OCaml: how to use batteries and ppx_deriving.* together?

Question

Currently I'm trying to use Batteries with ppx_deriving.show or something similar.
I am wondering how to use them together usefully.

To create dumping functions, I feel ppx_deriving.show is useful. But I have a little bit troubles using them together like the followings.

open Batteries
type t = { a: (int,int) Map.t }
[@@deriving show]

Now Map.pp is not defined, so it can't be compiled.

My adhoc fix is that I create module Map which include Batteries.Map and define function pp.

open Batteries
module Map = struct
  include Map
  let pp f g fmt t = ... (* create dump function by man hand *)
end

type t = { a: (int,int) Map.t }
[@@deriving show]

It works, but it is painful for me to adapt all of data structures...
Core with ppx_deriving.sexp is an alternative choice, but I prefer Batteries with ppx_deriving.show. Does anybody know how to solve the problem?

Answer 1

Your fix is the right way. If you want to use deriving for data types M.t declared without [@@deriving], you have to give its methods such as M.pp for show by yourself:

module M = struct
  include M
  let pp = ... (* code for pretty-printing M.t *)
end

There is a way to partially automate this:

module M = struct
  include M
  type t = M.t = ... (* the same type definition of M.t *)
    [@@deriving show]
end

It generates M.pp for type t using deriving.

With ppx_import, you can avoid copy-and-pasting of the definition:

module M = struct
  include M
  type t = [%import: M.t]
    [@@deriving show]
end

This should be expanded to the former code.

As you have found out, deriving show of Map.t is not really useful though: normally you do not want to see the binary tree representation of Map.t unless you are debugging Map module itself.