Instead of making a functor, you could try a first-class module:

module type Person = sig
  val name : string
  val age : int
end

let make_person ?(name="anon") (age : int) : (module Person) =
  (module struct
    let name = name
    let age = age
  end)

On Tue, Jun 23, 2015 at 4:43 AM, Jeremy Yallop <yallop@gmail.com> wrote:

> On 23 June 2015 at 09:26, Kenichi Asai <asai@is.ocha.ac.jp> wrote:
> > Using labeled arguments, one can supply default values for unspecified
> > arguments:
> >
> > let f ?(x = 3) y = x * y
> > in f 5
> >
> > will return 15 without specifying the value of x at the second line.
> > Is there a way to do a similar thing at the module level?  Namely, 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.
> >
> > Background: in the universe library for OCaml:
> >
> > http://pllab.is.ocha.ac.jp/~asai/Universe/
> >
> > one specifies various handlers for events (such as tick, mouse, and
> > key events).  Currently, these handlers are registered to the big_bang
> > function that accepts them using labeled arguments (with default
> > values).  I wonder if I can specify the interface as a module
> > signature and let the user write a module of that signature but only
> > those handlers that the user is interested in.
>
> Alain Frisch once had a patch to add exactly what you're asking for as
> a language feature:
>
>      http://alain.frisch.fr/soft.html#patches   (Scroll to 'optional
> fields in modules')
>      http://alain.frisch.fr/info/patch-option-announce
>
> In the absence of such a feature, one approach is to use 'include' to
> provide the defaults:
>
>     module M
>       include M_defaults
>       let y = 3
>    end
>
> Jeremy.
>
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