Example 1: BNF

simple

line_comment = "%".

Expr : expr -> ID PLUS expr.
Id : expr -> ID.

PLUS = "+".
ID = "x".

%{
  (* auto-generated by gt *)
  open Simple_syntax;;
  let parse_error s =
	 let error = 
		s^(Simple_util.string_of_pos 
			  (Simple_util.cur_pd())) 
	 in 
	 failwith error;;
%}

%start main
%token EOF
%token <Simple_syntax.__term_not_in_ast__> ID PLUS
%token <Simple_syntax.__terminal__>
%type <Simple_syntax.expr option> main
%type <Simple_syntax.expr> expr
%%

main:
  | expr { Some($1) }
  | EOF { None }

expr:
  | ID PLUS expr { Expr(get_term_pd_not_in_ast $1, $1, $2, $3) }

expr:
  | ID { Id(get_term_pd_not_in_ast $1, $1) }

(* auto-generated by gt *)

open Simple_util;;

(* This type is used for terminals.
	We do this to have better position data.*)
type __terminal__ = (pd * string);;
type __term_not_in_ast__ = pd;;
type dummy = Dummy 
and expr = 
  | Expr of pd * __term_not_in_ast__ * __term_not_in_ast__ * expr 
  | Id of pd * __term_not_in_ast__;;

(* pd stands for pos (position) *)
let rec dummy () = () 
and get_terminal_pd = function
  | (pd,_) -> pd 
and get_term_pd_not_in_ast = function
  | (pd) -> pd 
and pd_expr = function 
  | Expr(pd,_,_,_) -> pd
  | Id(pd,_) -> pd;;
let pd e = pd_expr e;;