Document %define lr.type and lr.default_rules.

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index b436896f6531a4b23a46369ab737cbe721afc8d0..a42ec81d6dec6a07b7a4c2e37ae17540d27f5d2b 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,35 @@ Bison News
 
 * Changes in version 2.5 (????-??-??):
 
+** IELR(1) and Canonical LR(1) Support
+
+  IELR(1) is a minimal LR(1) parser table generation algorithm.  That
+  is, given any context-free grammar, IELR(1) generates parser tables
+  with the full language recognition power of canonical LR(1) but with
+  nearly the same number of parser states as LALR(1).  This reduction in
+  parser states is often an order of magnitude.  More importantly,
+  because canonical LR(1)'s extra parser states may contain duplicate
+  conflicts in the case of non-LR(1) grammars, the number of conflicts
+  for IELR(1) is often an order of magnitude less as well.  This can
+  significantly reduce the complexity of developing of a grammar.
+
+  Bison can now generate IELR(1) and canonical LR(1) parser tables in
+  place of its traditional LALR(1) parser tables, which remain the
+  default.  You can specify the type of parser tables in the grammar
+  file with these directives:
+
+    %define lr.type "LALR"
+    %define lr.type "IELR"
+    %define lr.type "canonical LR"
+
+  The default rules optimization in the parser tables can also be
+  adjusted using `%define lr.default_rules'.  See the documentation for
+  `%define lr.type' and `%define lr.default_rules' in the section `Bison
+  Declaration Summary' in the Bison manual for the details.
+
+  These features are experimental.  More user feedback will help to
+  stabilize them.
+
 ** %define can now be invoked via the command line.
 
   Each of these bison command-line options
@@ -25,6 +54,27 @@ Bison News
   for any NAME and VALUE.  Omitting `=VALUE' on the command line is
   equivalent to omitting `"VALUE"' in the declaration.
 
+** Temporary hack for adding a semicolon to the user action.
+
+  Previously, Bison appended a semicolon to every user action for
+  reductions when the output language defaulted to C (specifically, when
+  neither %yacc, %language, %skeleton, or equivalent command-line
+  options were specified).  This allowed actions such as
+
+    exp: exp "+" exp { $$ = $1 + $3 };
+
+  instead of
+
+    exp: exp "+" exp { $$ = $1 + $3; };
+
+  As a first step in removing this misfeature, Bison now issues a
+  warning when it appends a semicolon.  Moreover, in cases where Bison
+  cannot easily determine whether a semicolon is needed (for example, an
+  action ending with a cpp directive or a braced compound initializer),
+  it no longer appends one.  Thus, the C compiler might now complain
+  about a missing semicolon where it did not before.  Future releases of
+  Bison will cease to append semicolons entirely.
+
 * Changes in version 2.4.2 (????-??-??):
 
 * Changes in version 2.4.1 (2008-12-11):