From: Akim Demaille <akim@epita.fr>
Date: Mon, 24 Apr 2006 09:45:57 +0000 (+0000)
Subject: * doc/bison.texinfo (Multiple start-symbols): New.
X-Git-Tag: v2.3b~422
X-Git-Url: https://git.saurik.com/bison.git/commitdiff_plain/ed2e6384ed2be6598e90d8551ef6d8daf2b65516?ds=sidebyside

* doc/bison.texinfo (Multiple start-symbols): New.
---

diff --git a/ChangeLog b/ChangeLog
index b4c238d5..758a5c36 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2006-04-24  Akim Demaille  <akim@lrde.epita.fr>
+
+	* doc/bison.texinfo (Multiple start-symbols): New.
+
 2006-04-24  Akim Demaille  <akim@lrde.epita.fr>
 
 	* etc/README, etc/bench.pl: New.
diff --git a/doc/bison.texinfo b/doc/bison.texinfo
index 01fa9bf2..9e65195d 100644
--- a/doc/bison.texinfo
+++ b/doc/bison.texinfo
@@ -311,6 +311,7 @@ Frequently Asked Questions
 * How Can I Reset the Parser:: @code{yyparse} Keeps some State
 * Strings are Destroyed::      @code{yylval} Loses Track of Strings
 * Implementing Gotos/Loops::   Control Flow in the Calculator
+* Multiple start-symbols::     Factoring closely related grammars
 * Secure?  Conform?::          Is Bison @acronym{POSIX} safe?
 * I can't build Bison::        Troubleshooting
 * Where can I find help?::     Troubleshouting
@@ -7727,6 +7728,7 @@ are addressed.
 * How Can I Reset the Parser::  @code{yyparse} Keeps some State
 * Strings are Destroyed::       @code{yylval} Loses Track of Strings
 * Implementing Gotos/Loops::    Control Flow in the Calculator
+* Multiple start-symbols::      Factoring closely related grammars
 * Secure?  Conform?::           Is Bison @acronym{POSIX} safe?
 * I can't build Bison::         Troubleshooting
 * Where can I find help?::      Troubleshouting
@@ -7927,6 +7929,55 @@ This topic is way beyond the scope of this manual, and the reader is
 invited to consult the dedicated literature.
 
 
+@node Multiple start-symbols
+@section Multiple start-symbols
+
+@display
+I have several closely related grammars, and I would like to share their
+implementations.  In fact, I could use a single grammar but with
+multiple entry points.
+@end display
+
+Bison does not support multiple start-symbols, but there is a very
+simple means to simulate them.  If @code{foo} and @code{bar} are the two
+pseudo start-symbols, then introduce two new tokens, say
+@code{START_FOO} and @code{START_BAR}, and use them as switches from the
+real start-symbol:
+
+@example
+%token START_FOO START_BAR;
+%start start;
+start: START_FOO foo
+     | START_BAR bar;
+@end example
+
+These tokens prevents the introduction of new conflicts.  As far as the
+parser goes, that is all that is needed.
+
+Now the difficult part is ensuring that the scanner will send these
+tokens first.  If your scanner is hand-written, that should be
+straightforward.  If your scanner is generated by Lex, them there is
+simple means to do it: recall that anything between @samp{%@{ ... %@}}
+after the first @code{%%} is copied verbatim in the top of the generated
+@code{yylex} function.  Make sure a variable @code{start_token} is
+available in the scanner (e.g., a global variable or using
+@code{%lex-param} etc.), and use the following:
+
+@example
+  /* @r{Prologue.}  */
+%%
+%@{
+  if (start_token)
+    @{
+      int t = start_token;
+      start_token = 0;
+      return t;
+    @}
+%@}
+  /* @r{The rules.}  */
+@end example
+
+
 @node Secure?  Conform?
 @section Secure?  Conform?