let parse_expr (s : string) : Ast.expr =
  let lexbuf = Lexing.from_string s in
  let string_source : Asai.Range.string_source = {
    title = None;
    content = s;
  } in
  let source = `String string_source in
  Eff.run ~env:source @@ fun () ->
  try Grammar.start_expr Lexer.token lexbuf with
  | Lexer.IllegalCharacter illegal_char ->
    Reporter.illegal_character ~loc:(Asai.Range.of_lexbuf ~source lexbuf) illegal_char
  | Grammar.Error ->
    Reporter.syntax_error ~loc:(Asai.Range.of_lexbuf ~source lexbuf)