type command = [ `Roundtrip ]

let command_of_yojson = function
  | `String "Roundtrip" -> Ok `Roundtrip
  | _ -> Error "invalid command"

type request = { expr : string; command : command }
[@@deriving of_yojson { exn = true }]

type response_expr = { expr : string } [@@deriving to_yojson { exn = true }]

type response = [ `Error of string | `Expr of response_expr ]
[@@deriving to_yojson { exn = true }]

let%expect_test _ =
  Yojson.Safe.to_channel stdout (response_to_yojson (`Error "foo"));
  [%expect {| ["Error","foo"] |}]

let%expect_test _ =
  Yojson.Safe.to_channel stdout (response_to_yojson (`Expr { expr = "foo" }));
  [%expect {| ["Expr",{"expr":"foo"}] |}]
(*
type position = [%import: Lexing.position] [@@deriving show]

type location = [%import: (Location.t[@with Lexing.position := position])]
[@@deriving show]

type constant = [%import: Parsetree.constant] [@@deriving show]
type expression_desc = [%import: Parsetree.expression_desc] [@@deriving show]
type expression = [%import: Parsetree.expression] [@@deriving show]

type structure_item_desc = [%import: Parsetree.structure_item_desc]
[@@deriving show]

type structure_item = [%import: Parsetree.structure_item] [@@deriving show]
type structure = [%import: Parsetree.structure] [@@deriving show]
type toplevel_phrase = [%import: Parsetree.toplevel_phrase] [@@deriving show]

let parse ~path (src : string) =
  let buf = Lexing.from_string src in
  buf.lex_start_p <- { buf.lex_start_p with pos_fname = path };
  buf.lex_curr_p <- { buf.lex_curr_p with pos_fname = path };
  Parse.expression buf

let () =
  parse ~path:"main.ml" " print_endline ((\"Hello, world!\") )"
  |> Format.fprintf Format.std_formatter "\n%a\n" Pprintast.expression
*)

let handle_request { expr; command } : response =
  try
    let buf = Lexing.from_string expr in
    let expr = Parse.expression buf in
    match command with
    | `Roundtrip ->
        let buf = Buffer.create 16 in
        (let fmt = Format.formatter_of_buffer buf in
         Pprintast.expression fmt expr;
         Format.pp_print_flush fmt ());
        `Expr { expr = Buffer.contents buf }
  with
  | Failure msg -> `Error msg
  | exn -> `Error ("uncaught exception: " ^ Printexc.to_string exn)