Codemirror.ml

open Bonsai_web
open Core_kernel
open Js_of_ocaml

module Change = struct

  type position =
    { ch: int
    ; line: int
    }

  type t =
    { from: position
    ; to_: position
    ; text: string array
    ; removed: string
    (* ; origin *)
    }

    (*
  type js =
    {
    }
  *)

end

class type codemirror_type = object
  method on : string -> (Change.t -> unit) Js.callback -> unit Js.meth
end

let codemirror : Vdom.Node.t
  = let widget_id = Type_equal.Id.create ~name:"codemirror widget" (fun _ -> Sexp.Atom "<codemirror>") in
    Vdom.Node.widget
      ~id:widget_id
      ~init:(fun () ->
        let wrapper = Dom_html.document##createElement (Js.string "div") in
        let textarea = Dom_html.document##createElement (Js.string "textarea") in
        Dom.appendChild wrapper textarea;
        let codemirror_obj = Js.Unsafe.get Dom_html.window (Js.string "CodeMirror") in
        let create_fn = Js.Unsafe.get codemirror_obj (Js.string "fromTextArea") in

        let editor : codemirror_type Js.t
          = Js.Unsafe.fun_call create_fn [| Js.Unsafe.inject textarea |]
        in

        (* Here is where I get an error:

          VM366:1 Uncaught TypeError: args.slice is not a function
            at eval (eval at caml_call_gen (stdlib.js:61), <anonymous>:1:6)
            at caml_call_gen (stdlib.js:61)
            at jslib_js_of_ocaml.js:115
            at codemirror.js:2080
            at fireCallbacksForOps (codemirror.js:2037)
            at finishOperation (codemirror.js:2051)
            at endOperation (codemirror.js:3791)
            at HTMLTextAreaElement.<anonymous> (codemirror.js:3928)

         *)
        editor##on "change" (Js.wrap_callback (fun _change_obj -> Printf.printf "change"));

        (), wrapper)
      ()

(You probably when Js.js_string Js.t instead of string for the on method.)

A fix for you issue is available here ocsigen/js_of_ocaml#993

You're right, I've now switched to Js.js_string Js.t locally.