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[Caml-list] extensible records again
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Date: 2004-03-21 (08:55)
From: Martin Jambon <martin_jambon@e...>
Subject: Re: [Caml-list] extensible records again
On Sat, 20 Mar 2004, Michael Vanier wrote:

> So I tried this.  Aside from the pain of having to change data to 'a data in
> lots of places (which I can live with), I got bitten by the polymorphic
> reference limitation.  Specifically, I have a mutable stack of data values
> which became a mutable stack of 'a data values, but the function which
> creates the mutable stack is of type '_a data, so it doesn't type check.  I
> can't figure out any way around this.  Basically, my main data type _cannot_
> be parameterized.

Keep your code polymorphic: create your stack in the same compilation
unit as the full type instanciation.
Then you have to pass your stack as a parameter
to all functions that use it.

I guess this is more or less what you do:

(* a.ml *)
let stack =
  let st = Stack.create () in
  Stack.push (`Int 1) st;
(* compiler complains about '_ *)

(* b.ml *)
let some_function x =
   match Stack.pop stack with
       `Int i -> print_int i
     | `Custom s -> print_string s
     | _ -> ()

Instead you should write:

(* a.ml *)
let init_stack () =
  let st = Stack.create () in
  Stack.push (`Int 1) st;

(* b.ml *)
let some_function stack x =
  match Stack.pop stack with
      `Int i -> print_int i
    | `Custom s -> print_string s
    | _ -> print_float x

let _ =
  let stack = A.init_stack () in
  some_function stack 1.2

I hope it answers your question...


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