-*- outline -*- * 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...). * Documentation Before releasing, make sure the documentation ("Understanding your parser") refers to the current `output' format. * lalr1.cc ** vector Move to using vector, drop stack.hh. ** 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 look-ahead 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. * 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. ** -D, --define-muscle NAME=VALUE To define muscles via cli. Or maybe support directly NAME=VALUE? ** 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 XML output for GNU Bison * 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. ** Java There are a couple of proposed outputs: - BYACC/J which is based on Byacc. <> - Bison Java which is based on Bison. <> Sebastien Serrurier (serrur_s@epita.fr) is working on this: he is expected to contact the authors, design the output, and implement it into Bison. * 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 ? ** %no-parser ** %token-table ** Skeleton strategy Must we keep %no-parser? %token-table? * src/print_graph.c Find the best graph parameters. * BTYacc See if we can integrate backtracking in Bison. Charles-Henri de Boysson <de-boy_c@epita.fr> is working on this, and already has some results. Vadim Maslow, the maintainer of BTYacc was contacted, and we stay in touch with him. 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++. ** 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. ----- Copyright (C) 2001, 2002, 2003, 2004,.