+
+* Debugging parsers
+
+From Greg McGary:
+
+akim demaille <akim.demaille@epita.fr> 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.
+
+* BTYacc
+See if we can integrate backtracking in Bison. Contact the BTYacc
+maintainers.
+
+* Automaton report
+Display more clearly the lookaheads for each item.
+
+* RR conflicts
+See if we can use precedence between rules to solve RR conflicts. See
+what POSIX says.
+
+* Precedence
+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.
+
+This will be possible with a Bison parser for the grammar, as it will
+make it much easier to extend the grammar.
+
+* Parsing grammars
+Rewrite the reader in Flex/Bison. There will be delicate parts, in
+particular, expect the scanner to be hard to write. Many interesting
+features cannot be implemented without such a new reader.
+
+* Presentation of the report file
+From: "Baum, Nathan I" <s0009525@chelt.ac.uk>
+Subject: Token Alias Bug
+To: "'bug-bison@gnu.org'" <bug-bison@gnu.org>
+
+I've also noticed something, that whilst not *wrong*, is inconvienient: I
+use the verbose mode to help find the causes of unresolved shift/reduce
+conflicts. However, this mode insists on starting the .output file with a
+list of *resolved* conflicts, something I find quite useless. Might it be
+possible to define a -v mode, and a -vv mode -- Where the -vv mode shows
+everything, but the -v mode only tells you what you need for examining
+conflicts? (Or, perhaps, a "*** This state has N conflicts ***" marker above
+each state with conflicts.)
+
+* $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.
+
+* Documenting C++ output
+Write a first documentation for C++ output.
+
+* 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.
+
+-----
+
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+
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projects / bison.git / blobdiff