test.ml

open Printf
open Parsetree
open Asttypes
open Ast_helper

let create_loc ?(loc = !default_loc) value = {
    txt = value
  ; loc = loc
  }

let structure_item mapper item =
  match item.pstr_desc with
  | Pstr_type declarations ->
    printf "found type\n";
    Str.attribute (create_loc "test", PStr [])
  | _ -> Ast_mapper.(default_mapper.structure_item mapper item)

let structure mapper strct =
  List.map (fun x -> x) strct


let item_mapper =
  Ast_mapper.{
    default_mapper with
    structure_item = structure_item;
    structure = structure
  }

let () =
  Ast_mapper.run_main (fun argv -> item_mapper)