Don't let maintainer-*-check targets force a version update.

[bison.git] / TODO

diff --git a/TODO b/TODO
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 -*- outline -*-
 
 -*- outline -*-
 

-* NEWS
-Sort from 1.31 NEWS.
-
-* Prologue
-The %union is declared after the user C declarations. It can be
-a problem if YYSTYPE is declared after the user part.  []
-
-Actually, the real problem seems that the %union ought to be output
-where it was defined.  For instance, in gettext/intl/plural.y, we
-have:
-
-       %{
-       ...
-       #include "gettextP.h"
-       ...
-       %}
-
-       %union {
-         unsigned long int num;
-         enum operator op;
-         struct expression *exp;
+* Short term
+** Document %define assert
+
+** Discuss about %printer/%destroy in the case of C++.
+It would be very nice to provide the symbol classes with an operator<<
+and a destructor.  Unfortunately the syntax we have chosen for
+%destroy and %printer make them hard to reuse.  For instance, the user
+is invited to write something like
+
+   %printer { debug_stream() << $$; } <my_type>;
+
+which is hard to reuse elsewhere since it wants to use
+"debug_stream()" to find the stream to use.  The same applies to
+%destroy: we told the user she could use the members of the Parser
+class in the printers/destructors, which is not good for an operator<<
+since it is no longer bound to a particular parser, it's just a
+(standalone symbol).
+
+** Rename LR0.cc
+as lr0.cc, why upper case?
+
+** bench several bisons.
+Enhance bench.pl with %b to run different bisons.
+
+** Use b4_symbol everywhere.
+Move its definition in the more standard places and deploy it in other
+skeletons.
+
+* Various
+** YYPRINT
+glr.c inherits its symbol_print function from c.m4, which supports
+YYPRINT.  But to use YYPRINT yytoknum is needed, which not defined by
+glr.c.
+
+Anyway, IMHO YYPRINT is obsolete and should be restricted to yacc.c.
+
+** YYERRCODE
+Defined to 256, but not used, not documented.  Probably the token
+number for the error token, which POSIX wants to be 256, but which
+Bison might renumber if the user used number 256.  Keep fix and doc?
+Throw away?
+
+We could (should?) also treat the case of the undef_token, which is
+numbered 257 for yylex, and 2 internal.  Both appear for instance in
+toknum:
+
+  const unsigned short int
+  parser::yytoken_number_[] =
+  {
+       0,   256,   257,   258,   259,   260,   261,   262,   263,   264,
+
+while here
+
+   enum yytokentype {
+     TOK_EOF = 0,
+     TOK_EQ = 258,
+
+so both 256 and 257 are "mysterious".
+
+  const char*
+  const parser::yytname_[] =
+  {
+  "\"end of command\"", "error", "$undefined", "\"=\"", "\"break\"",
+
+
+** YYFAIL
+It is seems to be *really* obsolete now, shall we remove it?
+
+** YYBACKUP
+There is no test about it, no examples in the doc, and I'm not sure
+what it should look like.  For instance what follows crashes.
+
+  %error-verbose
+  %debug
+  %pure-parser
+  %code {
+  # include <stdio.h>
+  # include <stdlib.h>
+  # include <assert.h>
+
+    static void yyerror (const char *msg);
+    static int yylex (YYSTYPE *yylval);
+  }
+  %%
+  exp:
+    'a'   { printf ("a: %d\n", $1); }
+  | 'b'   { YYBACKUP('a', 123); }
+  ;
+  %%
+  static int
+  yylex (YYSTYPE *yylval)
+  {
+    static char const input[] = "b";
+    static size_t toknum;
+    assert (toknum < sizeof input);
+    *yylval = (toknum + 1) * 10;
+    return input[toknum++];
+  }
+
+  static void
+  yyerror (const char *msg)
+  {
+    fprintf (stderr, "%s\n", msg);
+  }
+
+  int
+  main (void)
+  {
+    yydebug = !!getenv("YYDEBUG");
+    return yyparse ();
+  }
+
+** yychar == yyempty_
+The code in yyerrlab reads:
+
+      if (yychar <= YYEOF)
+       {
+         /* Return failure if at end of input.  */
+         if (yychar == YYEOF)
+           YYABORT;

        }
 
        }
 

-       %{
-       ...
-       static int yylex PARAMS ((YYSTYPE *lval, const char **pexp));
-       ...
-       %}
+There are only two yychar that can be <= YYEOF: YYEMPTY and YYEOF.
+But I can't produce the situation where yychar is YYEMPTY here, is it
+really possible?  The test suite does not exercise this case.
+
+This shows that it would be interesting to manage to install skeleton
+coverage analysis to the test suite.
+
+** Table definitions
+It should be very easy to factor the definition of the various tables,
+including the separation bw declaration and definition.  See for
+instance b4_table_define in lalr1.cc.  This way, we could even factor
+C vs. C++ definitions.
+
+* From lalr1.cc to yacc.c
+** Single stack
+Merging the three stacks in lalr1.cc simplified the code, prompted for
+other improvements and also made it faster (probably because memory
+management is performed once instead of three times).  I suggest that
+we do the same in yacc.c.
+
+** yysyntax_error
+In lalr1.cc we invoke it with the translated lookahead (yytoken), and
+yacc.c uses yychar.  I don't see why.
+
+** yysyntax_error
+The use of switch to select yyfmt in lalr1.cc seems simpler than
+what's done in yacc.c.
+
+* Header guards
+
+From Franc,ois: should we keep the directory part in the CPP guard?
+
+
+* Yacc.c: CPP Macros
+
+Do some people use YYPURE, YYLSP_NEEDED like we do in the test suite?
+They should not: it is not documented.  But if they need to, let's
+find something clean (not like YYLSP_NEEDED...).
+
+
+* Installation
+
+* Documentation
+Before releasing, make sure the documentation ("Understanding your
+parser") refers to the current `output' format.
+
+* Report
+
+**  GLR
+How would Paul like to display the conflicted actions?  In particular,
+what when two reductions are possible on a given lookahead token, but one is
+part of $default.  Should we make the two reductions explicit, or just
+keep $default?  See the following point.
+
+** Disabled Reductions
+See `tests/conflicts.at (Defaulted Conflicted Reduction)', and decide
+what we want to do.
+
+** Documentation
+Extend with error productions.  The hard part will probably be finding
+the right rule so that a single state does not exhibit too many yet
+undocumented ``features''.  Maybe an empty action ought to be
+presented too.  Shall we try to make a single grammar with all these
+features, or should we have several very small grammars?
+
+** --report=conflict-path
+Provide better assistance for understanding the conflicts by providing
+a sample text exhibiting the (LALR) ambiguity.  See the paper from
+DeRemer and Penello: they already provide the algorithm.
+
+** Statically check for potential ambiguities in GLR grammars.  See
+<http://www.i3s.unice.fr/~schmitz/papers.html#expamb> for an approach.
+
+
+* Extensions
+
+** Labeling the symbols
+Have a look at the Lemon parser generator: instead of $1, $2 etc. they
+can name the values.  This is much more pleasant.  For instance:
+
+       exp (res): exp (a) '+' exp (b) { $res = $a + $b; };
+
+I love this.  I have been bitten too often by the removal of the
+symbol, and forgetting to shift all the $n to $n-1.  If you are
+unlucky, it compiles...
+
+But instead of using $a etc., we can use regular variables.  And
+instead of using (), I propose to use `:' (again).  Paul suggests
+supporting `->' in addition to `:' to separate LHS and RHS. In other
+words:
+
+       r:exp -> a:exp '+' b:exp { r = a + b; };
+
+That requires an significant improvement of the grammar parser.  Using
+GLR would be nice.  It also requires that Bison know the type of the
+symbols (which will be useful for %include anyway).  So we have some
+time before...
+
+Note that there remains the problem of locations: `@r'?

 
 

-Where the first part defines struct expression, the second uses it to
-define YYSTYPE, and the last uses YYSTYPE.  Only this order is valid.
+
+** $-1
+We should find a means to provide an access to values deep in the
+stack.  For instance, instead of
+
+       baz: qux { $$ = $<foo>-1 + $<bar>0 + $1; }
+
+we should be able to have:
+
+  foo($foo) bar($bar) baz($bar): qux($qux) { $baz = $foo + $bar + $qux; }
+
+Or something like this.
+
+** %if and the like
+It should be possible to have %if/%else/%endif.  The implementation is
+not clear: should it be lexical or syntactic.  Vadim Maslow thinks it
+must be in the scanner: we must not parse what is in a switched off
+part of %if.  Akim Demaille thinks it should be in the parser, so as
+to avoid falling into another CPP mistake.
+
+** XML Output
+There are couple of available extensions of Bison targeting some XML
+output.  Some day we should consider including them.  One issue is
+that they seem to be quite orthogonal to the parsing technique, and
+seem to depend mostly on the possibility to have some code triggered
+for each reduction.  As a matter of fact, such hooks could also be
+used to generate the yydebug traces.  Some generic scheme probably
+exists in there.
+
+XML output for GNU Bison and gcc
+   http://www.cs.may.ie/~jpower/Research/bisonXML/
+
+XML output for GNU Bison
+   http://yaxx.sourceforge.net/
+
+* Unit rules
+Maybe we could expand unit rules, i.e., transform
+
+       exp: arith | bool;
+       arith: exp '+' exp;
+       bool: exp '&' exp;
+
+into
+
+       exp: exp '+' exp | exp '&' exp;
+
+when there are no actions.  This can significantly speed up some
+grammars.  I can't find the papers.  In particular the book `LR
+parsing: Theory and Practice' is impossible to find, but according to
+`Parsing Techniques: a Practical Guide', it includes information about
+this issue.  Does anybody have it?
+
+
+
+* Documentation
+
+** History/Bibliography
+Some history of Bison and some bibliography would be most welcome.
+Are there any Texinfo standards for bibliography?
+
+
+
+* Java, Fortran, etc.
+
+
+* Coding system independence
+Paul notes:
+
+       Currently Bison assumes 8-bit bytes (i.e. that UCHAR_MAX is
+       255).  It also assumes that the 8-bit character encoding is
+       the same for the invocation of 'bison' as it is for the
+       invocation of 'cc', but this is not necessarily true when
+       people run bison on an ASCII host and then use cc on an EBCDIC
+       host.  I don't think these topics are worth our time
+       addressing (unless we find a gung-ho volunteer for EBCDIC or
+       PDP-10 ports :-) but they should probably be documented
+       somewhere.
+
+       More importantly, Bison does not currently allow NUL bytes in
+       tokens, either via escapes (e.g., "x\0y") or via a NUL byte in
+       the source code.  This should get fixed.

 
 * --graph
 
 * --graph

-Show reductions.       []
+Show reductions.

 
 * Broken options ?
 
 * Broken options ?

-** %no-lines           [ok]
-** %no-parser          []
-** %pure-parser                []
-** %semantic-parser    []
-** %token-table                []
-** Options which could use parse_dquoted_param ().
-Maybe transfered in lex.c.
-*** %skeleton          [ok]
-*** %output            []
-*** %file-prefix       []
-*** %name-prefix       []
-
-** Skeleton strategy.  []
-Must we keep %no-parser?
-            %token-table?
-*** New skeletons.     []
-
-* src/print_graph.c
-Find the best graph parameters.        []
-
-* doc/bison.texinfo
-** Update
-informations about ERROR_VERBOSE.      []
-** Add explainations about
-skeleton muscles.      []
-%skeleton.             []
-
-* testsuite
-** tests/pure-parser.at        []
-New tests.
-
-* Debugging parsers
-
-From Greg McGary:
-
-akim demaille <akim.demaille@epita.fr> writes:
-
-> With great pleasure!  Nonetheless, things which are debatable
-> (or not, but just `big') should be discuss in `public': something
-> like help- or bug-bison@gnu.org is just fine.  Jesse and I are there,
-> but there is also Jim and some other people.
-
-I have no idea whether it qualifies as big or controversial, so I'll
-just summarize for you.  I proposed this change years ago and was
-surprised that it was met with utter indifference!
-
-This debug feature is for the programs/grammars one develops with
-bison, not for debugging bison itself.  I find that the YYDEBUG
-output comes in a very inconvenient format for my purposes.
-When debugging gcc, for instance, what I want is to see a trace of
-the sequence of reductions and the line#s for the semantic actions
-so I can follow what's happening.  Single-step in gdb doesn't cut it
-because to move from one semantic action to the next takes you through
-lots of internal machinery of the parser, which is uninteresting.
-
-The change I made was to the format of the debug output, so that it
-comes out in the format of C error messages, digestible by emacs
-compile mode, like so:
-
-grammar.y:1234: foo: bar(0x123456) baz(0x345678)
-
-where "foo: bar baz" is the reduction rule, whose semantic action
-appears on line 1234 of the bison grammar file grammar.y.  The hex
-numbers on the rhs tokens are the parse-stack values associated with
-those tokens.  Of course, yytype might be something totally
-incompatible with that representation, but for the most part, yytype
-values are single words (scalars or pointers).  In the case of gcc,
-they're most often pointers to tree nodes.  Come to think of it, the
-right thing to do is to make the printing of stack values be
-user-definable.  It would also be useful to include the filename &
-line# of the file being parsed, but the main filename & line# should
-continue to be that of grammar.y
-
-Anyway, this feature has saved my life on numerous occasions.  The way
-I customarily use it is to first run bison with the traces on, isolate
-the sequence of reductions that interests me, put those traces in a
-buffer and force it into compile-mode, then visit each of those lines
-in the grammar and set breakpoints with C-x SPACE.  Then, I can run
-again under the control of gdb and stop at each semantic action.
-With the hex addresses of tree nodes, I can inspect the values
-associated with any rhs token.
-
-You like?
+** %token-table
+** Skeleton strategy
+Must we keep %token-table?
+
+* BTYacc
+See if we can integrate backtracking in Bison.  Charles-Henri de
+Boysson <de-boy_c@epita.fr> has been working on this, but never gave
+the results.
+
+Vadim Maslow, the maintainer of BTYacc was once contacted.  Adjusting
+the Bison grammar parser will be needed to support some extra BTYacc
+features.  This is less urgent.
+
+** Keeping the conflicted actions
+First, analyze the differences between byacc and btyacc (I'm referring
+to the executables).  Find where the conflicts are preserved.
+
+** Compare with the GLR tables
+See how isomorphic the way BTYacc and the way the GLR adjustments in
+Bison are compatible.  *As much as possible* one should try to use the
+same implementation in the Bison executables.  I insist: it should be
+very feasible to use the very same conflict tables.
+
+** Adjust the skeletons
+Import the skeletons for C and C++.
+
+
+* Precedence
+
+** Partial order
+It is unfortunate that there is a total order for precedence.  It
+makes it impossible to have modular precedence information.  We should
+move to partial orders (sounds like series/parallel orders to me).
+
+** RR conflicts
+See if we can use precedence between rules to solve RR conflicts.  See
+what POSIX says.
+
+
+* $undefined
+From Hans:
+- If the Bison generated parser experiences an undefined number in the
+character range, that character is written out in diagnostic messages, an
+addition to the $undefined value.
+
+Suggest: Change the name $undefined to undefined; looks better in outputs.
+
+
+* Default Action
+From Hans:
+- For use with my C++ parser, I transported the "switch (yyn)" statement
+that Bison writes to the bison.simple skeleton file. This way, I can remove
+the current default rule $$ = $1 implementation, which causes a double
+assignment to $$ which may not be OK under C++, replacing it with a
+"default:" part within the switch statement.
+
+Note that the default rule $$ = $1, when typed, is perfectly OK under C,
+but in the C++ implementation I made, this rule is different from
+$<type_name>$ = $<type_name>1. I therefore think that one should implement
+a Bison option where every typed default rule is explicitly written out
+(same typed ruled can of course be grouped together).
+
+* Pre and post actions.
+From: Florian Krohm <florian@edamail.fishkill.ibm.com>
+Subject: YYACT_EPILOGUE
+To: bug-bison@gnu.org
+X-Sent: 1 week, 4 days, 14 hours, 38 minutes, 11 seconds ago
+
+The other day I had the need for explicitly building the parse tree. I
+used %locations for that and defined YYLLOC_DEFAULT to call a function
+that returns the tree node for the production. Easy. But I also needed
+to assign the S-attribute to the tree node. That cannot be done in
+YYLLOC_DEFAULT, because it is invoked before the action is executed.
+The way I solved this was to define a macro YYACT_EPILOGUE that would
+be invoked after the action. For reasons of symmetry I also added
+YYACT_PROLOGUE. Although I had no use for that I can envision how it
+might come in handy for debugging purposes.
+All is needed is to add
+
+#if YYLSP_NEEDED
+    YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen, yyloc, (yylsp - yylen));
+#else
+    YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen);
+#endif
+
+at the proper place to bison.simple. Ditto for YYACT_PROLOGUE.
+
+I was wondering what you think about adding YYACT_PROLOGUE/EPILOGUE
+to bison. If you're interested, I'll work on a patch.
+
+* Better graphics
+Equip the parser with a means to create the (visual) parse tree.
+
+-----
+
+Copyright (C) 2001, 2002, 2003, 2004, 2006, 2008 Free Software Foundation,
+Inc.
+
+This file is part of Bison, the GNU Compiler Compiler.
+
+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 3 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, see <http://www.gnu.org/licenses/>.