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Date: Sun, 23 Jun 2013 17:14:50 +0200
From: Xavier Leroy <Xavier.Leroy@inria.fr>
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Subject: Re: [Caml-list] Anonymous sum types in functors

On 23/06/13 09:16, David Allsopp wrote:

> a) Is there a way to do it where you can end up with type elt = A | B (I
> think the answer is no?)

No, because that wouldn't quite respect the generativity of sum type
definitions, but you can get close, see below.

> b) Is there a syntactically lighter way to write the module definition?

I would recommend naming your anonymous "struct":

module Flag = struct
  type t = A | B
  let compare = compare
  (* Other useful operations over flags, e.g. *)
  let to_string = function A -> "A" | B -> "B"
end

module FlagSet = Set.Make(Flag)

Then you get

FlagSet: sig
  type elt = Flag.t
  type t
  ...
end

The good thing about this solution is that it gives you a natural
place to put additional operations over flags, like "to_string" above,
shall you ever need them.

> the idea is that I'd
> be able to write Flag1Set.add Flag1Set.CommonFlag Flag1Set.empty and
> Flag2Set.add Flag2Set.CommonFlag Flag2Set.empty, etc.

Why not "Flag1Set.add Flag1.CommonFlag Flag1Set.empty"?  Flags morally
come from Flag1, not from Flag1Set.

As an aside, some ML-like module systems require functor arguments to
be module names or paths, e.g. Coq's.  OCaml accepts anonymous structs
as arguments, and does the best it can with them, but if the struct
contains generative type definitions, some information is necessarily
lost.

Hope this helps,

- Xavier Leroy