* Bison 3.6
** Questions
*** Java
- Should i18n be part of the Lexer?  Currently it's a static method of
  Lexer.

- is there a migration path that would allow to use TokenKinds in
  yylex?

*** D
- is there a way to attach yysymbol_name to the enum itself?  As we did
  in Java.

- It would be better to have TokenKind as return value.  Can we use
  reflection to support both output types?

** Documentation
- YYERRCODE, YYUNDEF, YYEOF
- YYERRCODE and translation

** yypcontext_expected_tokens
Beware that returning 0 is unclear: does it mean there are no possible
lookahead, or that there are too many?

** YYERRCODE
How about using "return YYERRCODE" to tell the parser to enter
error-recovery, but not emit an error message?  I have frequently felt the
need for the scanner to be able to do that.  In C++ we can, thanks to throw
syntax_error.

Some people seem to be aware that return YYERRCODE is possible...
https://github.com/search?q=%22return+YYERRCODE%22&type=Code

*** unixODBC
There are tons of copies on GitHub, I don't know where the original is.

https://github.com/faiz-akhtar/Calico/blob/01b49089a3eb535380877bbdc6c641b8b4e7ca6f/Drivers/nn/yylex.c

But they define YYERRCODE themselves!  They are stepping on "our" namespace.

```
# define	YYERRCODE	256
```

Besides, what they do is super-weird: they use YYERRCODE is several places
for their own error tracking.

```
int	nnsql_yylex(YYSTYPE* pyylval, yyenv_t* penv)
{
[...]
                len = getname(penv->texts_bufptr, -1, penv);

                if( len == YYERRCODE )
                        return len;
```

*** avrdude-6.3

Latest version: http://svn.savannah.gnu.org/viewvc/avrdude/trunk/avrdude/lexer.l?view=markup

We can certainly talk to them about their use of YYERRCODE.

Replicas on GitHub:

https://github.com/tam1035/Phys605-March27/blob/0f856baf20877b6f333f91ed17860bd752c7d68e/AVR/avrdude-6.3/lexer.l

They emit error messages from the scanner and force error recovery.  But
then, they get two error messages.

```
c: { yyerror("possible old-style config file entry\n"
             "  Update your config file (see " CONFIG_DIR
               "/avrdude.conf.sample for a sample)");
     return YYERRCODE; }
```

Unfortunately they also expect the error message at some places.  Here,
return YYUNDEFTOK (or just return yytext[0]) would have been better.

```
. { return YYERRCODE; }
```

*** Casacore  A suite of C++ libraries for radio astronomy data processing.

https://github.com/casacore/casacore/blob/110bf435c3fe292ee0c3e4bdd039853d257c9536/ms/MSSel/MSAntennaGram.ll

```
 /* Any other character is invalid */
.         { return YYERRCODE; }
```

A pattern they used several times.
https://github.com/casacore/casacore/search?q=YYERRCODE&unscoped_q=YYERRCODE

I don't understand how this works: AFAICT, we only define YYERRCODE in
y.tab.c, not y.tab.h.  That's the case in Apple's Bison 2.3 for instance.
Flex does not generate it either.

*** Some shell
Seems to be old versions of busybox.

https://github.com/luotao717/arsdk/blob/9f9004157afcd26b915125576a815f6b3e0cd715/apps/busybox-1.01/shell/msh.c


```
#define	YYERRCODE 300
```

Used only in one function (collect)

But today the parser is written by hand.

https://github.com/mirror/busybox/blob/master/shell/ash.c#L11527

*** glibc's intl

https://sourceware.org/git/?p=glibc.git;a=blob;f=intl/plural.y;h=f69fba7c602e9f890f8e5c3b2cd7fe1c487714ea;hb=HEAD

Their yylex returns YYERRCODE, but they don't want any error message.

```
 379 static void
 380 yyerror (struct parse_args *arg, const char *str)
 381 {
 382   /* Do nothing.  We don't print error messages here.  */
 383 }
```

This code was not updated in years.

** Java
*** Examples
Have an example with a push parser.  Use autocompletion in that case.

*** calc.at
Stop hard-coding "Calc".  Adjust local.at (look for FIXME).

** doc
I feel it's ugly to use the GNU style to declare functions in the doc.  It
generates tons of white space in the page, and may contribute to bad page
breaks.

** consistency
token vs terminal, variable vs non terminal.

** api.token.raw
The YYUNDEFTOK could be assigned a semantic value so that yyerror could be
used to report invalid lexemes.  See also the item "$undefined" below.

** push parsers
Consider deprecating impure push parsers.  They add a lot of complexity, for
a bad feature.  On the other hand, that would make it much harder to sit
push parsers on top of pull parser.  Which is currently not relevant, since
push parsers are measurably slower.

* Bison 3.7
** Counter example generation
See https://github.com/akimd/bison/pull/15.

** Clean up
Rename user_token_number for tokens as "code".  It's not a "user number",
it's the token code, and the user can control it, but this code always
exists.

See also USER_NUMBER_UNDEFINED and NUMBER_UNDEFINED.

Rename endtoken as eoftoken.

Also do it in data/skeletons.

Don't rename in Bison 3.6 (it would be logical to do so) because that
would probably create many conflicts in Vincent's work (see previous point).

* Bison 3.8
** Unit rules / Injection rules (Akim Demaille)
Maybe we could expand unit rules (or "injections", see
https://homepages.cwi.nl/~daybuild/daily-books/syntax/2-sdf/sdf.html), 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?

** clean up (Akim Demaille)
Do not work on these items now, as I (Akim) have branches with a lot of
changes in this area (hitting several files), and no desire to have to fix
conflicts.  Addressing these items will happen after my branches have been
merged.

*** lalr.c
Introduce a goto struct, and use it in place of from_state/to_state.
Rename states1 as path, length as pathlen.
Introduce inline functions for things such as nullable[*rp - ntokens]
where we need to map from symbol number to nterm number.

There are probably a significant part of the relations management that
should be migrated on top of a bitsetv.

*** closure
It should probably take a "state*" instead of two arguments.

*** traces
The "automaton" and "set" categories are not so useful.  We should probably
introduce lr(0) and lalr, just the way we have ielr categories.  The
"closure" function is too verbose, it should probably have its own category.

"set" can still be used for summariring the important sets.  That would make
tests easy to maintain.

*** complain.*
Rename these guys as "diagnostics.*" (or "diagnose.*"), since that's the
name they have in gcc, clang, etc.  Likewise for the complain_* series of
functions.

*** ritem
states/nstates, rules/nrules, ..., ritem/nritems
Fix the latter.

* Better error messages
The users are not provided with enough tools to forge their error messages.
See for instance "Is there an option to change the message produced by
YYERROR_VERBOSE?" by Simon Sobisch, on bison-help.

See also
https://www.cs.tufts.edu/~nr/cs257/archive/clinton-jefferey/lr-error-messages.pdf
https://research.swtch.com/yyerror
http://gallium.inria.fr/~fpottier/publis/fpottier-reachability-cc2016.pdf

* Modernization
Fix data/skeletons/yacc.c so that it defines YYPTRDIFF_T properly for modern
and older C++ compilers.  Currently the code defaults to defining it to
'long' for non-GCC compilers, but it should use the proper C++ magic to
define it to the same type as the C ptrdiff_t type.

* Completion
Several features are not available in all the backends.

- push parsers: glr.c, glr.cc, lalr1.cc
- lac: D, Java
- glr: D, Java
- token constructors: Java, C, D

* Bugs
** Autotest has quotation issues
tests/input.at:1730:AT_SETUP([%define errors])

->

$ ./tests/testsuite -l | grep errors | sed q
  38: input.at:1730      errors

* Short term
** Get rid of YYPRINT and b4_toknum
Besides yytoknum is wrong when api.token.raw is defined.

** Better design for diagnostics
The current implementation of diagnostics is ad hoc, it grew organically.
It works as a series of calls to several functions, with dependency of the
latter calls on the former.  For instance:

      complain (&sym->location,
                sym->content->status == needed ? complaint : Wother,
                _("symbol %s is used, but is not defined as a token"
                  " and has no rules; did you mean %s?"),
                quote_n (0, sym->tag),
                quote_n (1, best->tag));
      if (feature_flag & feature_caret)
        location_caret_suggestion (sym->location, best->tag, stderr);

We should rewrite this in a more FP way:

1. build a rich structure that denotes the (complete) diagnostic.
   "Complete" in the sense that it also contains the suggestions, the list
   of possible matches, etc.

2. send this to the pretty-printing routine.  The diagnostic structure
   should be sufficient so that we can generate all the 'format' of
   diagnostics, including the fixits.

If properly done, this diagnostic module can be detached from Bison and be
put in gnulib.  It could be used, for instance, for errors caught by
xgettext.

There's certainly already something alike in GCC.  At least that's the
impression I get from reading the "-fdiagnostics-format=FORMAT" part of this
page:

https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Message-Formatting-Options.html

** 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.

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);


** 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).

* Various
** Rewrite glr.cc in C++ (Valentin Tolmer)
As a matter of fact, it would be very interesting to see how much we can
share between lalr1.cc and glr.cc.  Most of the skeletons should be common.
It would be a very nice source of inspiration for the other languages.

Valentin Tolmer is working on this.

** 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.

(Some time later): it's also very nice to have three stacks: it's more dense
as we don't lose bits to padding.  For instance the typical stack for states
will use 8 bits, while it is likely to consume 32 bits in a struct.

We need trustworthy benchmarks for Bison, for all our backends.  Akim has a
few things scattered around; we need to put them in the repo, and make them
more useful.

* 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.lsv.fr/~schmitz/pub/expamb.pdf> for an approach.
An Experimental Ambiguity Detection Tool ∗ Sylvain Schmitz
LORIA, INRIA Nancy - Grand Est, Nancy, France

* Extensions
** More languages?
Well, only if there is really some demand for it.

https://github.com/scfc/bison-php/blob/master/data/lalr1.php

** Multiple start symbols
Would be very useful when parsing closely related languages.  The idea is to
declare several start symbols, for instance

    %start stmt expr
    %%
    stmt: ...
    expr: ...

and to generate parse(), parse_stmt() and parse_expr().  Technically, the
above grammar would be transformed into

   %start yy_start
   %token YY_START_STMT YY_START_EXPR
   %%
   yy_start: YY_START_STMT stmt | YY_START_EXPR expr

so that there are no new conflicts in the grammar (as would undoubtedly
happen with yy_start: stmt | expr).  Then adjust the skeletons so that this
initial token (YY_START_STMT, YY_START_EXPR) be shifted first in the
corresponding parse function.

** %include
This is a popular demand.  We already made many changes in the parser that
should make this reasonably easy to implement.

Bruce Mardle <marblypup@yahoo.co.uk>
https://lists.gnu.org/archive/html/bison-patches/2015-09/msg00000.html

However, there are many other things to do before having such a feature,
because I don't want a % equivalent to #include (which we all learned to
hate).  I want something that builds "modules" of grammars, and assembles
them together, paying attention to keep separate bits separated, in pseudo
name spaces.

** Push parsers
There is demand for push parsers in Java and C++.  And GLR I guess.

** Generate code instead of tables
This is certainly quite a lot of work.  See
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.50.4539.

** $-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.

(Later): I'm sure there's actually good case for this.  People who need that
feature can use m4/cpp on top of Bison.  I don't think it is worth the
trouble in Bison itself.

** 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/

** Counterexample generation
https://lists.gnu.org/archive/html/bug-bison/2016-06/msg00000.html
http://www.cs.cornell.edu/andru/papers/cupex/

Andrew Myers and Vincent Imbimbo are working on this item, see
https://github.com/akimd/bison/issues/12

* 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).

This is a prerequisite for modules.

* $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.


* 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.


Local Variables:
mode: outline
coding: utf-8
fill-column: 76
End:

-----

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