X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/817e9f41d135945f3caa781ceab68a112edc5613..026816664ff8283a55f91915843a8ff0ac5cf86c:/TODO diff --git a/TODO b/TODO index e5c3cc15..09fce089 100644 --- a/TODO +++ b/TODO @@ -1,149 +1,387 @@ --*- outline -*- +* Short term +** 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. + +Also, the underscore in print_graph.[ch] isn't very fitting considering the +dashes in the other filenames. + +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); + + +** stack.hh +Get rid of it. The original idea is nice, but actually it makes +the code harder to follow, and uselessly different from the other +skeletons. + +** 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. + +: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() << $$; } ; + +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). + +** Rename LR0.cc +as lr0.cc, why upper case? + +* 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? + +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 int + 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. + +** yysyntax_error +The code bw glr.c and yacc.c is really alike, we can certainly factor +some parts. + + +* 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 + for an approach. + + +* Extensions + +** $-1 +We should find a means to provide an access to values deep in the +stack. For instance, instead of + + baz: qux { $$ = $-1 + $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; + exp: arith | bool; + arith: exp '+' exp; + bool: exp '&' exp; into - exp: exp '+' exp | exp '&' exp; + exp: exp '+' exp | exp '&' exp; when there are no actions. This can significantly speed up some -grammars. +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? -* Huge Grammars -Currently, not only is Bison unable to handle huge grammars because of -internal limitations, but the test `big triangle' also demonstrates -that it can produce SEGVing executables! Push the limit beyond 124, -and have a core dump. Be my guest: fix this! -* 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. -* NEWS -Sort from 1.31 NEWS. +* Documentation -* Prologue -The %union is declared after the user C declarations. It can be -a problem if YYSTYPE is declared after the user part. [] +** History/Bibliography +Some history of Bison and some bibliography would be most welcome. +Are there any Texinfo standards for bibliography? -Actually, the real problem seems that the %union ought to be output -where it was defined. For instance, in gettext/intl/plural.y, we -have: +* Coding system independence +Paul notes: - %{ - ... - #include "gettextP.h" - ... - %} + 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. - %union { - unsigned long int num; - enum operator op; - struct expression *exp; - } + 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. - %{ - ... - static int yylex PARAMS ((YYSTYPE *lval, const char **pexp)); - ... - %} +* Broken options ? +** %token-table +** Skeleton strategy +Must we keep %token-table? -Where the first part defines struct expression, the second uses it to -define YYSTYPE, and the last uses YYSTYPE. Only this order is valid. +* Precedence -* --graph -Show reductions. [] +** 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). -* Broken options ? -** %no-lines [ok] -** %no-parser [] -** %pure-parser [] -** %semantic-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. - -* Debugging parsers - -From Greg McGary: - -akim demaille writes: - -> With great pleasure! Nonetheless, things which are debatable -> (or not, but just `big') should be discuss in `public': something -> like help- or bug-bison@gnu.org is just fine. Jesse and I are there, -> but there is also Jim and some other people. - -I have no idea whether it qualifies as big or controversial, so I'll -just summarize for you. I proposed this change years ago and was -surprised that it was met with utter indifference! - -This debug feature is for the programs/grammars one develops with -bison, not for debugging bison itself. I find that the YYDEBUG -output comes in a very inconvenient format for my purposes. -When debugging gcc, for instance, what I want is to see a trace of -the sequence of reductions and the line#s for the semantic actions -so I can follow what's happening. Single-step in gdb doesn't cut it -because to move from one semantic action to the next takes you through -lots of internal machinery of the parser, which is uninteresting. - -The change I made was to the format of the debug output, so that it -comes out in the format of C error messages, digestible by emacs -compile mode, like so: - -grammar.y:1234: foo: bar(0x123456) baz(0x345678) - -where "foo: bar baz" is the reduction rule, whose semantic action -appears on line 1234 of the bison grammar file grammar.y. The hex -numbers on the rhs tokens are the parse-stack values associated with -those tokens. Of course, yytype might be something totally -incompatible with that representation, but for the most part, yytype -values are single words (scalars or pointers). In the case of gcc, -they're most often pointers to tree nodes. Come to think of it, the -right thing to do is to make the printing of stack values be -user-definable. It would also be useful to include the filename & -line# of the file being parsed, but the main filename & line# should -continue to be that of grammar.y - -Anyway, this feature has saved my life on numerous occasions. The way -I customarily use it is to first run bison with the traces on, isolate -the sequence of reductions that interests me, put those traces in a -buffer and force it into compile-mode, then visit each of those lines -in the grammar and set breakpoints with C-x SPACE. Then, I can run -again under the control of gdb and stop at each semantic action. -With the hex addresses of tree nodes, I can inspect the values -associated with any rhs token. - -You like? - -* input synclines -Some users create their foo.y files, and equip them with #line. Bison -should recognize these, and preserve them. +** 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 $$ = $1, when typed, is perfectly OK under C, +but in the C++ implementation I made, this rule is different from +$$ = $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 +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. + +* Complaint submessage indentation. +We already have an implementation that works fairly well for named +reference messages, but it would be nice to use it consistently for all +submessages from Bison. For example, the "previous definition" +submessage or the list of correct values for a %define variable might +look better with indentation. + +However, the current implementation makes the assumption that the +location printed on the first line is not usually much shorter than the +locations printed on the submessage lines that follow. That assumption +may not hold true as often for some kinds of submessages especially if +we ever support multiple grammar files. + +Here's a proposal for how a new implementation might look: + + http://lists.gnu.org/archive/html/bison-patches/2009-09/msg00086.html + + +Local Variables: +mode: outline +coding: utf-8 +End: + +----- + +Copyright (C) 2001-2004, 2006, 2008-2013 Free Software Foundation, Inc. + +This file is part of Bison, the GNU Compiler Compiler. + +This program is free software: you can redistribute it and/or modify +it under the terms of the GNU General Public License as published by +the Free Software Foundation, either version 3 of the License, or +(at your option) any later version. + +This program is distributed in the hope that it will be useful, +but WITHOUT ANY WARRANTY; without even the implied warranty of +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +GNU General Public License for more details. + +You should have received a copy of the GNU General Public License +along with this program. If not, see .