Re: [Caml-list] Open datatypes

[Markus Mottl <markus.mottl@gmail.com>]

Hi,

it is possible to create open (i.e. extensible) datatypes with
polymorphic variants, even allowing recursion.  E.g.:

  type 't add_open = [ `Add of 't * 't ]
  type 't sub_open = [ `Sub of 't * 't ]

  let eval_add_open eval (`Add (l, r)) = eval l + eval r
  let eval_sub_open eval (`Sub (l, r)) = eval l - eval r

"add_open" and "sub_open" are clearly completely independent from each
other, both the datatypes and the evaluation functions.

Now we combine these two datatypes and evaluation functions, still
leaving the result extensible:

  type 't add_sub_open = [ 't add_open | 't sub_open ]

  let eval_add_sub_open eval = function
    | #add_open as t -> eval_add_open eval t
    | #sub_open as t -> eval_sub_open eval t

Eventually you will want to "close" the extensible definitions for
actual use.  This basically just means tying the open ends:

  type add_sub = add_sub add_sub_open

  let rec eval_add_sub t = eval_add_sub_open eval_add_sub t

In my experience using polymorphic variants for that purpose is hands
down the most elegant way of achieving extensibility and
composability.  It is especially useful for creating domain-specific
languages that can be quickly combined and extended.

Regards,
Markus

On Thu, Jul 14, 2011 at 09:38,  <lpw25@cam.ac.uk> wrote:
>
> Hi all,
>
> I was wondering whether there was any particular reason why OCaml
> doesn't allow the user to create open extensible datatypes like exn.
>
> I know that something similar can be created using local exceptions:
>
> module T = struct
>  type t = exn
>
>  type 'a tvariant = (('a -> t), (t -> 'a option))
>
>  let createVariant (type s) () =
>    let module M = struct exception E of s end in
>      (fun x -> M.E x), (function M.E x -> Some x | _ -> None)
>
>  let mkTVariant ((cnstr, _) :  'a tvariant) (x: 'a) = cnstr x
>
>  let matchTVariant ((_, destr) : 'a tvariant) (xt: t) = destr xt
> end
>
> but it isn't very neat, and it seems that it would not be that difficult to
> allow the user to declare types with the same properties as exn.
>
> Thanks,
>
> Leo
>
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-- 
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com