Serializing Hashtbls with [@to_yojson]: Unbound value Hashtbl.to_yojson

[dexterleng]

I am trying to serialize Hashtbls as associative lists in JSON.

Following the example in the docs:

let yojson_of_kgram_lookup m =
    [%to_yojson: (int * kgram list) list] (Hashtbl.to_alist m)

type project_file = {
  file_name: string;
  selected_kgrams_by_hash: ((int, kgram list) Hashtbl.t) [@to_yojson yojson_of_kgram_lookup];
} [@@deriving yojson]

I get this error:

project_compare.ml|1 col 1 error| Unbound value Hashtbl.to_yojson      

[ttamttam]

I was to report the following in a new issue, but it seems to me it is the same problem?

I would have expected

`Assoc [("f1", `Int 2); ("f2", `Int 2)]?

in the following?

─( 19:31:23 )─< command 0 >────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # #require "ppx_deriving_yojson";;
─( 19:31:24 )─< command 1 >────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # type ty = { f1: int [@to_yojson fun i -> `Int(succ i)]; f2: int} [@@deriving to_yojson];;
type ty = { f1 : int; f2 : int; }
val ty_to_yojson : ty -> Yojson.Safe.t = <fun>
─( 19:31:39 )─< command 2 >────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # ty_to_yojson {f1:1; f2:2};;
utop # ty_to_yojson {f1=1; f2=2};;
- : Yojson.Safe.t = `Assoc [("f1", `Int 1); ("f2", `Int 2)]