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Date: Tue, 23 Jun 2015 17:50:11 +0900
From: Oleg <oleg@okmij.org>
To: caml-list@inria.fr
Cc: asai@is.ocha.ac.jp
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Subject: [Caml-list]  Labels at the module level?


> I want to define a functor that accepts a module of sig:
> module type A_t = sig
>   val x : int
>   val y : int
> end
> but if the user did not specify some of the values in the signature,
> some default values will be used.

It is even easier at the module level: include comes in handy. Suppose
we have a consumer

module Universe(Params: A_t) = struct
  let r () = Printf.printf "x = %d y = %d\n" Params.x Params.y
end;;

First, we define defaults:

module Def : A_t = struct let x = 1 let y = 2 end;;

We can invoke it as

let module M = Universe(Def) in M.r ();;

and also as

let module M = Universe(struct include Def let x = 100 end) in M.r ();;

BTW, groups of related parameters could be further grouped into
submodules.