369 lines
12 KiB
Text
369 lines
12 KiB
Text
* Short term
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** Graphviz display code thoughts
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The code for the --graph option is over two files: print_graph, and
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graphviz. I believe this is because Bison used to also produce VCG graphs,
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but since this is no longer true, maybe we could consider these files for
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fusion.
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Little effort factoring seems to have been given to factoring in these files,
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and their print-xml and print counterpart. We would very much like to re-use
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the pretty format of states from .output in the .dot
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Also, the underscore in print_graph.[ch] isn't very fitting considering
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the dashes in the other filenames.
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** Variable names.
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What should we name `variant' and `lex_symbol'?
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** Use b4_symbol in all the skeleton
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Move its definition in the more standard places and deploy it in other
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skeletons. Then remove the older system, including the tables
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generated by output.c
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** Update the documentation on gnu.org
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** Get rid of fake #lines [Bison: ...]
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Possibly as simple as checking whether the column number is nonnegative.
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I have seen messages like the following from GCC.
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<built-in>:0: fatal error: opening dependency file .deps/libltdl/argz.Tpo: No such file or directory
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** Discuss about %printer/%destroy in the case of C++.
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It would be very nice to provide the symbol classes with an operator<<
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and a destructor. Unfortunately the syntax we have chosen for
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%destroy and %printer make them hard to reuse. For instance, the user
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is invited to write something like
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%printer { debug_stream() << $$; } <my_type>;
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which is hard to reuse elsewhere since it wants to use
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"debug_stream()" to find the stream to use. The same applies to
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%destroy: we told the user she could use the members of the Parser
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class in the printers/destructors, which is not good for an operator<<
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since it is no longer bound to a particular parser, it's just a
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(standalone symbol).
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** Rename LR0.cc
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as lr0.cc, why upper case?
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** bench several bisons.
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Enhance bench.pl with %b to run different bisons.
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* Various
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** Warnings
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Warnings about type tags that are used in printer and dtors, but not
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for symbols?
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** YYERRCODE
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Defined to 256, but not used, not documented. Probably the token
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number for the error token, which POSIX wants to be 256, but which
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Bison might renumber if the user used number 256. Keep fix and doc?
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Throw away?
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Also, why don't we output the token name of the error token in the
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output? It is explicitly skipped:
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/* Skip error token and tokens without identifier. */
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if (sym != errtoken && id)
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Of course there are issues with name spaces, but if we disable we have
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something which seems to be more simpler and more consistent instead
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of the special case YYERRCODE.
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enum yytokentype {
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error = 256,
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// ...
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};
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We could (should?) also treat the case of the undef_token, which is
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numbered 257 for yylex, and 2 internal. Both appear for instance in
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toknum:
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const unsigned short int
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parser::yytoken_number_[] =
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{
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0, 256, 257, 258, 259, 260, 261, 262, 263, 264,
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while here
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enum yytokentype {
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TOK_EOF = 0,
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TOK_EQ = 258,
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so both 256 and 257 are "mysterious".
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const char*
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const parser::yytname_[] =
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{
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"\"end of command\"", "error", "$undefined", "\"=\"", "\"break\"",
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** YYFAIL
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It is seems to be *really* obsolete now, shall we remove it?
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** yychar == yyempty_
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The code in yyerrlab reads:
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if (yychar <= YYEOF)
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{
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/* Return failure if at end of input. */
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if (yychar == YYEOF)
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YYABORT;
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}
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There are only two yychar that can be <= YYEOF: YYEMPTY and YYEOF.
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But I can't produce the situation where yychar is YYEMPTY here, is it
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really possible? The test suite does not exercise this case.
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This shows that it would be interesting to manage to install skeleton
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coverage analysis to the test suite.
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** Table definitions
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It should be very easy to factor the definition of the various tables,
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including the separation bw declaration and definition. See for
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instance b4_table_define in lalr1.cc. This way, we could even factor
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C vs. C++ definitions.
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* From lalr1.cc to yacc.c
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** Single stack
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Merging the three stacks in lalr1.cc simplified the code, prompted for
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other improvements and also made it faster (probably because memory
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management is performed once instead of three times). I suggest that
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we do the same in yacc.c.
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** yysyntax_error
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The code bw glr.c and yacc.c is really alike, we can certainly factor
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some parts.
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* Report
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** Figures
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Some statistics about the grammar and the parser would be useful,
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especially when asking the user to send some information about the
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grammars she is working on. We should probably also include some
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information about the variables (I'm not sure for instance we even
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specify what LR variant was used).
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** GLR
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How would Paul like to display the conflicted actions? In particular,
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what when two reductions are possible on a given lookahead token, but one is
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part of $default. Should we make the two reductions explicit, or just
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keep $default? See the following point.
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** Disabled Reductions
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See `tests/conflicts.at (Defaulted Conflicted Reduction)', and decide
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what we want to do.
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** Documentation
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Extend with error productions. The hard part will probably be finding
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the right rule so that a single state does not exhibit too many yet
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undocumented ``features''. Maybe an empty action ought to be
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presented too. Shall we try to make a single grammar with all these
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features, or should we have several very small grammars?
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** --report=conflict-path
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Provide better assistance for understanding the conflicts by providing
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a sample text exhibiting the (LALR) ambiguity. See the paper from
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DeRemer and Penello: they already provide the algorithm.
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** Statically check for potential ambiguities in GLR grammars. See
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<http://www.i3s.unice.fr/~schmitz/papers.html#expamb> for an approach.
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* Extensions
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** $-1
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We should find a means to provide an access to values deep in the
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stack. For instance, instead of
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baz: qux { $$ = $<foo>-1 + $<bar>0 + $1; }
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we should be able to have:
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foo($foo) bar($bar) baz($bar): qux($qux) { $baz = $foo + $bar + $qux; }
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Or something like this.
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** %if and the like
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It should be possible to have %if/%else/%endif. The implementation is
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not clear: should it be lexical or syntactic. Vadim Maslow thinks it
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must be in the scanner: we must not parse what is in a switched off
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part of %if. Akim Demaille thinks it should be in the parser, so as
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to avoid falling into another CPP mistake.
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** XML Output
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There are couple of available extensions of Bison targeting some XML
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output. Some day we should consider including them. One issue is
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that they seem to be quite orthogonal to the parsing technique, and
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seem to depend mostly on the possibility to have some code triggered
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for each reduction. As a matter of fact, such hooks could also be
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used to generate the yydebug traces. Some generic scheme probably
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exists in there.
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XML output for GNU Bison and gcc
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http://www.cs.may.ie/~jpower/Research/bisonXML/
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XML output for GNU Bison
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http://yaxx.sourceforge.net/
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* Unit rules
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Maybe we could expand unit rules, i.e., transform
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exp: arith | bool;
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arith: exp '+' exp;
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bool: exp '&' exp;
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into
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exp: exp '+' exp | exp '&' exp;
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when there are no actions. This can significantly speed up some
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grammars. I can't find the papers. In particular the book `LR
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parsing: Theory and Practice' is impossible to find, but according to
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`Parsing Techniques: a Practical Guide', it includes information about
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this issue. Does anybody have it?
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* Documentation
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** History/Bibliography
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Some history of Bison and some bibliography would be most welcome.
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Are there any Texinfo standards for bibliography?
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* Coding system independence
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Paul notes:
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Currently Bison assumes 8-bit bytes (i.e. that UCHAR_MAX is
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255). It also assumes that the 8-bit character encoding is
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the same for the invocation of 'bison' as it is for the
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invocation of 'cc', but this is not necessarily true when
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people run bison on an ASCII host and then use cc on an EBCDIC
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host. I don't think these topics are worth our time
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addressing (unless we find a gung-ho volunteer for EBCDIC or
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PDP-10 ports :-) but they should probably be documented
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somewhere.
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More importantly, Bison does not currently allow NUL bytes in
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tokens, either via escapes (e.g., "x\0y") or via a NUL byte in
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the source code. This should get fixed.
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* --graph
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Show reductions.
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* Broken options ?
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** %token-table
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** Skeleton strategy
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Must we keep %token-table?
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* Precedence
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** Partial order
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It is unfortunate that there is a total order for precedence. It
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makes it impossible to have modular precedence information. We should
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move to partial orders (sounds like series/parallel orders to me).
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** RR conflicts
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See if we can use precedence between rules to solve RR conflicts. See
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what POSIX says.
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* $undefined
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From Hans:
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- If the Bison generated parser experiences an undefined number in the
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character range, that character is written out in diagnostic messages, an
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addition to the $undefined value.
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Suggest: Change the name $undefined to undefined; looks better in outputs.
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* Default Action
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From Hans:
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- For use with my C++ parser, I transported the "switch (yyn)" statement
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that Bison writes to the bison.simple skeleton file. This way, I can remove
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the current default rule $$ = $1 implementation, which causes a double
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assignment to $$ which may not be OK under C++, replacing it with a
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"default:" part within the switch statement.
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Note that the default rule $$ = $1, when typed, is perfectly OK under C,
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but in the C++ implementation I made, this rule is different from
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$<type_name>$ = $<type_name>1. I therefore think that one should implement
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a Bison option where every typed default rule is explicitly written out
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(same typed ruled can of course be grouped together).
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* Pre and post actions.
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From: Florian Krohm <florian@edamail.fishkill.ibm.com>
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Subject: YYACT_EPILOGUE
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To: bug-bison@gnu.org
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X-Sent: 1 week, 4 days, 14 hours, 38 minutes, 11 seconds ago
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The other day I had the need for explicitly building the parse tree. I
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used %locations for that and defined YYLLOC_DEFAULT to call a function
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that returns the tree node for the production. Easy. But I also needed
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to assign the S-attribute to the tree node. That cannot be done in
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YYLLOC_DEFAULT, because it is invoked before the action is executed.
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The way I solved this was to define a macro YYACT_EPILOGUE that would
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be invoked after the action. For reasons of symmetry I also added
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YYACT_PROLOGUE. Although I had no use for that I can envision how it
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might come in handy for debugging purposes.
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All is needed is to add
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#if YYLSP_NEEDED
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YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen, yyloc, (yylsp - yylen));
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#else
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YYACT_EPILOGUE (yyval, (yyvsp - yylen), yylen);
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#endif
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at the proper place to bison.simple. Ditto for YYACT_PROLOGUE.
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I was wondering what you think about adding YYACT_PROLOGUE/EPILOGUE
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to bison. If you're interested, I'll work on a patch.
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* Better graphics
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Equip the parser with a means to create the (visual) parse tree.
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* Complaint submessage indentation.
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We already have an implementation that works fairly well for named
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reference messages, but it would be nice to use it consistently for all
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submessages from Bison. For example, the "previous definition"
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submessage or the list of correct values for a %define variable might
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look better with indentation.
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However, the current implementation makes the assumption that the
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location printed on the first line is not usually much shorter than the
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locations printed on the submessage lines that follow. That assumption
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may not hold true as often for some kinds of submessages especially if
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we ever support multiple grammar files.
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Here's a proposal for how a new implementation might look:
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http://lists.gnu.org/archive/html/bison-patches/2009-09/msg00086.html
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Local Variables:
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mode: outline
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coding: utf-8
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End:
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-----
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Copyright (C) 2001-2004, 2006, 2008-2012 Free Software Foundation, Inc.
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This file is part of Bison, the GNU Compiler Compiler.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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