let one_line text =
  match String.index_opt text '\n' with
  | Some index -> String.sub text 0 index
  | None -> text

let unexpected text lexbuf =
  let start = Lexing.lexeme_start_p lexbuf in
  let stop = Lexing.lexeme_end_p lexbuf in
  let token =
    if start.Lexing.pos_cnum >= String.length text then "end of file"
    else "`" ^ one_line (Lexing.lexeme lexbuf) ^ "`"
  in
  Diagnostic.error (Location.span_of_lexing start stop) "syntax error: unexpected %s" token

let expression text =
  let lexbuf = Lexing.from_string text in
  try Parser.expr Lexer.token lexbuf with Parsing.Parse_error -> unexpected text lexbuf