Re: [Caml-list] Implicit question about searching for implicit module definition in the functor argument.

["Gawlitza, Thomas" <thomas.gawlitza@sap.com>]

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Hi!

Your question inspired me to have a look at the modular implicits. I really like them. It seems that the argument of the implicit functor Show_logic also needs to be implicit, i.e., in curly braces. At least this worked for me.

Thomas

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

let show {S : SHOW} x = S.show x;;

implicit module Show_int : (SHOW with type t = int) = struct
    type t = int
    let show = string_of_int
end

module type CORE = sig
  type 'a logic

  implicit module Show_logic {X: SHOW} : (SHOW with type t = X.t logic)
end

module Make(MK: CORE) = struct
  open MK
  (* open implicit MK (* doesn't help *) *)

  let foo (x: int logic) =
    let s = show x in (* Error: No instance found for implicit S. *)
    s
end


---

Dr. rer. nat. Thomas Martin Gawlitza, Diplom-Informatiker
Development Architect
Technology Office
office location: Potsdam (DE), POT04, A2.01, SAP Innovation Center, Konrad-Zuse-Ring 10, 14469 Potsdam
email: Thomas.Gawlitza@sap.com
phone: +49 331 9799 5408
mobile: +49 151 168 10 726

From: <caml-list-request@inria.fr> on behalf of Kakadu <kakadu.hafanana@gmail.com>
Reply-To: Kakadu <kakadu.hafanana@gmail.com>
Date: Wednesday 3 August 2016 at 01:17
To: Caml List <caml-list@inria.fr>
Subject: [Caml-list] Implicit question about searching for implicit module definition in the functor argument.

Hey, hackers.

I'm missing something obvious. Any fresh eyes around?

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

let show {S : SHOW} x = S.show x;;

implicit module Show_int : (SHOW with type t = int) = struct
    type t = int
    let show = string_of_int
end

module type CORE = sig
  type 'a logic

  implicit module Show_logic(X: SHOW) : (SHOW with type t = X.t logic)
end

module Make(MK: CORE) = struct
  open MK
  open implicit MK (* doesn't help *)

  let foo (x: int logic) =
    let s = show x in (* Error: No instance found for implicit S. *)
    s
end


Kind regards,
Kakadu

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