implicits.ml

(* try this with 4.02.0+modular-implicits *)
module type Show = sig
  type t
  val show : t -> string
end

implicit module Lam{X:Show} = struct
  type t = [`Var of string | `App of X.t * X.t | `Abs of string * X.t]
  let show : t -> string = function
      |`Var(x) -> x
      |`App(t1,t2) -> "(" ^ X.show t1 ^ " " ^ X.show t2 ^ ")"
      |`Abs(x,t) -> "(λ" ^ x ^ "." ^ X.show t ^ ")"
end

let print {M:Show} x = print_endline (M.show x)
;;


type t = [`Var of string | `App of 'a * 'a | `Abs of string * 'a] as 'a

let rec v : t = `Abs("x",`App(v,`Var("x")));;

let _ = print v;; (* Error: Termination check failed when searching for implicit M. *)








(* try this on http://andrewray.github.io/iocamljs/modimp_show.html *)

module type Show = sig
  type t
  val show : t -> unit
end

let print (implicit S : Show) x = S.show x

module Lam(X:Show) = struct
  type t = [`Var of string | `App of X.t * X.t | `Abs of string * X.t]
  let show = function
      |`Var(x) -> print_string x
      |`App(t1,t2) -> print_string "(" ; X.show t1 ; print_string " " ; X.show t2 ; print_string ")"
      |`Abs(x,t) -> print_string "(λ" ; print_string x ; print_string "." ; X.show t ; print_string ")"
end

module rec LamF : Show
         with type t = [`Var of string | `App of 'a * 'a | `Abs of string * 'a] as 'a =
  Lam(LamF)

implicit module LamF_ = LamF;;  

print (`Var("x") : LamF.t);;  (* OK *)
print (`Var("x"));; (* FAIL *)