diff --git a/ChangeLog b/ChangeLog
index 572ebdc2..02d15de9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2009-09-09  Akim Demaille  <demaille@gostai.com>
+
+	lalr1.cc: syntax_error as exceptions.
+	It is common to use sort of factories in the user actions.  These
+	factories may check some "syntactic" constraints that are not
+	enforced by the grammar itself.  This is possible using YYERROR
+	within the action itself.  Provide the user with a means to throw
+	a syntax_error exception.
+
+	* data/c++.m4 (b4_public_types_declare, b4_public_types_define):
+	Declare and define yy::parser::syntax_error.
+	* data/lalr1.cc: Include stdexcept.
+	(yy::parser::parse): Wrap the user action within a try/catch.
+	* data/glr.cc: Include stdexcept.
+
 2009-09-09  Akim Demaille  <demaille@gostai.com>
 
 	lalr1.cc: add missing "inline".
diff --git a/data/c++.m4 b/data/c++.m4
index cfe8a6cb..3157e47b 100644
--- a/data/c++.m4
+++ b/data/c++.m4
@@ -126,6 +126,13 @@ m4_define([b4_public_types_declare],
     /// Symbol locations.
     typedef b4_percent_define_get([[location_type]]) location_type;])[
 
+    /// Syntax errors thrown from user actions.
+    struct syntax_error : std::runtime_error
+    {
+      syntax_error (]b4_locations_if([const location_type& l, ])[const std::string& m);]b4_locations_if([
+      location_type location;])[
+    };
+
     /// Tokens.
     struct token
     {
@@ -195,8 +202,15 @@ m4_define([b4_public_types_declare],
 # ----------------------
 # Provide the implementation needed by the public types.
 m4_define([b4_public_types_define],
-[[  // symbol_base_type.
+[[  inline
+  ]b4_parser_class_name[::syntax_error::syntax_error (]b4_locations_if([const location_type& l, ])[const std::string& m)
+    : std::runtime_error (m)]b4_locations_if([
+    , location (l)])[
+  {}
+
+  // symbol_base_type.
   template <typename Exact>
+  inline
   ]b4_parser_class_name[::symbol_base_type<Exact>::symbol_base_type ()
     : value()]b4_locations_if([
     , location()])[
diff --git a/data/glr.cc b/data/glr.cc
index 0d9e0496..87e3eab1 100644
--- a/data/glr.cc
+++ b/data/glr.cc
@@ -224,6 +224,7 @@ b4_copyright([Skeleton interface for Bison GLR parsers in C++],
 
 ]b4_percent_code_get([[requires]])[
 
+#include <stdexcept>
 #include <string>
 #include <iostream>
 
diff --git a/data/lalr1.cc b/data/lalr1.cc
index 8e79333e..ac2fa35b 100644
--- a/data/lalr1.cc
+++ b/data/lalr1.cc
@@ -147,6 +147,7 @@ dnl FIXME: This is wrong, we want computed header guards.
 ]b4_percent_code_get([[requires]])[
 
 ]b4_parse_assert_if([#include <cassert>])[
+#include <stdexcept>
 #include <string>
 #include <iostream>
 #include "stack.hh"
@@ -814,12 +815,21 @@ m4_ifdef([b4_lex_param], [, ]b4_lex_param)));])[
 
     // Perform the reduction.
     YY_REDUCE_PRINT (yyn);
-    switch (yyn)
+    try
+    {
+      switch (yyn)
       {
 ]b4_user_actions[
 	default:
           break;
       }
+    }
+    catch (const syntax_error& yyexc)
+    {
+      error (]b4_args(b4_locations_if([yyexc.location]),
+                      [[yyexc.what()]])[);
+      YYERROR;
+    }
     YY_SYMBOL_PRINT ("-> $$ =", yylhs);
 ]b4_variant_if([[
     // Destroy the rhs symbols.