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310 lines
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310 lines
10 KiB
Plaintext
-*- outline -*-
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* Header guards
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From Franc,ois: should we keep the directory part in the CPP guard?
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* Yacc.c: CPP Macros
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Do some people use YYPURE, YYLSP_NEEDED like we do in the test suite?
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They should not: it is not documented. But if they need to, let's
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find something clean (not like YYLSP_NEEDED...).
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* Installation
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* Documentation
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Before releasing, make sure the documentation ("Understanding your
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parser") refers to the current `output' format.
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* lalr1.cc
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** vector
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Move to using vector, drop stack.hh.
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** I18n
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Catch up with yacc.c.
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* Report
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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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** Labeling the symbols
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Have a look at the Lemon parser generator: instead of $1, $2 etc. they
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can name the values. This is much more pleasant. For instance:
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exp (res): exp (a) '+' exp (b) { $res = $a + $b; };
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I love this. I have been bitten too often by the removal of the
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symbol, and forgetting to shift all the $n to $n-1. If you are
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unlucky, it compiles...
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But instead of using $a etc., we can use regular variables. And
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instead of using (), I propose to use `:' (again). Paul suggests
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supporting `->' in addition to `:' to separate LHS and RHS. In other
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words:
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r:exp -> a:exp '+' b:exp { r = a + b; };
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That requires an significant improvement of the grammar parser. Using
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GLR would be nice. It also requires that Bison know the type of the
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symbols (which will be useful for %include anyway). So we have some
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time before...
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Note that there remains the problem of locations: `@r'?
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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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** -D, --define-muscle NAME=VALUE
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To define muscles via cli. Or maybe support directly NAME=VALUE?
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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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* Java, Fortran, etc.
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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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* src/print_graph.c
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Find the best graph parameters.
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* BTYacc
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See if we can integrate backtracking in Bison. Charles-Henri de
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Boysson <de-boy_c@epita.fr> is working on this, and already has some
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results. Vadim Maslow, the maintainer of BTYacc was contacted, and we
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stay in touch with him. Adjusting the Bison grammar parser will be
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needed to support some extra BTYacc features. This is less urgent.
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** Keeping the conflicted actions
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First, analyze the differences between byacc and btyacc (I'm referring
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to the executables). Find where the conflicts are preserved.
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** Compare with the GLR tables
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See how isomorphic the way BTYacc and the way the GLR adjustments in
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Bison are compatible. *As much as possible* one should try to use the
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same implementation in the Bison executables. I insist: it should be
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very feasible to use the very same conflict tables.
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** Adjust the skeletons
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Import the skeletons for C and C++.
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** Improve the skeletons
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Have them support yysymprint, yydestruct and so forth.
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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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** Correlation b/w precedence and associativity
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Also, I fail to understand why we have to assign the same
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associativity to operators with the same precedence. For instance,
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why can't I decide that the precedence of * and / is the same, but the
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latter is nonassoc?
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If there is really no profound motivation, we should find a new syntax
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to allow specifying this.
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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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Note: Robert Anisko handles this. He knows how to do it.
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* Warnings
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It would be nice to have warning support. See how Autoconf handles
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them, it is fairly well described there. It would be very nice to
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implement this in such a way that other programs could use
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lib/warnings.[ch].
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Don't work on this without first announcing you do, as I already have
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thought about it, and know many of the components that can be used to
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implement it.
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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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-----
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Copyright (C) 2001, 2002, 2003, 2004, 2006 Free Software Foundation,
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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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