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Date: Wed, 17 Mar 2010 18:36:29 +0100
From: Alain Frisch <alain@frisch.fr>
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To: Dario Teixeira <darioteixeira@yahoo.com>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Lazy modules
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X-Spam: no; 0.00; frisch:01 frisch:01 functor:01 sig:01 ocaml:01 functor:01 struct:01 sig:01 val:01 struct:01 foo:01 val:01 config:01 config:01 wrote:01 

On 17/03/2010 18:04, Dario Teixeira wrote:
> In a sense I need a lazy module.  Also, note that I find the solution
> of declaring all values in that module as lazy a bit inelegant.
>
> I do have a solution which though hackish and relying on a language
> extension (local modules) seems to work: I create the module on demand
> via a functor parameterised over an empty sig:

Here is a variant of your version with first-class modules (which will 
be available in OCaml 3.12). Compared to your version, we get the 
benefit of laziness (the "functor body" is evaluated only once).


module Config =
   struct
     let x = ref "XXX"
   end

module type S =
   sig
     val x: string
   end

let lazy_module =
   lazy
     (
      let module M =
        struct
          let x = !Config.x
        end
      in
      (module M : S)
     )

let foo () =
   let module M = (val Lazy.force lazy_module : S) in
   M.x


-- Alain