yychar cannot be empty in yyerrlab.

[bison.git] / TODO

-*- outline -*-

* 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?

** 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;
        }

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.

* 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.

* lalr1.cc
** I18n
Catch up with yacc.c.

* 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'?


** $-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
Show reductions.

* Broken options ?
** %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.

-----

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