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Extending modules and signatures
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Date: -- (:)
From: Ashish Agarwal <agarwal1975@g...>
Subject: Re: [Caml-list] Extending modules and signatures
This is a commonly requested feature. One issue is that a file a.ml creates
a module A. However, a file a.mli does not create a module type A. I'm not
sure why this is the case. Does anyone know if there is a specific reason?

On Fri, Apr 17, 2009 at 4:51 PM, Peter Hawkins <hawkinsp@cs.stanford.edu>wrote:

> Hi...
>
> I have a quick question. I want to extend the List module with various
> functions that I want that aren't present in the standard library,
> much as the Batteries ExtList library does.
>
> I might write the following code in "mylibrary.ml":
> module MyList = struct
>  include List
>  let foo x = ... code here
>  let bar y = ... code here
> end
>
> That's ok so far, but now suppose I want to write a "mylibrary.mli"
> interface file corresponding to "mylibrary.ml". Ideally I'd write
> something like this in "mylibrary.mli":
> module MyList : sig
>  include List  (* unknown module type List *)
>  val foo : ...
>  val bar : ...
> end
>
> Unfortunately I can't include "List" here since it is a structure, not
> a signature. I don't think there is a way to say "include the
> signature associated with List".
>
> I can think of three solutions:
> a) Copy the complete signature of List into MyList. This is a bad idea
> since the List module might change in the future. This is what the
> Batteries ExtList module does.
> b) Alter the List module to define a signature, say List.S, in
> addition to its other contents. I can't easily do this since I didn't
> write the List module.
> c) Don't write a .mli file at all.
>
> Are there any other alternatives?
>
> Cheers,
> Peter
>
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