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From selkovjr@mcs.anl.gov Sat Jul 25 05:31:05 1998
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Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
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by candle.pha.pa.us (8.8.5/8.8.5) with ESMTP id FAA16564
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for <maillist@candle.pha.pa.us>; Sat, 25 Jul 1998 05:31:03 -0400 (EDT)
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id EAA28698 for <maillist@candle.pha.pa.us>; Sat, 25 Jul 1998 04:27:05 -0500
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Sender: selkovjr@mcs.anl.gov
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Message-ID: <35B9968D.21CF60A2@mcs.anl.gov>
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Date: Sat, 25 Jul 1998 08:25:49 +0000
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From: "Gene Selkov, Jr." <selkovjr@mcs.anl.gov>
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Organization: MCS, Argonne Natl. Lab
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X-Mailer: Mozilla 4.03 [en] (X11; I; Linux 2.0.32 i586)
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MIME-Version: 1.0
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To: Bruce Momjian <maillist@candle.pha.pa.us>
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Subject: position-aware scanners
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References: <199807250524.BAA07296@candle.pha.pa.us>
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Content-Type: text/plain; charset=us-ascii
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Content-Transfer-Encoding: 7bit
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Status: RO
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Bruce,
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I attached here (trough the web links) a couple examples, totally
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irrelevant to postgres but good enough to discuss token locations. I
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might as well try to patch the backend parser, though not sure how soon.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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1.
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The first c parser I wrote,
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http://wit.mcs.anl.gov/~selkovjr/unit-troff.tgz, is not very
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sophisticated, so token locations reported by yyerr() may be slightly
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incorrect (+/- one position depending on the existence and type of the
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lookahead token. It is a filter used to typeset the units of measurement
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with eqn. To use it, unpack the tar file and run make. The Makefile is
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not too generic but I built it on various systems including linux,
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freebsd and sunos 4.3. The invocation can be something like this:
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./check 0 parse "l**3/(mmoll*min)"
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parse error, expecting `BASIC_UNIT' or `INTEGER' or `POSITIVE_NUMBER' or
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`'(''
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l**3/(mmoll*min)
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^^^^^
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Now to the guts. As far as I can imagine, the only way to consistently
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keep track of each character read by the scanner (regardless of the
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length of expressions it will match) is to redefine its YY_INPUT like
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this:
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#undef YY_INPUT
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#define YY_INPUT(buf,result,max_size) \
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{ \
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int c = (int) buffer[pos++]; \
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result = (c == '\0') ? YY_NULL : (buf[0] = c, 1); \
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}
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Here, buffer is the pointer to the origin of the string being scanned
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and pos is a global variable, similar in usage to a file pointer (you
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can both read and manipulate it at will). The buffer and the pointer are
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initialized by the function
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void setString(char *s)
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{
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buffer = s;
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pos = 0;
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}
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each time the new string is to be parsed. This (exportable) function is
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part of the interface.
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In this simplistic design, yyerror() is part of the scanner module and
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it uses the pos variable to report the location of unexpected tokens.
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The downside of such arrangement is that in case of error condition, you
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can't easily tell whether your context is current or lookahead token, it
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just reports the position of the last token read (be it $ (end of
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buffer) or something else):
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./check 0 convert "mol/foo"
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parse error, expecting `BASIC_UNIT' or `INTEGER' or `POSITIVE_NUMBER' or
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`'(''
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mol/foo
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^^^
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(should be at the beginning of "foo")
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./check 0 convert "mmol//l"
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parse error, expecting `BASIC_UNIT' or `INTEGER' or `POSITIVE_NUMBER' or
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`'(''
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mmol//l
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^
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(should be at the second '/')
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I believe this is why most simple parsers made with yacc would report
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parse errors being "at or near" some token, which is fair enough if the
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expression is not too complex.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2. The second version of the same scanner,
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http://wit.mcs.anl.gov/~selkovjr/scanner-example.tgz, addresses this
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problem by recording exact locations of the tokens in each instance of
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the token semantic data structure. The global,
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UNIT_YYSTYPE unit_yylval;
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would be normally used to export the token semantics (including its
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original or modified text and location data) to the parser.
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Unfortunately, I cannot show you the parser part in c, because that's
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about when I stopped writing parsers in c. Instead, I included a small
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test program, test.c, that mimics the parser's expectations for the
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scanner data pretty well. I am assuming here that you are not interested
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in digging someone else's ugly guts for relatively small bit of
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information; let me know if I am wrong and I will send you the complete
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perl code (also generated with bison).
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To run this example, unpack the tar file and run Make. Then do
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gcc test.c scanner.o
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and run a.out
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Note the line
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yylval = unit_getyylval();
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in test.c. You will not normally need it in a c parser. It is enough to
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define yylval as an external variable and link it to yylval in yylex()
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In the bison-generated parser, yylval gets pushed into a stack (pointed
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to by yylsp) each time a new token is read. For each syntax rule, the
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bison macros @1, @2, ... are just shortcuts to locations in the stack 1,
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2, ... levels deep. In following code fragment, @3 refers to the
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location info for the third term in the rule (INTEGER):
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(sorry about perl, but I think you can do the same things in c without
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significant changes to your existing parser)
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term: base {
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$$ = $1;
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$$->{'order'} = 1;
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}
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| base EXP INTEGER {
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$$ = $1;
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$$->{'order'} = @3->{'text'};
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$$->{'scale'} = $$->{'scale'} ** $$->{'order'};
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if ( $$->{'order'} == 0 ) {
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yyerror("Error: expecting a non-zero
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integer exponent");
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YYERROR;
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}
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}
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which translates to:
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($yyn == 10) && do {
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$yyval = $yyvsa[-1];
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$yyval->{'order'} = 1;
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last SWITCH;
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};
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($yyn == 11) && do {
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$yyval = $yyvsa[-3];
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$yyval->{'order'} = $yylsa[-1]->{'text'}
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$yyval->{'scale'} = $yyval->{'scale'} ** $yyval->{'order'};
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if ( $yyval->{'order'} == 0 ) {
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yyerror("Error: expecting a non-zero integer
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exponent");
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goto yyerrlab1 ;
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}
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last SWITCH;
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};
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In c, you will have a bit more complicated pointer arithmetic to adress
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the stack, but the usage of objects will be the same. Note here that it
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is convenient to keep all information about the token in its location
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info, (yylsa, yylsp, yylval, @n), while everything relating to the value
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of the expression, or to the parse tree, is better placed in the
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semantic stack (yyssa, yyssp, yysval, $n). Also note that in some cases
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you can do semantic checks inside rules and report useful messages
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before or instead of invoking yyerror();
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Finally, it is useful to make the following wrapper function around
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external yylex() in order to maintain your own token stack. Unlike the
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parser's internal stack which is only as deep as the rule being reduced,
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this one can hold all tokens recognized during the current run, and that
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can be extremely helpful for error reporting and any transformations you
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may need. In this way, you can even scan (tokenize) the whole buffer
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before handing it off to the parser (who knows, you may need a token
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ahead of what is currently seen by the parser):
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sub tokenize {
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undef @tokenTable;
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my ($tok, $text, $name, $unit, $first_line, $first_column,
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$last_line, $last_column);
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while ( ($tok = &UnitLex::yylex()) > 0 ) { # this is where the
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c-coded yylex is called,
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# UnitLex is the perl
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extension encapsulating it
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( $text, $name, $unit, $first_line, $first_column, $last_line,
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$last_column ) = &UnitLex::getyylval;
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push(@tokenTable,
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Unit::yyltype->new (
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'token' => $tok,
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'text' => $text,
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'name' => $name,
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'unit' => $unit,
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'first_line' => $first_line,
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'first_column' => $first_column,
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'last_line' => $last_line,
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'last_column' => $last_column,
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)
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)
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}
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}
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It is now a lot easier to handle various state-related problems, such as
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backtracking and error reporting. The yylex() function as seen by the
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parser might be constructed somewhat like this:
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sub yylex {
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$yylloc = $tokenTable[$tokenNo]; # $tokenNo is a global; now
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instead of a "file pointer",
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# as in the first example, we have
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a "token pointer"
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undef $yylval;
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# disregard this; name this block "computing semantic values"
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if ( $yylloc->{'token'} == UNIT) {
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$yylval = Unit::Operand->new(
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'unit' => Unit::Dict::unit($yylloc->{'unit'}),
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'base' => Unit::Dict::base($yylloc->{'unit'}),
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'scale' => Unit::Dict::scale($yylloc->{'unit'}),
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'scaleToBase' => Unit::Dict::scaleToBase($yylloc->{'unit'}),
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'loc' => $yylloc,
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);
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}
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elsif ( ($yylloc->{'token'} == INTEGER ) || ($yylloc->{'token'} ==
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POSITIVE_NUMBER) ) {
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$yylval = Unit::Operand->new(
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'unit' => '1',
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'base' => '1',
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'scale' => 1,
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'scaleToBase' => 1,
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'loc' => $yylloc,
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);
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}
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$tokenNo++;
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return(%{$yylloc}->{'token'}); # This is all the parser needs to
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know about this token.
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# But we already made sure we saved
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everything we need to know.
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}
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Now the most interesting part, the error reporting routine:
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sub yyerror {
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my ($str) = @_;
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my ($message, $start, $end, $loc);
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$loc = $tokenTable[$tokenNo-1]; # This is the same as to say,
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# "obtain the location info for the
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current token"
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# You may use this routine for your own purposes or let parser use
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it
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if( $str ne 'parse error' ) {
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$message = "$str instead of `" . $loc->{'name'} . "' <" .
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$loc->{'text'} . ">, at line " . $loc->{'first_line'} . ":\n\
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n";
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}
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else {
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$message = "unexpected token `" . $loc->{'name'} . "' <" .
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$loc->{'text'} . ">, at line " . loc->{'first_line'} . ":\n
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\n";
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}
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$message .= $parseBuffer . "\n"; # that's the original string that
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was used to set the parser buffer
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$message .= ( ' ' x ($loc->{'first_column'} + 1) ) . ( '^' x
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length($loc->{'text'}) ). "\n";
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if( $str ne 'parse error' ) {
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print STDERR "$str instead of `", $loc->{'name'}, "' {",
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$loc->{'text'}, "}, at line ", $loc->{'first_line'}, ":\n\n";
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}
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else {
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print STDERR "unexpected token `", $loc->{'name'}, "' {",
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$loc->{'text'}, "}, at line ", $loc->{'first_line'}, ":\n\n";
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}
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print STDERR "$parseBuffer\n";
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print STDERR ' ' x ($loc->{'first_column'} + 1), '^' x
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length($loc->{'text'}), "\n";
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}
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Scanners used in these examples assume there is a single line of text on
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the input (the first_line and last_line elements of yylloc are simply
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ignored). If you want to be able to parse multi-line buffers, just add a
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lex rule for '\n' that will increment the line count and reset the pos
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variable to zero.
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Ugly as it may seem, I find this approach extremely liberating. If the
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grammar becomes too complicated for a LALR(1) parser, I can cascade
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multiple parsers. The token table can then be used to reassemble parts
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of original expression for subordinate parsers, preserving the location
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info all the way down, so that subordinate parsers can report their
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problems consistently. You probably don't need this, as SQL is very well
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thought of and has parsable grammar. But it may be of some help, for
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error reporting.
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--Gene
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