Update.

[bison.git] / TODO

-*- outline -*-

* URGENT: Documenting C++ output
Write a first documentation for C++ output.


* Error messages
Some are really funky.  For instance

        type clash (`%s' `%s') on default action

is really weird.  Revisit them all.

* Stupid error messages
An example shows it easily:

src/bison/tests % ./testsuite -k calc,location,error-verbose -l
GNU Bison 1.49a test suite test groups:

 NUM: FILENAME:LINE      TEST-GROUP-NAME
      KEYWORDS

  51: calc.at:440        Calculator --locations --yyerror-verbose
  52: calc.at:442        Calculator --defines --locations --name-prefix=calc --verbose --yacc --yyerror-verbose
  54: calc.at:445        Calculator --debug --defines --locations --name-prefix=calc --verbose --yacc --yyerror-verbose
src/bison/tests % ./testsuite 51 -d
## --------------------------- ##
## GNU Bison 1.49a test suite. ##
## --------------------------- ##
 51: calc.at:440       ok
## ---------------------------- ##
## All 1 tests were successful. ##
## ---------------------------- ##
src/bison/tests % cd ./testsuite.dir/51
tests/testsuite.dir/51 % echo "()" | ./calc
1.2-1.3: parse error, unexpected ')', expecting error or "number" or '-' or '('


* read_pipe.c
This is not portable to DOS for instance.  Implement a more portable
scheme.  Sources of inspiration include GNU diff, and Free Recode.


* value_components_used
Was defined but not used: where was it coming from?  It can't be to
check if %union is used, since the user is free to $<foo>n on her
union, doesn't she?


* Report

**  GLR
How would Paul like to display the conflicted actions?  In particular,
what when two reductions are possible on a given lookahead, 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.


* Extentions

** yyerror, yysymprint interface
It should be improved, in particular when using Bison features such as
locations, and YYPARSE_PARAMS.  For the time being, it is recommended
to #define yyerror and yyprint to steal internal variables...

** Several %unions
I think this is a pleasant (but useless currently) feature, but in the
future, I want a means to %include other bits of grammars, and _then_
it will be important for the various bits to define their needs in
%union.

When implementing multiple-%union support, bare the following in mind:

- when --yacc, this must be flagged as an error.  Don't make it fatal
  though.

- The #line must now appear *inside* the definition of yystype.
  Something like

        {
        #line 12 "foo.y"
          int ival;
        #line 23 "foo.y"
          char *sval;
        }

* 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

** Vocabulary
Explain $axiom (and maybe change its name: BTYacc names it `goal',
byacc `$accept' probably based on AT&T Yacc, Meta `Start'...).
Complete the glossary (item, axiom, ?).  Should we also rename `$'?
BYacc uses `$end'.  `$eof' is attracting, but after all we may be
parsing a string, a stream etc.

** 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.



* --graph
Show reductions.        []

* Broken options ?
** %no-lines            [ok]
** %no-parser           []
** %pure-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.

* input synclines
Some users create their foo.y files, and equip them with #line.  Bison
should recognize these, and preserve them.

* BTYacc
See if we can integrate backtracking in Bison.  Contact the BTYacc
maintainers.

** 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 adjustements 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++.

** Improve the skeletons
Have them support yysymprint, yydestruct and so forth.


* 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 will be possible with a Bison parser for the grammar, as it will
make it much easier to extend the grammar.

** Correlation b/w precedence and associativity
Also, I fail to understand why we have to assign the same
associativity to operators with the same precedence.  For instance,
why can't I decide that the precedence of * and / is the same, but the
latter is nonassoc?

If there is really no profound motivation, we should find a new syntax
to allow specifying this.

** 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).

Note: Robert Anisko handles this.  He knows how to do it.


* Warnings
It would be nice to have warning support.  See how Autoconf handles
them, it is fairly well described there.  It would be very nice to
implement this in such a way that other programs could use
lib/warnings.[ch].

Don't work on this without first announcing you do, as I already have
thought about it, and know many of the components that can be used to
implement it.


* 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.

* Move to Graphviz
Well, VCG seems really dead.  Move to Graphviz instead.  Also, equip
the parser with a means to create the (visual) parse tree.

-----

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