OK the other option I was hoping for was that I was missing a simple
solution, and one was provided, so thank you!


On Thu, Aug 27, 2015 at 9:35 AM, Gabriel Scherer <gabriel.scherer@gmail.com>
wrote:

> I don't follow. Doesn't
>
>   module MakeBetter (K : sig include Map.OrderedType val show : t ->
> string end) =
>   struct
>     include Map.Make(K)
>     let show show_val m = List.show (Pair.show K.show show_val) (bindings
> m)
>   end
>
> suit your need?
>
> The extensibility problem I see is that you cannot use the map's
> internal representation because it is an abstract type -- which
> prevents from efficiently implementing certain operations -- but this
> is unrelated to being a functor.
>
> On Thu, Aug 27, 2015 at 3:29 PM, Yotam Barnoy <yotambarnoy@gmail.com>
> wrote:
> > One problem I've commonly encountered in OCaml is the inability to expand
> > the interface of functors after they've been created (I'm not talking
> about
> > post-application). For example, Map.Make in the stdlib takes an
> OrderedType
> > module which contains only the compare function. What happens if I want
> to
> > add something to this interface, such as a show function? I have to copy
> the
> > whole implementation of Map.Make into my own file to modify it. Compare
> this
> > to the ability to 'include' a regular module and just add the new
> > functionality, and to take the type of a module and expand that type as
> > needed. Functors are severely lacking in this regard.
> >
> > What do people think of this idea -- of allowing functors to be expanded?
> > Ideally, expanding a functor would allow for both replacing its argument
> > type (as in the example I gave) and for adding a second/third functor
> > argument type (so Map.Make(OrderedType) would become
> > Map.Make(OrderedType)(Show).
> >
> > -Yotam
>