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(* arch-tag: Documentation for pa_bparser *)
(*
Copyright (C) 2004 John Goerzen <jgoerzen@complete.org>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*)
(** Backtracking Parser Syntax Extension *)
(** {1 Introduction}
NOTE: THIS MODULE IS ONLY USABLE AS A CAMLP4 EXTENSION; SEE BELOW FOR
DETAILS.
The standard camlp4 system comes with the syntax extension [pa_op.cmo] which
adds support for the [parser] keyword to the OCaml language.
This is nice, but it lacks backtracking. Here is an illustration of
the problem. Assume you have the following parser defined:
{[let p = parser
[< '3; '1; '4 >] -> "hey"
| [< '3; '4 >] -> "there";;]}
Calling the parser with the stream [[< '3; '1; '4 >]] will produce the result
["hey"] as expected. However, calling it with the stream [[< '3; '4 >]] will
produce an error.
The built-in [parser] will use only the first element in a stream to determine
which of the alternative rules to pursue. Since the first element here is
[3], it will choose the first stream; the second one will be completely
ignored.
The [pa_bparser] module is a complete replacement for [pa_op]. It supports
everything [pa_op] does but adds a new keyword: [bparser]. A [bparser] is
defined just like a [parser], but it supports a key feature: backtracking.
Thus, it solves this problem. You could define your parser like this:
{[let p = bparser
[< '3; '1; '4 >] -> "hey"
| [< '3; '4 >] -> "there";;]}
and it will work as expected.
{1 Use}
This module defines the [ocamlfind] module [missinglib.pa_bparser]. You can
use it like this:
{[ocamlfind ocamlc -linkpkg -o test -package missinglib.pa_bparser -syntax camlp4o test.ml]}
*)
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