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From: Michael Vanier <mvanier@cs.caltech.edu>
To: martin_jambon@emailuser.net
Cc: caml-list@inria.fr
In-reply-to: <Pine.LNX.4.40.0403211617410.1400-100000@localhost> (message from
	Martin Jambon on Sun, 21 Mar 2004 16:53:38 +0800 (HKT))
Subject: Re: [Caml-list] extensible records again
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Interesting.  I'll try it.  Thanks!

Mike

> Date: Sun, 21 Mar 2004 16:53:38 +0800 (HKT)
> From: Martin Jambon <martin_jambon@emailuser.net>
> Cc: caml-list@inria.fr
> 
> 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;
>   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;
>   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...
> 
> Martin
> 
> 

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