parser: use %empty

[bison.git] / TODO

* Short term
** Graphviz display code thoughts
The code for the --graph option is over two files: print_graph, and
graphviz. This is because Bison used to also produce VCG graphs, but since
this is no longer true, maybe we could consider these files for fusion.

An other consideration worth noting is that print_graph.c (correct me if I
am wrong) should contain generic functions, whereas graphviz.c and other
potential files should contain just the specific code for that output
format. It will probably prove difficult to tell if the implementation is
actually generic whilst only having support for a single format, but it
would be nice to keep stuff a bit tidier: right now, the construction of the
bitset used to show reductions is in the graphviz-specific code, and on the
opposite side we have some use of \l, which is graphviz-specific, in what
should be generic code.

Little effort seems to have been given to factoring these files and their
rint{,-xml} counterpart. We would very much like to re-use the pretty format
of states from .output for the graphs, etc.

Also, the underscore in print_graph.[ch] isn't very fitting considering the
dashes in the other filenames.

Since graphviz dies on medium-to-big grammars, maybe consider an other tool?

** push-parser
Check it too when checking the different kinds of parsers.  And be
sure to check that the initial-action is performed once per parsing.

** m4 names
b4_shared_declarations is no longer what it is.  Make it
b4_parser_declaration for instance.

** yychar in lalr1.cc
There is a large difference bw maint and master on the handling of
yychar (which was removed in lalr1.cc).  See what needs to be
back-ported.


    /* User semantic actions sometimes alter yychar, and that requires
       that yytoken be updated with the new translation.  We take the
       approach of translating immediately before every use of yytoken.
       One alternative is translating here after every semantic action,
       but that translation would be missed if the semantic action
       invokes YYABORT, YYACCEPT, or YYERROR immediately after altering
       yychar.  In the case of YYABORT or YYACCEPT, an incorrect
       destructor might then be invoked immediately.  In the case of
       YYERROR, subsequent parser actions might lead to an incorrect
       destructor call or verbose syntax error message before the
       lookahead is translated.  */

    /* Make sure we have latest lookahead translation.  See comments at
       user semantic actions for why this is necessary.  */
    yytoken = yytranslate_ (yychar);


** stack.hh
Get rid of it.  The original idea is nice, but actually it makes
the code harder to follow, and uselessly different from the other
skeletons.

** Get rid of fake #lines [Bison: ...]
Possibly as simple as checking whether the column number is nonnegative.

I have seen messages like the following from GCC.

<built-in>:0: fatal error: opening dependency file .deps/libltdl/argz.Tpo: No such file or directory


** 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?

* Various
** 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?

Also, why don't we output the token name of the error token in the
output?  It is explicitly skipped:

      /* Skip error token and tokens without identifier.  */
      if (sym != errtoken && id)

Of course there are issues with name spaces, but if we disable we have
something which seems to be more simpler and more consistent instead
of the special case YYERRCODE.

   enum yytokentype {
     error = 256,
     // ...
   };


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\"",


** yychar == yyempty_
The code in yyerrlab reads:

      if (yychar <= YYEOF)
        {
          /* Return failure if at end of input.  */
          if (yychar == YYEOF)
            YYABORT;
        }

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.

* 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
The code bw glr.c and yacc.c is really alike, we can certainly factor
some parts.


* Report

** Figures
Some statistics about the grammar and the parser would be useful,
especially when asking the user to send some information about the
grammars she is working on.  We should probably also include some
information about the variables (I'm not sure for instance we even
specify what LR variant was used).

**  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

** $-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?

* 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.

* Broken options ?
** %token-table
** Skeleton strategy
Must we keep %token-table?

* 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.

* Complaint submessage indentation.
We already have an implementation that works fairly well for named
reference messages, but it would be nice to use it consistently for all
submessages from Bison.  For example, the "previous definition"
submessage or the list of correct values for a %define variable might
look better with indentation.

However, the current implementation makes the assumption that the
location printed on the first line is not usually much shorter than the
locations printed on the submessage lines that follow.  That assumption
may not hold true as often for some kinds of submessages especially if
we ever support multiple grammar files.

Here's a proposal for how a new implementation might look:

  http://lists.gnu.org/archive/html/bison-patches/2009-09/msg00086.html


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