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Extending modules and signatures
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Date: 2009-04-17 (20:51)
From: Peter Hawkins <hawkinsp@c...>
Subject: Extending modules and signatures

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

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 : ...

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?