TODO

* Bison 3.6
** doc
I feel its 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.

Also, we seem to teach YYPRINT very early on, although it should be
considered deprecated: %printer is superior.

** improve syntax errors (UTF-8, internationalization)
Bison depends on the current locale.  For instance:

%define parse.error verbose
%code top {
 #include <stdio.h>
 #include <stdlib.h>
 void yyerror(const char* msg) { fprintf(stderr, "%s\n", msg); }
 int yylex() { return 0; }
}
%%
exp: "↦" | "🎅🐃" | '\n'
%%
int main() { return yyparse(); }

gives different results with/without LC_ALL=C.

$ LC_ALL=C /opt/local/bin/bison /tmp/mangle.y -o ascii.c
$ /opt/local/bin/bison /tmp/mangle.y -o utf8.c
$ diff -u ascii.c utf8.c -I#line
--- ascii.c     2019-01-12 08:15:35.878010093 +0100
+++ utf8.c      2019-01-12 08:15:38.856495929 +0100
@@ -415,9 +415,8 @@
    First, the terminals, then, starting at YYNTOKENS, nonterminals.  */
 static const char *const yytname[] =
 {
-  "$end", "error", "$undefined", "\"\\342\\206\\246\"",
-  "\"\\360\\237\\216\\205\\360\\237\\220\\203\"", "'\\n'", "$accept",
-  "exp", YY_NULLPTR
+  "$end", "error", "$undefined", "\"↦\"", "\"🎅🐃\"", "'\\n'",
+  "$accept", "exp", YY_NULLPTR
 };
 #endif

$ gcc ascii.c -o ascii && ./ascii
syntax error, unexpected $end, expecting "\342\206\246" or "\360\237\216\205\360\237\220\203" or '\n'
$ gcc utf8.c -o utf8 && ./utf8
syntax error, unexpected $end, expecting ↦ or 🎅🐃 or '\n'


While at it, we should stop using "$end" by default, in favor of "end of
file", or "end of input", whatever.  See how lalr1.java does that.

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

** Stop indentation in diagnostics
Before Bison 2.7, we printed "flatly" the dependencies in long diagnostics:

    input.y:2.7-12: %type redeclaration for exp
    input.y:1.7-12: previous declaration

In Bison 2.7, we indented them

    input.y:2.7-12: error: %type redeclaration for exp
    input.y:1.7-12:     previous declaration

Later we quoted the source in the diagnostics, and today we have:

    /tmp/foo.y:1.12-14: warning: symbol FOO redeclared [-Wother]
        1 | %token FOO FOO
          |            ^~~
    /tmp/foo.y:1.8-10:      previous declaration
        1 | %token FOO FOO
          |        ^~~

The indentation is no longer helping.  We should probably get rid of it, or
maybe keep it only when -fno-caret. GCC displays this as a "note":

    $ g++-mp-9 -Wall /tmp/foo.c -c
    /tmp/foo.c:1:10: error: redefinition of 'int foo'
        1 | int foo, foo;
          |          ^~~
    /tmp/foo.c:1:5: note: 'int foo' previously declared here
        1 | int foo, foo;
          |     ^~~

Likewise for Clang, contrary to what I believed (because "note:" is written
in black, so it doesn't show in my terminal :-)

    $ clang++-mp-8.0 -Wall /tmp/foo.c -c
    clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
    /tmp/foo.c:1:10: error: redefinition of 'foo'
    int foo, foo;
             ^
    /tmp/foo.c:1:5: note: previous definition is here
    int foo, foo;
        ^
    1 error generated.

See also the item "Complaint submessage indentation" below.

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

** C++
Move to int everywhere instead of unsigned?  stack_size, etc.  The parser
itself uses int (for yylen for instance), yet stack is based on size_t.

Maybe locations should also move to ints.

Paul Eggert already covered most of this.  But before publishing these
changes, we need to ask our C++ users if they agree with that change, or if
we need some migration path.  Could be a %define variable, or simply
%require "3.5".

* Bison 3.7
** 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.

* 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.cc, lalr1.cc
- ielr: C++ and Java
- glr: Java
- token constructors: Java and C

* 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 adhoc, 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.

** 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
  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.

(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.

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

This should be worked on when we also address the expected improvements for
error generation (e.g., i18n).


* 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
** 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.

** 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
and https://research.swtch.com/yyerror.

** %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:
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