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Date: Tue, 18 Nov 2008 23:07:33 +0100
From: Alain Frisch <alain@frisch.fr>
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To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Wanted: your feedback on the hierarchy of OCaml	Batteries
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X-Spam: no; 0.00; frisch:01 frisch:01 ocaml:01 struct:01 sig:01 val:01 struct:01 idioms:01 wrote:01 extensible:01 caml-list:01 int:01 alain:01 alain:01 interfaces:01 

On 11/18/2008 7:17 PM, Jon Harrop wrote:
> I don't follow. Can you not use "include" to extend an existing module:
>
> # module Array = struct
>      include Array
>      let empty = [||]
>    end;;
> module Array :
>    sig
>      external length : 'a array ->  int = "%array_length"
>      ...
>      val empty : 'a array
>    end

In addition to this being non-modular, this extension scheme does not 
work well with hiararchy as it forces you to mention all the siblings of 
the ancestors of the module you want to extend.

E.g. if you start from:

module M = struct
   module M1 = struct
    module M11 = struct ... end
    module M12 = struct ... end
    module M13 = struct ... end
    ...
   end
   module M2 = struct
    ...
   end
   module M3 = struct
    ...
   end
   ...
end

and you want to extend M11, you need to write:

module M' = struct
   module M1 = struct
     module M11 = struct include M.M1.M11 (* extension here *) end
     module M12 = M.M1.M12
     module M13 = M.M1.M13
     ...
   end
   module M2 = M.M2
   module M3 = M.M3
   ...
end


Frankly, I don't think that having a nice and well-organized hierarchy 
of modules really matters. Things like having uniform interfaces, 
consistent idioms and compatible types across libraries seem much more 
important to me. Anyway, if a hierarchy is desired, I fail to see any 
advantage of using "." instead of e.g. "_" (easily extensible + does not 
force you to link everything).

-- Alain