Re: [Caml-list] Implicit module question

[Jeremy Yallop <yallop@gmail.com>]

On 9 September 2014 16:32, Yotam Barnoy <yotambarnoy@gmail.com> wrote:
> I have a question about the implicit module implementation: Is there any way to
> combine modules automatically? For example, suppose I have a Show implicit
> module and an Ord implicit module, and a function receives both, and wants
> to infer both functionalities for an incoming type so we can run both show
> and compare on the same type. Does the current model cover such a use-case?

Yes, it does.  The implicits language is essentially the same as
OCaml's module language, so you can use constraints/equations on the
module types in the regular way.  For example, suppose you have module
types for SHOW and ORD together with corresponding top-level
functions:

  module type SHOW =
  sig
    type t
    val show : t -> string
  end

  let show (implicit Show : SHOW) (x : Show.t) = Show.show x

  module type ORD =
  sig
    type t
    val compare : t -> t -> int
  end

  let compare (implicit Ord : ORD) (x : Ord.t) (y : Ord.t) = Ord.compare x y

You can write a function which operates on a value with implicit
instances for both SHOW and ORD by adding some kind of type
constraint.  For example, you might write:

   let f (implicit Show: SHOW) (implicit Ord: ORD with type t =
Show.t) (x : Show.t) y =
     if compare x y < 0 then show x else show y

or perhaps

   let f (type a) (implicit Show: SHOW with type t = a) (implicit Ord:
ORD with type t = a) (x : a) y =
     if compare x y < 0 then show x else show y

The inferred type is just as you'd expect:

  val f :
    (implicit Show : SHOW with type t = 'a) ->
    (implicit Ord : ORD with type t = 'a) ->
    'a -> 'a -> string

You might like to try out the implementation for yourself, either via
the opam switch or via the IOCaml top-level that Andrew Ray's set up:

   http://andrewray.github.io/iocamljs/modimp_show.html