Question

I’m using ocamlyacc and ocamllex. I have an error production in my grammar that signals a custom exception. So far, I can get it to report the error position:

| error { raise (Parse_failure (string_of_position (symbol_start_pos ()))) }

But, I also want to know which token was read. There must be a way—anyone know?

Thanks.

Answer 1

Tokens are generated by lexer, hence you can use the current lexer token when error occurs :

  let parse_buf_exn lexbuf =
    try
      T.input T.rule lexbuf
    with exn ->
      begin
        let curr = lexbuf.Lexing.lex_curr_p in
        let line = curr.Lexing.pos_lnum in
        let cnum = curr.Lexing.pos_cnum - curr.Lexing.pos_bol in
        let tok = Lexing.lexeme lexbuf in
        let tail = Sql_lexer.ruleTail "" lexbuf in
        raise (Error (exn,(line,cnum,tok,tail)))
      end

Lexing.lexeme lexbuf is what you need. Other parts are not necessary but useful.
ruleTail will concat all remaining tokens into string for the user to easily locate error position. lexbuf.Lexing.lex_curr_p should be updated in the lexer to contain correct positions. (source)