [`B of b_data] list ref and [`C of c_data] list
> ref.

Did you try something along the lines of

(* data.mli *)
type 'a t = {data : 'a list ref}

(* a.mli *)
type 'a t = 'a Data.t constraint 'a = [> `A of a_data]

(* b.mli *)
type 'a t = 'a Data.t constraint 'a = [> `B of b_data]

(* central.ml *)
let data = {data = ref []}
let a = data : 'a A.t
let b = data : 'a B.t

Best regards,

	- Andreas

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids; I mean if Pac Man affected us
 as kids, we would all be running around in darkened rooms, munching
 magic pills, and listening to repetitive electronic music."
 - Kristian Wilson, Nintendo Inc.
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