Allow specification of semantic predicates.

These changes allow users to prefix an action with %? to indicate that it
is a semantic predicate---an expression that is evaluated immediately (not
deferred, even in GLR nondeterministic mode) and causes a syntax error if
false.  In GLR parsers, this has the effect of killing one of a set of
split-off parses, just as would an ordinary syntax error.

Changelog:

    * NEWS: Describe new semantic-predicate feature.
    * data/c.m4 (b4_predicate_case): New definition.
    * data/java.m4 (b4_predicate_case): New definition.
    * data/glr.c (yyimmediate): Add definition.
    (yydoAction): Remove comment, now obsolete.
    Do YY_REDUCE_PRINT here.
    (yyglrReduce): Alter comment to indicate that semantic values
    need not be deferred.
    Remove YY_REDUCE_PRINT from here; done in yydoAction.
    (yyprocessOneStack): Pass immediate flag.
    Delete stacks rejected by predicates in newly split-off parsers.
    Change handling of yyerr so that only current stack gets deleted
    when semantic predicate fails.
    (yyfillin): Don't crash if a semantic value is unresolved (as may
    happen in predicate rules).
    Copy lr state as well in debugging mode.
    Update comment on setting of yysval to include yyloc as well.
    (yy_reduce_print): Add yynormal argument.  Perform fillin properly.
    Report unresolved RHS values.
    (yyimmediate): New table.
    * src/gram.h (struct rule): Add is_predicate field.
    * src/output.c (user_actions_output): Use b4_predicate_case for
    predicates.
    (prepare_symbols): Output yyimmediate.
    * src/scan-gram.l: Add %? token, SC_PREDICATE state.
    * src/scan-code.l (code_props_rule_action_init): Add is_predicate
    argument.
    * src/scan-code.h (struct code_props): Add is_predicate field.
    (code_props_rule_action_init): New interface.
    * src/parse-gram.y (%?{...}): New token.
    (rhs): Add %?{...} rule.
    * src/parse-gram.c: Regenerate.
    * src/parse-gram.h: Regenerate.
    * src/reader.c (grammar_current_rule_action_append): Add
    immediate argument.
    (grammar_midrule_action): Use new interface for
    code_props_rule_action_init.
    (grammar_current_rule_action_append): Ditto.
    (packgram): Transfer is_predicate value.
    * src/reader.h (grammar_current_rule_action_append): New interface.
    * doc/bison.texinfo: Document semantic predicates (%?).

    * data/glr.c (yylhsNonterm, yyisDefaultedState,yyDefaultAction)
    (yygetLRActions,yynewGLRStackItem,yyaddDeferredAction,yyinitStateSet)
    (yyinitGLRStack,yyexpandGLRStack,yyupdateSplit,yymarkStackDeleted)
    (yyundeleteLastStack,yyglrShift,yyglrShiftDefer,yydoAction,yyglrReduce)
    (yyidenticalOptions,yymergeOptionSets,yyresolveStates,yyresolveAction)
    (yyresolveLocations,yyresolveValue,yyreducePrint): Update parameter
    names in comments and mention all parameters.
    (struct yyGLRState): Fix description of yyposn field.
    (yyresolveLocations): Correct comment so as not to imply action when
    yyn1==0.

ChangeLog

@@ -1,3 +1,58 @@
+2010-07-23  Paul Hilfinger  <hilfingr@EECS.Berkeley.EDU>
+
+	* NEWS: Describe new semantic-predicate feature.
+        * data/c.m4 (b4_predicate_case): New definition.
+        * data/java.m4 (b4_predicate_case): New definition.
+        * data/glr.c (yyimmediate): Add definition.
+        (yydoAction): Remove comment, now obsolete.
+        Do YY_REDUCE_PRINT here.
+        (yyglrReduce): Alter comment to indicate that semantic values
+        need not be deferred.
+        Remove YY_REDUCE_PRINT from here; done in yydoAction.
+        (yyprocessOneStack): Pass immediate flag.
+        Delete stacks rejected by predicates in newly split-off parsers.
+        Change handling of yyerr so that only current stack gets deleted
+        when semantic predicate fails.
+        (yyfillin): Don't crash if a semantic value is unresolved (as may
+        happen in predicate rules).
+        Copy lr state as well in debugging mode.
+        Update comment on setting of yysval to include yyloc as well.
+        (yy_reduce_print): Add yynormal argument.  Perform fillin properly.
+        Report unresolved RHS values.
+        (yyimmediate): New table.
+        * src/gram.h (struct rule): Add is_predicate field.
+        * src/output.c (user_actions_output): Use b4_predicate_case for
+        predicates.
+        (prepare_symbols): Output yyimmediate.
+        * src/scan-gram.l: Add %? token, SC_PREDICATE state.
+        * src/scan-code.l (code_props_rule_action_init): Add is_predicate
+        argument.
+        * src/scan-code.h (struct code_props): Add is_predicate field.
+        (code_props_rule_action_init): New interface.
+        * src/parse-gram.y (%?{...}): New token.
+        (rhs): Add %?{...} rule.
+        * src/parse-gram.c: Regenerate.
+        * src/parse-gram.h: Regenerate.
+        * src/reader.c (grammar_current_rule_action_append): Add
+        immediate argument.
+        (grammar_midrule_action): Use new interface for
+        code_props_rule_action_init.
+        (grammar_current_rule_action_append): Ditto.
+        (packgram): Transfer is_predicate value.
+        * src/reader.h (grammar_current_rule_action_append): New interface.
+        * doc/bison.texinfo: Document semantic predicates (%?).
+    
+        * data/glr.c (yylhsNonterm, yyisDefaultedState,yyDefaultAction)
+        (yygetLRActions,yynewGLRStackItem,yyaddDeferredAction,yyinitStateSet)
+        (yyinitGLRStack,yyexpandGLRStack,yyupdateSplit,yymarkStackDeleted)
+        (yyundeleteLastStack,yyglrShift,yyglrShiftDefer,yydoAction,yyglrReduce)
+        (yyidenticalOptions,yymergeOptionSets,yyresolveStates,yyresolveAction)
+        (yyresolveLocations,yyresolveValue,yyreducePrint): Update parameter
+        names in comments and mention all parameters.
+        (struct yyGLRState): Fix description of yyposn field.
+        (yyresolveLocations): Correct comment so as not to imply action when
+        yyn1==0.
+	
 2010-06-17  Paul Eggert  <eggert@cs.ucla.edu>
 
 	Update from GFDL GFDL 1.2 to 1.3.

NEWS

@@ -51,6 +51,15 @@ Bison News
   use of the %error-verbose directive is deprecated in favor of
   %define parse.error "verbose".
 
+** Semantic predicates
+
+  The new, experimental, semantic-predicate feature allows actions of
+  the form %?{ BOOLEAN-EXPRESSION }, which cause syntax errors (as for
+  YYERROR) if the expression evaluates to 0, and are evaluated immediately
+  in GLR parsers, rather than being deferred.  The result is that they
+  allow the programmer to prune possible parses based on the values of
+  runtime expressions.
+
 * Changes in version 2.5 (????-??-??):
 
 ** Named References Support

data/c.m4

@@ -417,6 +417,15 @@ b4_syncline([@oline@], [@ofile@])
     break;])
 
 
+# b4_predicate_case(LABEL, CONDITIONS)
+# ------------------------------------
+m4_define([b4_predicate_case],
+[  case $1:
+    if (! ($2)) YYERROR;
+b4_syncline([@oline@], [@ofile@])
+    break;])
+
+
 # b4_yydestruct_generate(FUNCTION-DECLARATOR)
 # -------------------------------------------
 # Generate the "yydestruct" function, which declaration is issued using

data/glr.c

@@ -402,6 +402,13 @@ static const ]b4_int_type_for([b4_merger])[ yymerger[] =
   ]b4_merger[
 };
 
+/* YYIMMEDIATE[RULE-NUM] -- True iff rule #RULE-NUM is not to be deferred, as
+   in the case of predicates.  */
+static const yybool yyimmediate[] =
+{
+  ]b4_immediate[
+};
+
 /* YYCONFLP[YYPACT[STATE-NUM]] -- Pointer into YYCONFL of start of
    list of conflicting reductions corresponding to action entry for
    state STATE-NUM in yytable.  0 means no conflicts.  The list in
@@ -693,7 +700,7 @@ struct yyGLRState {
   yyStateNum yylrState;
   /** Preceding state in this stack */
   yyGLRState* yypred;
-  /** Source position of the first token produced by my symbol */
+  /** Source position of the last token produced by my symbol */
   size_t yyposn;
   union {
     /** First in a chain of alternative reductions producing the
@@ -806,9 +813,16 @@ yyfillin (yyGLRStackItem *yyvsp, int yylow0, int yylow1)
   s = yyvsp[yylow0].yystate.yypred;
   for (i = yylow0-1; i >= yylow1; i -= 1)
     {
-      YYASSERT (s->yyresolved);
+#if YYDEBUG
-      yyvsp[i].yystate.yyresolved = yytrue;
+      yyvsp[i].yystate.yylrState = s->yylrState;
-      yyvsp[i].yystate.yysemantics.yysval = s->yysemantics.yysval;
+#endif
+      yyvsp[i].yystate.yyresolved = s->yyresolved;
+      if (s->yyresolved)
+	yyvsp[i].yystate.yysemantics.yysval = s->yysemantics.yysval;
+      else
+	/* The effect of using yysval or yyloc (in an immediate rule) is
+         * undefined.  */
+	yyvsp[i].yystate.yysemantics.yyfirstVal = NULL;
       yyvsp[i].yystate.yyloc = s->yyloc;
       s = yyvsp[i].yystate.yypred = s->yypred;
     }
@@ -948,7 +962,7 @@ yydestroyGLRState (char const *yymsg, yyGLRState *yys]b4_user_formals[)
     }
 }
 
-/** Left-hand-side symbol for rule #RULE.  */
+/** Left-hand-side symbol for rule #YYRULE.  */
 static inline yySymbol
 yylhsNonterm (yyRuleNum yyrule)
 {
@@ -958,7 +972,7 @@ yylhsNonterm (yyRuleNum yyrule)
 #define yypact_value_is_default(yystate) \
   ]b4_table_value_equals([[pact]], [[yystate]], [b4_pact_ninf])[
 
-/** True iff LR state STATE has only a default reduction (regardless
+/** True iff LR state YYSTATE has only a default reduction (regardless
  *  of token).  */
 static inline yybool
 yyisDefaultedState (yyStateNum yystate)
@@ -966,7 +980,7 @@ yyisDefaultedState (yyStateNum yystate)
   return yypact_value_is_default (yypact[yystate]);
 }
 
-/** The default reduction for STATE, assuming it has one.  */
+/** The default reduction for YYSTATE, assuming it has one.  */
 static inline yyRuleNum
 yydefaultAction (yyStateNum yystate)
 {
@@ -981,8 +995,8 @@ yydefaultAction (yyStateNum yystate)
  *    R < 0:  Reduce on rule -R.
  *    R = 0:  Error.
  *    R > 0:  Shift to state R.
- *  Set *CONFLICTS to a pointer into yyconfl to 0-terminated list of
+ *  Set *YYCONFLICTS to a pointer into yyconfl to a 0-terminated list
- *  conflicting reductions.
+ *  of conflicting reductions.
  */
 static inline void
 yygetLRActions (yyStateNum yystate, int yytoken,
@@ -1031,9 +1045,10 @@ yyisErrorAction (int yyaction)
 
 				/* GLRStates */
 
-/** Return a fresh GLRStackItem.  Callers should call
+/** Return a fresh GLRStackItem in YYSTACKP.  The item is an LR state
- * YY_RESERVE_GLRSTACK afterwards to make sure there is sufficient
+ *  if YYISSTATE, and otherwise a semantic option.  Callers should call
- * headroom.  */
+ *  YY_RESERVE_GLRSTACK afterwards to make sure there is sufficient
+ *  headroom.  */
 
 static inline yyGLRStackItem*
 yynewGLRStackItem (yyGLRStack* yystackp, yybool yyisState)
@@ -1046,9 +1061,9 @@ yynewGLRStackItem (yyGLRStack* yystackp, yybool yyisState)
 }
 
 /** Add a new semantic action that will execute the action for rule
- *  RULENUM on the semantic values in RHS to the list of
+ *  YYRULE on the semantic values in YYRHS to the list of
- *  alternative actions for STATE.  Assumes that RHS comes from
+ *  alternative actions for YYSTATE.  Assumes that YYRHS comes from
- *  stack #K of *STACKP. */
+ *  stack #YYK of *YYSTACKP. */
 static void
 yyaddDeferredAction (yyGLRStack* yystackp, size_t yyk, yyGLRState* yystate,
 		     yyGLRState* yyrhs, yyRuleNum yyrule)
@@ -1073,7 +1088,7 @@ yyaddDeferredAction (yyGLRStack* yystackp, size_t yyk, yyGLRState* yystate,
 
 				/* GLRStacks */
 
-/** Initialize SET to a singleton set containing an empty stack.  */
+/** Initialize YYSET to a singleton set containing an empty stack.  */
 static yybool
 yyinitStateSet (yyGLRStateSet* yyset)
 {
@@ -1099,8 +1114,8 @@ static void yyfreeStateSet (yyGLRStateSet* yyset)
   YYFREE (yyset->yylookaheadNeeds);
 }
 
-/** Initialize STACK to a single empty stack, with total maximum
+/** Initialize *YYSTACKP to a single empty stack, with total maximum
- *  capacity for all stacks of SIZE.  */
+ *  capacity for all stacks of YYSIZE.  */
 static yybool
 yyinitGLRStack (yyGLRStack* yystackp, size_t yysize)
 {
@@ -1122,7 +1137,7 @@ yyinitGLRStack (yyGLRStack* yystackp, size_t yysize)
 # define YYRELOC(YYFROMITEMS,YYTOITEMS,YYX,YYTYPE) \
   &((YYTOITEMS) - ((YYFROMITEMS) - (yyGLRStackItem*) (YYX)))->YYTYPE
 
-/** If STACK is expandable, extend it.  WARNING: Pointers into the
+/** If *YYSTACKP is expandable, extend it.  WARNING: Pointers into the
     stack from outside should be considered invalid after this call.
     We always expand when there are 1 or fewer items left AFTER an
     allocation, so that we can avoid having external pointers exist
@@ -1192,9 +1207,9 @@ yyfreeGLRStack (yyGLRStack* yystackp)
   yyfreeStateSet (&yystackp->yytops);
 }
 
-/** Assuming that S is a GLRState somewhere on STACK, update the
+/** Assuming that YYS is a GLRState somewhere on *YYSTACKP, update the
- *  splitpoint of STACK, if needed, so that it is at least as deep as
+ *  splitpoint of *YYSTACKP, if needed, so that it is at least as deep as
- *  S.  */
+ *  YYS.  */
 static inline void
 yyupdateSplit (yyGLRStack* yystackp, yyGLRState* yys)
 {
@@ -1202,7 +1217,7 @@ yyupdateSplit (yyGLRStack* yystackp, yyGLRState* yys)
     yystackp->yysplitPoint = yys;
 }
 
-/** Invalidate stack #K in STACK.  */
+/** Invalidate stack #YYK in *YYSTACKP.  */
 static inline void
 yymarkStackDeleted (yyGLRStack* yystackp, size_t yyk)
 {
@@ -1211,8 +1226,8 @@ yymarkStackDeleted (yyGLRStack* yystackp, size_t yyk)
   yystackp->yytops.yystates[yyk] = NULL;
 }
 
-/** Undelete the last stack that was marked as deleted.  Can only be
+/** Undelete the last stack in *YYSTACKP that was marked as deleted.  Can
-    done once after a deletion, and only when all other stacks have
+    only be done once after a deletion, and only when all other stacks have
     been deleted.  */
 static void
 yyundeleteLastStack (yyGLRStack* yystackp)
@@ -1261,8 +1276,9 @@ yyremoveDeletes (yyGLRStack* yystackp)
     }
 }
 
-/** Shift to a new state on stack #K of STACK, corresponding to LR state
+/** Shift to a new state on stack #YYK of *YYSTACKP, corresponding to LR
- * LRSTATE, at input position POSN, with (resolved) semantic value SVAL.  */
+ * state YYLRSTATE, at input position YYPOSN, with (resolved) semantic
+ * value *YYVALP and source location *YYLOCP.  */
 static inline void
 yyglrShift (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
 	    size_t yyposn,
@@ -1281,7 +1297,7 @@ yyglrShift (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
   YY_RESERVE_GLRSTACK (yystackp);
 }
 
-/** Shift stack #K of YYSTACK, to a new state corresponding to LR
+/** Shift stack #YYK of *YYSTACKP, to a new state corresponding to LR
  *  state YYLRSTATE, at input position YYPOSN, with the (unresolved)
  *  semantic value of YYRHS under the action for YYRULE.  */
 static inline void
@@ -1301,11 +1317,51 @@ yyglrShiftDefer (yyGLRStack* yystackp, size_t yyk, yyStateNum yylrState,
   yyaddDeferredAction (yystackp, yyk, yynewState, yyrhs, yyrule);
 }
 
-/** Pop the symbols consumed by reduction #RULE from the top of stack
+#if !YYDEBUG
- *  #K of STACK, and perform the appropriate semantic action on their
+# define YY_REDUCE_PRINT(Args)
+#else
+# define YY_REDUCE_PRINT(Args)		\
+do {					\
+  if (yydebug)				\
+    yy_reduce_print Args;		\
+} while (YYID (0))
+
+/*----------------------------------------------------------------------.
+| Report that stack #YYK of *YYSTACKP is going to be reduced by YYRULE. |
+`----------------------------------------------------------------------*/
+
+/*ARGSUSED*/ static inline void
+yy_reduce_print (int yynormal, yyGLRStackItem* yyrhsVals, size_t yyk, yyRuleNum yyrule]b4_user_formals[)
+{
+  int yynrhs = yyrhsLength (yyrule);
+  int yyi;
+]b4_parse_param_use[]dnl
+[  YYFPRINTF (stderr, "Reducing stack %lu by rule %d (line %lu):\n",
+	     (unsigned long int) yyk, yyrule - 1,
+	     (unsigned long int) yyrline[yyrule]);
+  if (! yynormal)
+    yyfillin (yyrhsVals, 1, -yynrhs);
+  /* The symbols being reduced.  */
+  for (yyi = 0; yyi < yynrhs; yyi++)
+    {
+      YYFPRINTF (stderr, "   $%d = ", yyi + 1);
+      yy_symbol_print (stderr,
+		       yystos[yyrhsVals[yyi - yynrhs + 1].yystate.yylrState],
+		       &yyrhsVals[yyi - yynrhs + 1].yystate.yysemantics.yysval
+		       ]b4_locations_if([, &]b4_rhs_location(yynrhs, yyi + 1))[]dnl
+		       b4_user_args[);
+      if (!yyrhsVals[yyi - yynrhs + 1].yystate.yyresolved)
+        YYFPRINTF (stderr, " (unresolved)");
+      YYFPRINTF (stderr, "\n");
+    }
+}
+#endif
+
+/** Pop the symbols consumed by reduction #YYRULE from the top of stack
+ *  #YYK of *YYSTACKP, and perform the appropriate semantic action on their
  *  semantic values.  Assumes that all ambiguities in semantic values
- *  have been previously resolved.  Set *VALP to the resulting value,
+ *  have been previously resolved.  Set *YYVALP to the resulting value,
- *  and *LOCP to the computed location (if any).  Return value is as
+ *  and *YYLOCP to the computed location (if any).  Return value is as
  *  for userAction.  */
 static inline YYRESULTTAG
 yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
@@ -1321,15 +1377,12 @@ yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
       yystackp->yynextFree -= yynrhs;
       yystackp->yyspaceLeft += yynrhs;
       yystackp->yytops.yystates[0] = & yystackp->yynextFree[-1].yystate;
+      YY_REDUCE_PRINT ((1, yyrhs, yyk, yyrule]b4_user_args[));
       return yyuserAction (yyrule, yynrhs, yyrhs,
 			   yyvalp, yylocp, yystackp]b4_user_args[);
     }
   else
     {
-      /* At present, doAction is never called in nondeterministic
-       * mode, so this branch is never taken.  It is here in
-       * anticipation of a future feature that will allow immediate
-       * evaluation of selected actions in nondeterministic mode.  */
       int yyi;
       yyGLRState* yys;
       yyGLRStackItem yyrhsVals[YYMAXRHS + YYMAXLEFT + 1];
@@ -1345,60 +1398,21 @@ yydoAction (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
 	}
       yyupdateSplit (yystackp, yys);
       yystackp->yytops.yystates[yyk] = yys;
+      YY_REDUCE_PRINT ((0, yyrhsVals + YYMAXRHS + YYMAXLEFT - 1, yyk, yyrule]b4_user_args[));
       return yyuserAction (yyrule, yynrhs, yyrhsVals + YYMAXRHS + YYMAXLEFT - 1,
 			   yyvalp, yylocp, yystackp]b4_user_args[);
     }
 }
 
-#if !YYDEBUG
+/** Pop items off stack #YYK of *YYSTACKP according to grammar rule YYRULE,
-# define YY_REDUCE_PRINT(Args)
-#else
-# define YY_REDUCE_PRINT(Args)		\
-do {					\
-  if (yydebug)				\
-    yy_reduce_print Args;		\
-} while (YYID (0))
-
-/*----------------------------------------------------------.
-| Report that the RULE is going to be reduced on stack #K.  |
-`----------------------------------------------------------*/
-
-/*ARGSUSED*/ static inline void
-yy_reduce_print (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule]b4_user_formals[)
-{
-  int yynrhs = yyrhsLength (yyrule);
-  yybool yynormal __attribute__ ((__unused__)) =
-    (yystackp->yysplitPoint == NULL);
-  yyGLRStackItem* yyvsp = (yyGLRStackItem*) yystackp->yytops.yystates[yyk];
-  int yylow = 1;
-  int yyi;
-]b4_parse_param_use[]dnl
-[  YYFPRINTF (stderr, "Reducing stack %lu by rule %d (line %lu):\n",
-	     (unsigned long int) yyk, yyrule - 1,
-	     (unsigned long int) yyrline[yyrule]);
-  /* The symbols being reduced.  */
-  for (yyi = 0; yyi < yynrhs; yyi++)
-    {
-      YYFPRINTF (stderr, "   $%d = ", yyi + 1);
-      yy_symbol_print (stderr,
-                       ]yystos@{b4_rhs_data(yynrhs, yyi + 1).yylrState@}[,
-		       &]b4_rhs_value(yynrhs, yyi + 1)[
-		       ]b4_locations_if([, &]b4_rhs_location(yynrhs, yyi + 1))[]dnl
-		       b4_user_args[);
-      YYFPRINTF (stderr, "\n");
-    }
-}
-#endif
-
-/** Pop items off stack #K of STACK according to grammar rule RULE,
  *  and push back on the resulting nonterminal symbol.  Perform the
- *  semantic action associated with RULE and store its value with the
+ *  semantic action associated with YYRULE and store its value with the
- *  newly pushed state, if FORCEEVAL or if STACK is currently
+ *  newly pushed state, if YYFORCEEVAL or if *YYSTACKP is currently
  *  unambiguous.  Otherwise, store the deferred semantic action with
  *  the new state.  If the new state would have an identical input
  *  position, LR state, and predecessor to an existing state on the stack,
- *  it is identified with that existing state, eliminating stack #K from
+ *  it is identified with that existing state, eliminating stack #YYK from
- *  the STACK.  In this case, the (necessarily deferred) semantic value is
+ *  *YYSTACKP.  In this case, the semantic value is
  *  added to the options for the existing state's semantic value.
  */
 static inline YYRESULTTAG
@@ -1409,12 +1423,19 @@ yyglrReduce (yyGLRStack* yystackp, size_t yyk, yyRuleNum yyrule,
 
   if (yyforceEval || yystackp->yysplitPoint == NULL)
     {
+      YYRESULTTAG yyflag;
       YYSTYPE yysval;
       YYLTYPE yyloc;
 
-      YY_REDUCE_PRINT ((yystackp, yyk, yyrule]b4_user_args[));
+      yyflag = yydoAction (yystackp, yyk, yyrule, &yysval,
-      YYCHK (yydoAction (yystackp, yyk, yyrule, &yysval,
+			   &yyloc]b4_user_args[);
-			 &yyloc]b4_user_args[));
+      if (yyflag == yyerr)
+        {
+	  YYDPRINTF ((stderr, "Parse on stack %lu rejected by rule #%d.\n",
+		     (unsigned long int) yyk, yyrule - 1));
+        }
+      if (yyflag != yyok)
+        return yyflag;      
       YY_SYMBOL_PRINT ("-> $$ =", yyr1[yyrule], &yysval, &yyloc);
       yyglrShift (yystackp, yyk,
 		  yyLRgotoState (yystackp->yytops.yystates[yyk]->yylrState,
@@ -1508,7 +1529,7 @@ yysplitStack (yyGLRStack* yystackp, size_t yyk)
   return yystackp->yytops.yysize-1;
 }
 
-/** True iff Y0 and Y1 represent identical options at the top level.
+/** True iff YYY0 and YYY1 represent identical options at the top level.
  *  That is, they represent the same rule applied to RHS symbols
  *  that produce the same terminal symbols.  */
 static yybool
@@ -1530,8 +1551,8 @@ yyidenticalOptions (yySemanticOption* yyy0, yySemanticOption* yyy1)
     return yyfalse;
 }
 
-/** Assuming identicalOptions (Y0,Y1), destructively merge the
+/** Assuming identicalOptions (YYY0,YYY1), destructively merge the
- *  alternative semantic values for the RHS-symbols of Y1 and Y0.  */
+ *  alternative semantic values for the RHS-symbols of YYY1 and YYY0.  */
 static void
 yymergeOptionSets (yySemanticOption* yyy0, yySemanticOption* yyy1)
 {
@@ -1612,11 +1633,11 @@ static YYRESULTTAG yyresolveValue (yyGLRState* yys,
 				   yyGLRStack* yystackp]b4_user_formals[);
 
 
-/** Resolve the previous N states starting at and including state S.  If result
+/** Resolve the previous YYN states starting at and including state YYS
- *  != yyok, some states may have been left unresolved possibly with empty
+ *  on *YYSTACKP. If result != yyok, some states may have been left
- *  semantic option chains.  Regardless of whether result = yyok, each state
+ *  unresolved possibly with empty semantic option chains.  Regardless
- *  has been left with consistent data so that yydestroyGLRState can be invoked
+ *  of whether result = yyok, each state has been left with consistent
- *  if necessary.  */
+ *  data so that yydestroyGLRState can be invoked if necessary.  */
 static YYRESULTTAG
 yyresolveStates (yyGLRState* yys, int yyn,
 		 yyGLRStack* yystackp]b4_user_formals[)
@@ -1631,9 +1652,10 @@ yyresolveStates (yyGLRState* yys, int yyn,
   return yyok;
 }
 
-/** Resolve the states for the RHS of OPT, perform its user action, and return
+/** Resolve the states for the RHS of YYOPT on *YYSTACKP, perform its
- *  the semantic value and location.  Regardless of whether result = yyok, all
+ *  user action, and return the semantic value and location in *YYVALP
- *  RHS states have been destroyed (assuming the user action destroys all RHS
+ *  and *YYLOCP.  Regardless of whether result = yyok, all RHS states
+ *  have been destroyed (assuming the user action destroys all RHS
  *  semantic values if invoked).  */
 static YYRESULTTAG
 yyresolveAction (yySemanticOption* yyopt, yyGLRStack* yystackp,
@@ -1743,9 +1765,9 @@ yyreportAmbiguity (yySemanticOption* yyx0,
   return yyabort;
 }
 
-/** Starting at and including state S1, resolve the location for each of the
+/** Resolve the locations for each of the YYN1 states in *YYSTACKP,
- *  previous N1 states that is unresolved.  The first semantic option of a state
+ *  ending at YYS1.  Has no effect on previously resolved states.
- *  is always chosen.  */
+ *  The first semantic option of a state is always chosen.  */
 static void
 yyresolveLocations (yyGLRState* yys1, int yyn1,
 		    yyGLRStack *yystackp]b4_user_formals[)
@@ -1801,11 +1823,12 @@ yyresolveLocations (yyGLRState* yys1, int yyn1,
     }
 }
 
-/** Resolve the ambiguity represented in state S, perform the indicated
+/** Resolve the ambiguity represented in state YYS in *YYSTACKP,
- *  actions, and set the semantic value of S.  If result != yyok, the chain of
+ *  perform the indicated actions, and set the semantic value of YYS.
- *  semantic options in S has been cleared instead or it has been left
+ *  If result != yyok, the chain of semantic options in YYS has been
- *  unmodified except that redundant options may have been removed.  Regardless
+ *  cleared instead or it has been left unmodified except that
- *  of whether result = yyok, S has been left with consistent data so that
+ *  redundant options may have been removed.  Regardless of whether
+ *  result = yyok, YYS has been left with consistent data so that
  *  yydestroyGLRState can be invoked if necessary.  */
 static YYRESULTTAG
 yyresolveValue (yyGLRState* yys, yyGLRStack* yystackp]b4_user_formals[)
@@ -1950,6 +1973,7 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
   int yyaction;
   const short int* yyconflicts;
   yyRuleNum yyrule;
+  YYRESULTTAG yyflag;
 
   while (yystackp->yytops.yystates[yyk] != NULL)
     {
@@ -1969,11 +1993,21 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 	      yymarkStackDeleted (yystackp, yyk);
 	      return yyok;
 	    }
-	  YYCHK (yyglrReduce (yystackp, yyk, yyrule, yyfalse]b4_user_args[));
+	  yyflag = yyglrReduce (yystackp, yyk, yyrule, yyimmediate[yyrule]]b4_user_args[);
+          if (yyflag == yyerr)
+            {
+	      YYDPRINTF ((stderr, "Stack %lu dies (predicate failure or explicit user error).\n",
+			  (unsigned long int) yyk));
+	      yymarkStackDeleted (yystackp, yyk);
+	      return yyok;
+	    }
+	  if (yyflag != yyok)
+            return yyflag;
 	}
       else
 	{
 	  yySymbol yytoken;
+
 	  yystackp->yytops.yylookaheadNeeds[yyk] = yytrue;
 	  if (yychar == YYEMPTY)
 	    {
@@ -1996,14 +2030,25 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 
 	  while (*yyconflicts != 0)
 	    {
+	      YYRESULTTAG yyflag;
 	      size_t yynewStack = yysplitStack (yystackp, yyk);
 	      YYDPRINTF ((stderr, "Splitting off stack %lu from %lu.\n",
 			  (unsigned long int) yynewStack,
 			  (unsigned long int) yyk));
-	      YYCHK (yyglrReduce (yystackp, yynewStack,
+	      yyflag = yyglrReduce (yystackp, yynewStack,
-				  *yyconflicts, yyfalse]b4_user_args[));
+				    *yyconflicts,
-	      YYCHK (yyprocessOneStack (yystackp, yynewStack,
+				    yyimmediate[*yyconflicts]]b4_user_args[);
-					yyposn]b4_pure_args[));
+              if (yyflag == yyok)
+	        YYCHK (yyprocessOneStack (yystackp, yynewStack,
+					  yyposn]b4_pure_args[));
+	      else if (yyflag == yyerr)
+		{
+		  YYDPRINTF ((stderr, "Stack %lu dies.\n", 
+			      (unsigned long int) yynewStack));
+		  yymarkStackDeleted (yystackp, yynewStack);
+		}
+	      else
+		return yyflag;
 	      yyconflicts += 1;
 	    }
 
@@ -2017,8 +2062,19 @@ yyprocessOneStack (yyGLRStack* yystackp, size_t yyk,
 	      break;
 	    }
 	  else
-	    YYCHK (yyglrReduce (yystackp, yyk, -yyaction,
+            {
-				yyfalse]b4_user_args[));
+	      yyflag = yyglrReduce (yystackp, yyk, -yyaction,
+		  		    yyimmediate[-yyaction]]b4_user_args[);
+	      if (yyflag == yyerr)
+                {
+	          YYDPRINTF ((stderr, "Stack %lu dies (predicate failure or explicit user error).\n",
+			      (unsigned long int) yyk));
+	          yymarkStackDeleted (yystackp, yyk);
+                  break;
+		}
+	      else if (yyflag != yyok)
+                return yyflag;
+            }
 	}
     }
   return yyok;
@@ -2249,7 +2305,6 @@ yyrecoverSyntaxError (yyGLRStack* yystackp]b4_user_formals[)
     }									     \
   } while (YYID (0))
 
-
 /*----------.
 | yyparse.  |
 `----------*/

data/java.m4

@@ -169,6 +169,13 @@ m4_define([b4_case], [  case $1:
   break;
     ])
 
+# b4_predicate_case(LABEL, CONDITIONS)
+# ------------------------------------
+m4_define([b4_predicate_case], [  case $1:
+     if (! ($2)) YYERROR;
+    break;
+    ])
+
 
 ## ---------------- ##
 ## Default values.  ##

doc/bison.texinfo

@@ -136,6 +136,7 @@ Writing @acronym{GLR} Parsers
 * Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
 * Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
 * GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Semantic Predicates::    Controlling a parse with arbitrary computations.
 * Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
 
 Examples
@@ -758,6 +759,7 @@ merged result.
 * Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
 * Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
 * GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Semantic Predicates::    Controlling a parse with arbitrary computations.
 * Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
 @end menu
 
@@ -1163,6 +1165,65 @@ In a deferred semantic action, its effect is undefined.
 Also, see @ref{Location Default Action, ,Default Action for Locations}, which
 describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers.
 
+@node Semantic Predicates
+@subsection Controlling a Parse with Arbitrary Predicates
+@findex %?
+@cindex Semantic predicates in @acronym{GLR} parsers
+
+In addition to the @code{%dprec} and @code{%merge} directives,
+@acronym{GLR} parsers
+allow you to reject parses on the basis of arbitrary computations executed
+in user code, without having Bison treat this rejection as an error
+if there are alternative parses. (This feature is experimental and may
+evolve.  We welcome user feedback.)  For example,
+
+@smallexample
+widget :
+          %?@{ new_syntax @} "widget" id new_args   @{ $$ = f($3, $4); @}
+       |  %?@{ !new_syntax @} "widget" id old_args  @{ $$ = f($3, $4); @}
+       ;
+@end smallexample
+
+@noindent
+is one way to allow the same parser to handle two different syntaxes for 
+widgets.  The clause preceded by @code{%?} is treated like an ordinary
+action, except that its text is treated as an expression and is always
+evaluated immediately (even when in nondeterministic mode).  If the 
+expression yields 0 (false), the clause is treated as a syntax error,
+which, in a nondeterministic parser, causes the stack in which it is reduced 
+to die.  In a deterministic parser, it acts like YYERROR.
+
+As the example shows, predicates otherwise look like semantic actions, and
+therefore you must be take them into account when determining the numbers
+to use for denoting the semantic values of right-hand side symbols.
+Predicate actions, however, have no defined value, and may not be given
+labels.
+
+There is a subtle difference between semantic predicates and ordinary
+actions in nondeterministic mode, since the latter are deferred.
+For example, we could try to rewrite the previous example as 
+
+@smallexample
+widget :
+          @{ if (!new_syntax) YYERROR; @} "widget" id new_args  @{ $$ = f($3, $4); @}
+       |  @{ if (new_syntax) YYERROR; @} "widget" id old_args   @{ $$ = f($3, $4); @}
+       ;
+@end smallexample
+
+@noindent
+(reversing the sense of the predicate tests to cause an error when they are
+false).  However, this
+does @emph{not} have the same effect if @code{new_args} and @code{old_args}
+have overlapping syntax.
+Since the mid-rule actions testing @code{new_syntax} are deferred, 
+a @acronym{GLR} parser first encounters the unresolved ambiguous reduction 
+for cases where @code{new_args} and @code{old_args} recognize the same string
+@emph{before} performing the tests of @code{new_syntax}.  It therefore
+reports an error.
+
+Finally, be careful in writing predicates: deferred actions have not been
+evaluated, so that using them in a predicate will have undefined effects.
+
 @node Compiler Requirements
 @subsection Considerations when Compiling @acronym{GLR} Parsers
 @cindex @code{inline}
@@ -10610,6 +10671,19 @@ file.  @xref{Grammar Outline, ,Outline of a Bison
 Grammar}.
 @end deffn
 
+@deffn {Directive} %?@{@var{expression}@}
+Predicate actions.  This is a type of action clause that may appear in
+rules. The expression is evaluated, and if false, causes a syntax error.  In
+@acronym{GLR} parsers during nondeterministic operation,  
+this silently causes an alternative parse to die.  During deterministic
+operation, it is the same as the effect of YYERROR.
+@xref{Semantic Predicates}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+@end deffn
+
 @deffn {Construct} /*@dots{}*/
 Comment delimiters, as in C.
 @end deffn

src/gram.h

@@ -194,6 +194,7 @@ typedef struct
 
   location location;
   bool useful;
+  bool is_predicate;
 
   const char *action;
   location action_location;

src/output.c

@@ -204,7 +204,7 @@ prepare_symbols (void)
 
 /*----------------------------------------------------------------.
 | Prepare the muscles related to the rules: r1, r2, rline, dprec, |
-| merger.                                                         |
+| merger, immediate.                                              |
 `----------------------------------------------------------------*/
 
 static void
@@ -215,6 +215,7 @@ prepare_rules (void)
   unsigned int *r2 = xnmalloc (nrules, sizeof *r2);
   int *dprec = xnmalloc (nrules, sizeof *dprec);
   int *merger = xnmalloc (nrules, sizeof *merger);
+  int *immediate = xnmalloc (nrules, sizeof *immediate);
 
   rule_number r;
   for (r = 0; r < nrules; ++r)
@@ -229,6 +230,8 @@ prepare_rules (void)
       dprec[r] = rules[r].dprec;
       /* Merger-function index (GLR).  */
       merger[r] = rules[r].merger;
+      /* Immediate reduction flags (GLR).  */
+      immediate[r] = rules[r].is_predicate;
     }
 
   muscle_insert_unsigned_int_table ("rline", rline, 0, 0, nrules);
@@ -236,6 +239,7 @@ prepare_rules (void)
   muscle_insert_unsigned_int_table ("r2", r2, 0, 0, nrules);
   muscle_insert_int_table ("dprec", dprec, 0, 0, nrules);
   muscle_insert_int_table ("merger", merger, 0, 0, nrules);
+  muscle_insert_int_table ("immediate", immediate, 0, 0, nrules);
 
   MUSCLE_INSERT_INT ("rules_number", nrules);
   MUSCLE_INSERT_INT ("max_left_semantic_context", max_left_semantic_context);
@@ -349,8 +353,9 @@ user_actions_output (FILE *out)
   for (r = 0; r < nrules; ++r)
     if (rules[r].action)
       {
-	fprintf (out, "b4_case(%d, [b4_syncline(%d, ", r + 1,
+	fprintf (out, "b4_%scase(%d, [b4_syncline(%d, ",
-		 rules[r].action_location.start.line);
+		 rules[r].is_predicate ? "predicate_" : "",
+		 r + 1, rules[r].action_location.start.line);
 	escaped_output (out, rules[r].action_location.start.file);
 	fprintf (out, ")\n[    %s]])\n\n", rules[r].action);
       }

src/parse-gram.h

@@ -1,8 +1,9 @@
-/* A Bison parser, made by GNU Bison 2.4.483-4ad39-dirty.  */
+/* A Bison parser, made by GNU Bison 2.4.516-804e.  */
 
 /* Interface for Bison's Yacc-like parsers in C
 
-   Copyright (C) 1984, 1989-1990, 2000-2010 Free Software Foundation, Inc.
+   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
+   2007, 2008, 2009, 2010 Free Software Foundation, Inc.
 
    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
@@ -31,7 +32,7 @@
    version 2.2 of Bison.  */
 /* "%code requires" blocks.  */
 /* Line 1658 of yacc.c  */
-#line 201 "src/parse-gram.y"
+#line 202 "src/parse-gram.y"
 
 # ifndef PARAM_TYPE
 #  define PARAM_TYPE
@@ -46,7 +47,7 @@
 
 
 /* Line 1658 of yacc.c  */
-#line 50 "src/parse-gram.h"
+#line 51 "src/parse-gram.h"
 
 /* Tokens.  */
 #ifndef YYTOKENTYPE
@@ -93,21 +94,22 @@
      PERCENT_VERBOSE = 293,
      PERCENT_YACC = 294,
      BRACED_CODE = 295,
-     BRACKETED_ID = 296,
+     BRACED_PREDICATE = 296,
-     CHAR = 297,
+     BRACKETED_ID = 297,
-     EPILOGUE = 298,
+     CHAR = 298,
-     EQUAL = 299,
+     EPILOGUE = 299,
-     ID = 300,
+     EQUAL = 300,
-     ID_COLON = 301,
+     ID = 301,
-     PERCENT_PERCENT = 302,
+     ID_COLON = 302,
-     PIPE = 303,
+     PERCENT_PERCENT = 303,
-     PROLOGUE = 304,
+     PIPE = 304,
-     SEMICOLON = 305,
+     PROLOGUE = 305,
-     TAG = 306,
+     SEMICOLON = 306,
-     TAG_ANY = 307,
+     TAG = 307,
-     TAG_NONE = 308,
+     TAG_ANY = 308,
-     PERCENT_PARAM = 309,
+     TAG_NONE = 309,
-     PERCENT_UNION = 310
+     PERCENT_PARAM = 310,
+     PERCENT_UNION = 311
    };
 #endif
 /* Tokens.  */
@@ -150,21 +152,22 @@
 #define PERCENT_VERBOSE 293
 #define PERCENT_YACC 294
 #define BRACED_CODE 295
-#define BRACKETED_ID 296
+#define BRACED_PREDICATE 296
-#define CHAR 297
+#define BRACKETED_ID 297
-#define EPILOGUE 298
+#define CHAR 298
-#define EQUAL 299
+#define EPILOGUE 299
-#define ID 300
+#define EQUAL 300
-#define ID_COLON 301
+#define ID 301
-#define PERCENT_PERCENT 302
+#define ID_COLON 302
-#define PIPE 303
+#define PERCENT_PERCENT 303
-#define PROLOGUE 304
+#define PIPE 304
-#define SEMICOLON 305
+#define PROLOGUE 305
-#define TAG 306
+#define SEMICOLON 306
-#define TAG_ANY 307
+#define TAG 307
-#define TAG_NONE 308
+#define TAG_ANY 308
-#define PERCENT_PARAM 309
+#define TAG_NONE 309
-#define PERCENT_UNION 310
+#define PERCENT_PARAM 310
+#define PERCENT_UNION 311
 
 
 
@@ -173,7 +176,7 @@
 typedef union YYSTYPE
 {
 /* Line 1658 of yacc.c  */
-#line 87 "src/parse-gram.y"
+#line 88 "src/parse-gram.y"
 
   assoc assoc;
   char *code;
@@ -186,13 +189,13 @@ typedef union YYSTYPE
   unsigned char character;
 
 /* Line 1658 of yacc.c  */
-#line 225 "src/parse-gram.y"
+#line 226 "src/parse-gram.y"
 
   param_type param;
 
 
 /* Line 1658 of yacc.c  */
-#line 196 "src/parse-gram.h"
+#line 199 "src/parse-gram.h"
 } YYSTYPE;
 # define YYSTYPE_IS_TRIVIAL 1
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */

src/parse-gram.y

@@ -118,7 +118,6 @@ static char const *char_name (char);
 %token PERCENT_DPREC         "%dprec"
 %token PERCENT_MERGE         "%merge"
 
-
 /*----------------------.
 | Global Declarations.  |
 `----------------------*/
@@ -151,6 +150,7 @@ static char const *char_name (char);
 ;
 
 %token BRACED_CODE     "{...}"
+%token BRACED_PREDICATE "%?{...}"
 %token BRACKETED_ID    "[identifier]"
 %token CHAR            "char"
 %token EPILOGUE        "epilogue"
@@ -171,7 +171,7 @@ static char const *char_name (char);
 /* braceless is not to be used for rule or symbol actions, as it
    calls code_props_plain_init.  */
 %type <chars> STRING "%{...%}" EPILOGUE braceless content.opt
-%type <code> "{...}"
+%type <code> "{...}" "%?{...}"
 %printer { fputs (quotearg_style (c_quoting_style, $$), stderr); }
 	 STRING
 %printer { fprintf (stderr, "{\n%s\n}", $$); }
@@ -586,7 +586,9 @@ rhs:
 | rhs symbol named_ref.opt
     { grammar_current_rule_symbol_append ($2, @2, $3); }
 | rhs "{...}" named_ref.opt
-    { grammar_current_rule_action_append ($2, @2, $3); }
+    { grammar_current_rule_action_append ($2, @2, $3, false); }
+| rhs "%?{...}" 
+    { grammar_current_rule_action_append ($2, @2, NULL, true); }
 | rhs "%prec" symbol
     { grammar_current_rule_prec_set ($3, @3); }
 | rhs "%dprec" INT
@@ -601,7 +603,6 @@ named_ref.opt:
   BRACKETED_ID   { $$ = named_ref_new($1, @1); }
 ;
 
-
 /*---------------------------.
 | variable and content.opt.  |
 `---------------------------*/

src/reader.c

@@ -377,7 +377,8 @@ grammar_midrule_action (void)
   code_props_rule_action_init (&midrule->action_props,
                                current_rule->action_props.code,
                                current_rule->action_props.location,
-                               midrule, 0);
+                               midrule, 0,
+			       current_rule->action_props.is_predicate);
   code_props_none_init (&current_rule->action_props);
 
   if (previous_rule_end)
@@ -468,14 +469,14 @@ grammar_current_rule_symbol_append (symbol *sym, location loc,
 
 void
 grammar_current_rule_action_append (const char *action, location loc,
-				    named_ref *name)
+				    named_ref *name, bool is_predicate)
 {
   if (current_rule->action_props.code)
     grammar_midrule_action ();
   /* After all symbol declarations have been parsed, packgram invokes
      code_props_translate_code.  */
   code_props_rule_action_init (&current_rule->action_props, action, loc,
-                               current_rule, name);
+                               current_rule, name, is_predicate);
 }
 
 
@@ -516,6 +517,7 @@ packgram (void)
       rules[ruleno].useful = true;
       rules[ruleno].action = p->action_props.code;
       rules[ruleno].action_location = p->action_props.location;
+      rules[ruleno].is_predicate = p->action_props.is_predicate;
 
       /* If the midrule's $$ is set or its $n is used, remove the `$' from the
 	 symbol name so that it's a user-defined symbol so that the default

src/reader.h

@@ -53,7 +53,7 @@ void grammar_current_rule_merge_set (uniqstr name, location loc);
 void grammar_current_rule_symbol_append (symbol *sym, location loc,
 					 named_ref *named_ref);
 void grammar_current_rule_action_append (const char *action, location loc,
-					 named_ref *named_ref);
+					 named_ref *named_ref, bool);
 void reader (void);
 void free_merger_functions (void);
 

src/scan-code.h

@@ -63,6 +63,13 @@ typedef struct code_props {
    */
   bool is_value_used;
 
+  /**
+   * \c true iff this code is an action that is not to be deferred in
+   * a non-deterministic parser.
+   */
+  bool is_predicate;
+
+
   /** \c NULL iff \c code_props::kind is not \c CODE_PROPS_RULE_ACTION.  */
   struct symbol_list *rule;
 
@@ -80,7 +87,7 @@ void code_props_none_init (code_props *self);
 
 /** Equivalent to \c code_props_none_init.  */
 #define CODE_PROPS_NONE_INIT \
-  {CODE_PROPS_NONE, NULL, EMPTY_LOCATION_INIT, false, NULL, NULL}
+  {CODE_PROPS_NONE, NULL, EMPTY_LOCATION_INIT, false, false, NULL, NULL}
 
 /** Initialized by \c CODE_PROPS_NONE_INIT with no further modification.  */
 extern code_props const code_props_none;
@@ -134,7 +141,7 @@ void code_props_symbol_action_init (code_props *self, char const *code,
  */
 void code_props_rule_action_init (code_props *self, char const *code,
                                   location code_loc, struct symbol_list *rule,
-                                  named_ref *name);
+                                  named_ref *name, bool is_predicate);
 
 /**
  * \pre

src/scan-code.l

@@ -902,7 +902,7 @@ code_props_symbol_action_init (code_props *self, char const *code,
 void
 code_props_rule_action_init (code_props *self, char const *code,
                              location code_loc, symbol_list *rule,
-			     named_ref *name)
+			     named_ref *name, bool is_predicate)
 {
   self->kind = CODE_PROPS_RULE_ACTION;
   self->code = code;
@@ -910,6 +910,7 @@ code_props_rule_action_init (code_props *self, char const *code,
   self->is_value_used = false;
   self->rule = rule;
   self->named_ref = name;
+  self->is_predicate = is_predicate;
 }
 
 void

src/scan-gram.l

@@ -106,11 +106,12 @@ static void unexpected_newline (boundary, char const *);
  /* A complex tag, with nested angles brackets. */
 %x SC_TAG
 
- /* Three types of user code:
+ /* Four types of user code:
     - prologue (code between `%{' `%}' in the first section, before %%);
     - actions, printers, union, etc, (between braced in the middle section);
-    - epilogue (everything after the second %%). */
+    - epilogue (everything after the second %%). 
-%x SC_PROLOGUE SC_BRACED_CODE SC_EPILOGUE
+    - predicate (code between `%?{' and `{' in middle section); */
+%x SC_PROLOGUE SC_BRACED_CODE SC_EPILOGUE SC_PREDICATE
  /* C and C++ comments in code. */
 %x SC_COMMENT SC_LINE_COMMENT
  /* Strings and characters in code. */
@@ -285,6 +286,13 @@ splice	 (\\[ \f\t\v]*\n)*
     BEGIN SC_BRACED_CODE;
   }
 
+  /* Semantic predicate. */
+  "%?"[ \f\n\t\v]*"{" {
+    nesting = 0;
+    code_start = loc->start;
+    BEGIN SC_PREDICATE;
+  }
+
   /* A type. */
   "<*>"       return TAG_ANY;
   "<>"        return TAG_NONE;
@@ -661,7 +669,7 @@ splice	 (\\[ \f\t\v]*\n)*
   | Strings, comments etc. can be found in user code.  |
   `---------------------------------------------------*/
 
-<SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE>
+<SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE,SC_PREDICATE>
 {
   "'" {
     STRING_GROW;
@@ -691,14 +699,32 @@ splice	 (\\[ \f\t\v]*\n)*
 
 
   /*-----------------------------------------------------------.
-  | Scanning some code in braces (actions). The initial "{" is |
+  | Scanning some code in braces (actions, predicates). The    |
-  | already eaten.                                             |
+  | initial "{" is already eaten.                              |
   `-----------------------------------------------------------*/
 
-<SC_BRACED_CODE>
+<SC_BRACED_CODE,SC_PREDICATE>
 {
   "{"|"<"{splice}"%"  STRING_GROW; nesting++;
   "%"{splice}">"      STRING_GROW; nesting--;
+
+  /* Tokenize `<<%' correctly (as `<<' `%') rather than incorrrectly
+     (as `<' `<%').  */
+  "<"{splice}"<"  STRING_GROW;
+
+  <<EOF>> {
+    int token = (YY_START == SC_BRACED_CODE) ? BRACED_CODE : BRACED_PREDICATE;
+    unexpected_eof (code_start, "}");
+    STRING_FINISH;
+    loc->start = code_start;
+    val->code = last_string;
+    BEGIN INITIAL;
+    return token;
+  }
+}
+
+<SC_BRACED_CODE>
+{
   "}" {
     obstack_1grow (&obstack_for_string, '}');
 
@@ -712,21 +738,24 @@ splice	 (\\[ \f\t\v]*\n)*
 	return BRACED_CODE;
       }
   }
-
-  /* Tokenize `<<%' correctly (as `<<' `%') rather than incorrrectly
-     (as `<' `<%').  */
-  "<"{splice}"<"  STRING_GROW;
-
-  <<EOF>> {
-    unexpected_eof (code_start, "}");
-    STRING_FINISH;
-    loc->start = code_start;
-    val->code = last_string;
-    BEGIN INITIAL;
-    return BRACED_CODE;
-  }
 }
 
+<SC_PREDICATE>
+{
+  "}" {
+    --nesting;
+    if (nesting < 0)
+      {
+	STRING_FINISH;
+	loc->start = code_start;
+	val->code = last_string;
+	BEGIN INITIAL;
+	return BRACED_PREDICATE;
+      }
+    else
+      obstack_1grow (&obstack_for_string, '}');
+  }
+}
 
   /*--------------------------------------------------------------.
   | Scanning some prologue: from "%{" (already scanned) to "%}".  |
@@ -774,8 +803,8 @@ splice	 (\\[ \f\t\v]*\n)*
   | By default, grow the string obstack with the input.  |
   `-----------------------------------------------------*/
 
-<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE,SC_STRING,SC_CHARACTER,SC_ESCAPED_STRING,SC_ESCAPED_CHARACTER>.	|
+<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PREDICATE,SC_PROLOGUE,SC_EPILOGUE,SC_STRING,SC_CHARACTER,SC_ESCAPED_STRING,SC_ESCAPED_CHARACTER>.	|
-<SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PROLOGUE,SC_EPILOGUE>\n	STRING_GROW;
+  <SC_COMMENT,SC_LINE_COMMENT,SC_BRACED_CODE,SC_PREDICATE,SC_PROLOGUE,SC_EPILOGUE>\n	STRING_GROW;
 
 %%