Parse::Eyapp::Driver - The LR parser

This class has the method YYParse implementing the LR generic parsing algorithm plus the methods that give support to the generated parser.

The YYParse methods implements the generic LR parsing algorithm. It very much works Parse::Yapp::YYParse and as yacc/bison yyparse. It accepts almost the same arguments as Class->new (Being Class the name of the generated class).

The parser uses two tables and a stack. The two tables are called the action table and the goto table. The stack is used to keep track of the states visited.

At each step the generated parser consults the action table and takes one decision: To shift to a new state consuming one token (and pushing the current state in the stack) or to reduce by some production rule. In the last case the parser pops from its stack as many states as symbols are on the right hand side of the production rule. Here is a Perl/C like pseudocode summarizing the activity of YYParse:

     1   my $parser = shift; # The parser object
     2   push(@stack, $parser->{startstate});
     3   $b = $parser->YYLexer(); # Get the first token
     4   FOREVER: {
     5     $s = top(0);  # Get the state on top of the stack
     6     $a = $b;
     7     switch ($parser->action[$s->state][$a]) {
     8       case "shift t" : 
     9         my $t;
    10         $t->{state} = t;
    11         $t->{attr}  = $a->{attr};
    12         push($t); 
    13         $b = $parser->YYLexer(); # Call the lexical analyzer
    14         break;
    15       case "reduce A->alpha" : 
    16         # Call the semantic action with the attributes of the rhs as args
    17         my $semantic  = $parser->Semantic{A ->alpha}; # The semantic action
    18         my $r;
    19         $r->{attr} = $semantic->($parser, top(|alpha|-1)->attr, ... , top(0)->attr); 
    20  
    21         # Pop as many states as symbols on the rhs of A->alpha
    22         pop(|alpha|);  
    23  
    24         # Goto next state 
    25         $r->{state} = $parser->goto[top(0)][A]; 
    26         push($r); 
    27         break;
    28       case "accept" : return (1); 
    29       default : $parser->YYError("syntax error"); 
    30     }
    31     redo FOREVER;
    32   }

Here |alpha| stands for the length of alpha. Function top(k) returns the state in position k from the top of the stack, i.e. the state at depth k. Function pop(k) extracts k states from the stack. The call $state->attr returns the attribute associated with $state. The call $parser->Semantic{A ->alpha} returns the semantic action associated with production A ->alpha.

Let us see a trace for the small grammar in examples/debuggingtut/aSb.yp:

  pl@nereida:~/LEyapp/examples$ /usr/local/bin/paste.pl aSb.yp aSb.output | head -5
  %%                                             | Rules:
  S:                 { print "S -> epsilon\n" }  | ------
      |   'a' S 'b'  { print "S -> a S b\n" }    | 0:    $start -> S $end
  ;                                              | 1:    S -> /* empty */
  %%                                             | 2:    S -> 'a' S 'b'

The tables in file aSb.output describe the actions and transitions to take:

  pl@nereida:~/LEyapp/examples$ cat -n aSb.output
     .  .........................................
     7  States:
     8  -------
     9  State 0:
    10
    11          $start -> . S $end      (Rule 0)
    12
    13          'a'     shift, and go to state 2
    14
    15          $default        reduce using rule 1 (S)
    16
    17          S       go to state 1
    18
    19  State 1:
    20
    21          $start -> S . $end      (Rule 0)
    22
    23          $end    shift, and go to state 3
    24
    25  State 2:
    26
    27          S -> 'a' . S 'b'        (Rule 2)
    28
    29          'a'     shift, and go to state 2
    30
    31          $default        reduce using rule 1 (S)
    32
    33          S       go to state 4
    34
    35  State 3:
    36
    37          $start -> S $end .      (Rule 0)
    38
    39          $default        accept
    40
    41  State 4:
    42
    43          S -> 'a' S . 'b'        (Rule 2)
    44
    45          'b'     shift, and go to state 5
    46
    47  State 5:
    48
    49          S -> 'a' S 'b' .        (Rule 2)
    50
    51          $default        reduce using rule 2 (S)
    52
    53
    54  Summary:
    55  --------
    56  Number of rules         : 3
    57  Number of terminals     : 3
    58  Number of non-terminals : 2
    59  Number of states        : 6

When executed with yydebug set and input aabb we obtain the following output:

  pl@nereida:~/LEyapp/examples/debuggingtut$ eyapp -b '' -o use_aSb.pl aSb
  pl@nereida:~/LEyapp/examples/debuggingtut$ ./use_aSb.pl -d
  Provide a statement like "a a b b" and press <CR><CTRL-D>: aabb
  ----------------------------------------                       
  In state 0:                                                    
  Stack:[0]                                                      
  Need token. Got >a<                                            
  Shift and go to state 2.                                       
  ----------------------------------------                       
  In state 2:                                                    
  Stack:[0,2]                                                    
  Need token. Got >a<
  Shift and go to state 2.
  ----------------------------------------
  In state 2:
  Stack:[0,2,2]
  Need token. Got >b<
  Reduce using rule 1 (S --> /* empty */): S -> epsilon
  Back to state 2, then go to state 4.
  ----------------------------------------
  In state 4:
  Stack:[0,2,2,4]
  Shift and go to state 5.
  ----------------------------------------
  In state 5:
  Stack:[0,2,2,4,5]
  Don't need token.
  Reduce using rule 2 (S --> a S b): S -> a S b
  Back to state 2, then go to state 4.
  ----------------------------------------

As a result of reducing by rule 2 the three last visited states are popped from the stack, and the stack becomes [0,2]. But that means that we are now in state 2 seeing a S. If you look at the table above being in state 2 and seeing a S we go to state 4.

  In state 4:
  Stack:[0,2,4]
  Need token. Got >b<
  Shift and go to state 5.
  ----------------------------------------
  In state 5:
  Stack:[0,2,4,5]
  Don't need token.
  Reduce using rule 2 (S --> a S b): S -> a S b
  Back to state 0, then go to state 1.
  ----------------------------------------
  In state 1:
  Stack:[0,1]
  Need token. Got ><
  Shift and go to state 3.
  ----------------------------------------
  In state 3:
  Stack:[0,1,3]
  Don't need token.
  Accept.

The class containing the parser generated by Parse::Eyapp inherits from Parse::Eyapp::Driver. Therefore all the methods in Parse::Eyapp::Driver are available in the generated class.

This section describes the methods and objects belonging to the class generated either using eyapp or Parse::Eyapp->new_grammar. In the incoming paragraphs we will assume that Class was the value selected for the classname argument when Parse::Eyapp->new_grammar was called. Objects belonging to Class are the actual parsers for the input grammar.

The method Class->new returns a new LALR parser object. Here Class stands for the name of the class containing the parser. See an example of call:

  my $parser = main->new(yyprefix => 'Parse::Eyapp::Node::',
                         yylex    => \&main::_Lexer,
                         yyerror  => \&main::_Error,
                         yydebug => 0x1F,
  );

The meaning of the arguments used in the example are as follows:

- yyprefix

Used with %tree or %metatree. When used, the type names of the nodes of the syntax tree will be build prefixing the value associated to yyprefix to the name of the production rule. The name of the production rule is either explicitly given through a %name directive or the concatenation of the left hand side of the rule with the ordinal of the right hand side of the production. See section Compiling with eyapp and treereg in Parse::Eyapp for an example.

- yylex

Reference to the lexical analyzer subroutine

- yyerror

Reference to the error subroutine. The error subroutine receives as first argument the reference to the Class parser object. This way it can take advantage of methods like YYCurval and YYExpect (see below):

  sub _Error {
    my($token)=$_[0]->YYCurval;
    my($what)= $token ? "input: '$token'" : "end of input";
    my @expected = $_[0]->YYExpect();

    local $" = ', ';
    die "Syntax error near $what. Expected one of these tokens: @expected\n";
  }

- yydebug

Controls the level of debugging. Must be a number.

The package produced from the grammar has several methods.

The parser object has the following methods that work at parsing time exactly as in Parse::Yapp. These methods can be found in the module Parse::Eyapp::Driver. Assume you have in $parser the reference to your parser object:

Receives the name of a production and a subroutine reference implementing the new semantic action. If no subroutine reference is set returns the reference to the current semantic action. See the tutorial Parse::Eyapp::defaultaction and the examples in the examples/recycle/ directory

Works as yacc/bison YYACCEPT. The parser finishes returning the current semantic value to indicate success.

Works as yacc/bison YYABORT. The parser finishes returning undef to indicate failure.

Is not a method. Receives as input a Class name. Introduces Parse::Eyapp::Node as an ancestor class of Class. To work correctly, objects belonging to Class must be hashes with a children key whose value must be a reference to the array of children. The children must be also Parse::Eyapp::Node nodes. Actually you can circumvent this call by directly introducing Parse::Eyapp::Node in the ancestors of Class:

         push @{$class."::ISA"}, "Parse::Eyapp::Node" 

Sometimes the best time to decorate a node with some attributes is just after being built. In such cases the programmer can take manual control building the node with YYBuildAST to immediately proceed to decorate it.

The following example from the file lib/Simple/Types.eyp in the tarball in examples/typechecking/Simple-Types-XXX.tar.gz illustrates the idea:

 Variable:
     %name  VARARRAY
     $ID ('[' binary ']') <%name INDEXSPEC +>
       {
         my $self = shift;
         my $node =  $self->YYBuildAST(@_);
         $node->{line} = $ID->[1];
         return $node;
       }

Actually, the %tree directive is semantically equivalent to:

  %default action { goto &Parse::Eyapp::Driver::YYBuildAST }

Influences the semantic of list operators. If true the action associated with X+ will be to build a Parse::Eyapp::Node node with all the attributes of the elements in the list as children. This is the appropriate semantic when working under the %tree directive. If set to false the semantic action will return an anonymous list with the attributes associated with the X in the plus list. Same thing with the operators * and ?.

Similar to $parser->YYBuildAST but builds nodes for translation schemes.

Returns TRUE if running under the %tree bypass clause

Returns TRUE if the production being used for reduction was marked to be bypassed.

Gives the current token

Gives the attribute associated with the current token

Use it as defaultaction if you want to recycle your grammar. It is equivalent to:

  sub YYDelegateaction {
    my $self = shift;

    my $action = $self->YYName;

    $self->$action(@_);
  }

For a full example illustrating how to use it, see files examples/recycle/NoacInh.eyp and examples/recycle/icalcu_and_ipost.pl in the Parse::Eyapp distribution

True if the pos() of the input being scanned in ${$parser->input} is at the end

Works as yacc/bison yyerrok. Modifies the error status so that subsequent error messages will be emitted.

Works as yacc/bison YYERROR. Pretends that a syntax error has been detected.

Returns the list of tokens the parser expected when the failure occurred

 pl@nereida:~/src/perl/YappWithDefaultAction/examples$ \
                            sed -ne '26,33p' Postfix.eyp
 sub _Error {
   my($token)=$_[0]->YYCurval;
   my($what)= $token ? "input: '$token'" : "end of input";
   my @expected = $_[0]->YYExpect();

   local $" = ', ';
   die "Syntax error near $what. Expected one of these tokens: @expected\n";
 }

See the tutorial Parse::Eyapp::datagenerationtut and the section TOKENS DEPENDING ON THE SYNTACTIC CONTEXT in the tutorial Parse::Eyapp::debuggingtut for more detailed examples of use of YYExpect.

First line of the input string describing the grammar

Return the list of grammar items. Each item is an anonymous list containing

* The name of the production

* The LHS of the production

* An anonymous list containing the symbols in the RHS

If it receives an index as argument returns the corresponding item The following debugger session explain its use:

  pl@europa:~/LEyapp/examples/recycle$ perl -wd usepostfix.pl
  main::(usepostfix.pl:5):        my $parser = new Postfix();
    DB<1> n
  main::(usepostfix.pl:6):        $parser->Run;
    DB<1> x $parser->YYGrammar
  0  ARRAY(0xde5e20)
     0  '_SUPERSTART'
     1  '$start'
     2  ARRAY(0xc85e80)
        0  'line'
        1  '$end'
     3  0
  1  ARRAY(0xe2b6b0)
     0  'line_1'
     1  'line'
     2  ARRAY(0xe3abc0)
        0  'exp'
     3  0
  2  ARRAY(0xa05530)
     0  'exp_2'
     1  'exp'
     2  ARRAY(0x75bdc0)
        0  'NUM'
     3  0

     ...  etc, etc

If an index is provided it returns the item for such number:

    DB<2> x $parser->YYGrammar(10)
  0  'exp_10'
  1  'exp'
  2  ARRAY(0xa05f80)
     0  '('
     1  'exp'
     2  ')'
  3  0

You can also use a production name as argument:

    DB<3> x $parser->YYGrammar('exp_7')
  0  'exp_7'
  1  'exp'
  2  ARRAY(0xa05890)
     0  'exp'
     1  '*'
     2  'exp'
  3  0

Returns the shift-reduce action for state $state and token $token. A positive number must be interpreted as a shift to the state with that number. A negative number -m indicates a reduction by production with index m. Returns undef if no action is defined for such combination ($state, $token).

See example DynamicallyChangingTheParser.eyp in the directory examples/debuggintut for an example of use.

Returns TRUE if the terminal is semantic. Semantics token can be declared using the directive %semantic token. The opposite of a Semantic token is a Syntactic token. Syntactic tokens can be declared using the directive %syntactic token.

When using the %tree directive all the nodes corresponding to syntactic tokens are pruned from the tree. Under this directive tokens in the text delimited by simple quotes (like '+') are, by default, considered syntactic tokens.

When using the %metatree directive all the tokens are considered, by default, semantic tokens. Thus, no nodes will be - by default- pruned when construction the code augmented tree. The exception are string tokens used as separators in the definition of lists, like in S <* ';'>. If you want the separating string token to appear include an explicit semantic declaration for it (example %semantic token ';').

Receives the name of production (right hand side). Returns the index in the grammar of the production with such name. When called in a list context and without a name return the hash containing the relation

           production name => production index

The following debugger session illustrates its use:

  pl@europa:~/LEyapp/examples/recycle$ perl -wd usepostfix.pl
  main::(usepostfix.pl:5):        my $parser = new Postfix();
  main::(usepostfix.pl:6):        $parser->Run;
  DB<1> x $parser->YYIndex
  0  'line_1'
  1  1
  2  'exp_3'
  3  3
  4  'exp_6'
  5  6
  6  'exp_4'
  7  4
  8  'exp_10'
  9  10
  10  'exp_8'
  11  8
  12  'exp_5'
  13  5
  14  'exp_7'
  15  7
  16  'exp_2'
  17  2
  18  '_SUPERSTART'
  19  0
  20  'exp_9'
  21  9

We can specify a list of names:

  DB<2> x $parser->YYIndex(qw{exp_4 exp_7})
  0  4
  1  7
  DB<3> x $parser->YYIndex(qw{exp_4})
  0  4

Alias input. If an argument is provided, sets the input for the parser object. The argument is a string or a reference to a string. It returns a reference to the input string or undef if not set.

Returns TRUE if the symbol given as argument is a terminal. Example:

  DB<0> x $self->YYIsterm('exp')
 0  ''
  DB<1> x $self->YYIsterm('*')
 0  1

An example of combined use of YYRightside, YYRuleindex, YYLhs and YYIsterm can be found examples/Eyapp/Rule3.yp:

 nereida:~/src/perl/YappWithDefaultAction/examples> sed -n -e '4,22p' Rule3.yp | cat -n
  1  sub build_node {
  2    my $self = shift;
  3    my @children = @_;
  4    my @right = $self->YYRightside();
  5    my $var = $self->YYLhs;
  6    my $rule = $self->YYRuleindex();
  7
  8    for(my $i = 0; $i < @right; $i++) {
  9      $_ = $right[$i];
 10      if ($self->YYIsterm($_)) {
 11        $children[$i] = bless { token => $_, attr => $children[$i] },
 12                                            __PACKAGE__.'::TERMINAL';
 13      }
 14    }
 15    bless {
 16            children => \@children,
 17            info => "$var -> @right"
 18          }, __PACKAGE__."::${var}_$rule"
 19  }

when executed an output similar to this is produced:

 nereida:~/src/perl/YappWithDefaultAction/examples> userule3.pl
 2*3
 $VAR1 = bless( {
   'info' => 'exp -> exp * exp',
   'children' => [
     bless( {
       'info' => 'exp -> NUM',
       'children' => [ bless( { 'attr' => '2', 'token' => 'NUM' }, 'Rule3::TERMINAL' ) ]
     }, 'Rule3::exp_6' ),
     bless( { 'attr' => '*', 'token' => '*' }, 'Rule3::TERMINAL' ),
     bless( {
       'info' => 'exp -> NUM',
       'children' => [ bless( { 'attr' => '3', 'token' => 'NUM' }, 'Rule3::TERMINAL' )
       ]
     }, 'Rule3::exp_6' )
   ]
 }, 'Rule3::exp_11' );

Returns a reference to the lexical analyzer

Returns the identifier of the left hand side of the current production (the one that is being used for reduction/reverse derivation. An example of use can be found in examples/Eyapp/Lhs1.yp:

  %defaultaction { print $_[0]->YYLhs,"\n" }

Alias is also main.

Other than the package, it has as optional arguments the prompt (shown each time it ask for input), the name of the input file (if it wasn't specified in the command line using --file filename) and also the input string.

This method provides a default main for testing the generated parser. It parses the commandline searching for a number of options. See an example of use:

  pl@nereida:~/LEyapp/examples/eyapplanguageref$ cat use_list2.pl
  #!/usr/bin/env perl
  use warnings;
  use strict;
  use List2;

  unshift @ARGV, '--noslurp';
  List2->new->main("Try input 'aacbb': ");
  pl@nereida:~/LEyapp/examples/eyapplanguageref$ ./use_list2.pl --help
  Available options:
      --debug                    sets yydebug on
      --nodebug                  sets yydebug off
      --file filepath            read input from filepath
      --commandinput string      read input from string
      --tree                     prints $tree->str
      --notree                   does not print $tree->str
      --info                     When printing $tree->str shows the value of TERMINALs
      --help                     shows this help
      --slurp                    read until EOF reached
      --noslurp                  read until CR is reached
      --argfile                  main() will take the input string from its @_
      --noargfile                main() will not take the input string from its @_
      --yaml                     dumps YAML for $tree: YAML module must be installed
      --margin=i                 controls the indentation of $tree->str (i.e. $Parse::Eyapp::Node::INDENT)

Returns the name of the current rule (The production whose reduction gave place to the execution of the current semantic action).

  DB<12> x $self->YYName
 0  'exp_11'

Return the list of production names. In a scalar context returns a reference to such list.

  pl@europa:~/LEyapp/examples/recycle$ eyapp Postfix
  pl@europa:~/LEyapp/examples/recycle$ perl -wd usepostfix.pl
  main::(usepostfix.pl:5):        my $parser = new Postfix();
  main::(usepostfix.pl:6):        $parser->Run;
  DB<1> x $parser->YYNames
  0  '_SUPERSTART'
  1  'line_1'
  2  'exp_2'
  3  'exp_3'
  4  'exp_4'
  5  'exp_5'
  6  'exp_6'
  7  'exp_7'
  8  'exp_8'
  9  'exp_9'
  10  'exp_10'

The current number of errors

If called inside a semantic action, returns the state after the reduction by the current production.

Provide a token if called from any other side:

  $parser->YYNextState($token);

It will return the state given by the action table for the state in the top of the stack and the given token.

For an example, see the program DynamicallyChangingTheParser.eyp in the directory examples/debuggintut/.

Return and/or sets the yyprefix attribute. This a string that will be concatenated as a prefix to any Parse::Eyapp::Node nodes in the syntax tree.

It very much works Parse::Yapp::YYParse and as yacc/bison yyparse. It accepts almost the same arguments as Class->new with the exception of yyprefix which can be used only with new.

Works as yacc/bison YYRECOVERING. Returns TRUE if the parser is recovering from a syntax error.

This method has been designed to solve shift-reduce and reduce-reduce conflicts at parsing-time using the postponed conflict strategy. It has to be called inside the semantic action associated with the postponed conflict rule. The LALR table is changed so that the action in the presence of the token $token is restored the one before the last call to

  $parser->YYSetReduce($token, $productionname )

See the examples in examples/debuggingtut/ in files DynamicallyChangingTheParser2.eyp and Cplusplus.eyp.

Also:

  $parser->YYRHSLength

returns the length of the right hand side (the number of symbols) of $productionindex. The name of the production can be used instead of its index. If no index or name is provided and the method is called inside a semantic action, the length of the current production is returned.

Also:

  $parser->YYRightside($index)

Returns an array of strings describing the right hand side of the rule. The name of the production can be given instead of $index. If no $index is provided and the method is called inside a semantic action the right hand side of the current production is returned.

To be called inside a semantic action. Returns the index of the current production rule, counting the super rule as rule 0.

To know the numbers have a look at the .output file. To get a .output file use the option -v of eyapp or the outputfile parameter when using method new_grammar (see the documentation for eyapp).

Return the list of rules. The following debugger session illustrates its use:

  pl@europa:~/LEyapp/examples/recycle$ perl -wd usepostfix.pl
  main::(usepostfix.pl:5):        my $parser = new Postfix();
  main::(usepostfix.pl:6):        $parser->Run;
  0  ARRAY(0xa068e0)
     0  '$start'
     1  2
     2  undef
  1  ARRAY(0xa06940)
     0  'line'
     1  1
     2  CODE(0xc22360)
        -> &Postfix::__ANON__[Postfix.eyp:10] in Postfix.eyp:227-10
  ... etc, etc.

Each item has three components: the LHS of the production, the number of symbols in the RHS and the CODE reference to the semantic action.

If an index is specified as argument it returns the corresponding item:

     DB<2> x $parser->YYRule(7)
  0  'exp'
  1  3
  2  CODE(0xc1fce0)
     -> &Postfix::__ANON__[Postfix.eyp:7] in Postfix.eyp:276-7

To know to what production an item is associated we can use the YYGrammar method:

     DB<3> x $parser->YYGrammar('exp_7')
  0  'exp_7'
  1  'exp'
  2  ARRAY(0xa05290)
     0  'exp'
     1  '*'
     2  'exp'
  3  0

We can also use the name of the rule to get the item:

   DB<4> x $parser->YYRule('exp_7')
  0  'exp'
  1  3
  2  CODE(0xc1fce0)
     -> &Postfix::__ANON__[Postfix.eyp:7] in Postfix.eyp:276-7

Receives a hash with keys the names of the production rules (right hand sides) and values the new semantic actions. Used to reuse a grammar without overwriting all the semantic actions. See section Reusing Grammars by Dynamic Substitution of Semantic Actions in Parse::Eyapp::defaultactionsintro.

It also accepts the syntax:

  $parser->YYSetLRAction($conflictstate, [$token1, ... ], $shiftreduceaction )

This method has been designed to solve shift-reduce and reduce-reduce conflicts at parsing-time (not at parser-generation time).

The LR table is changed so that the action in state $conflictstate in the presence of the token $token will be given by $shiftreduceaction. The current shift-reduce action isn't saved.

See an example in Cplusplus2.eyp in the directory examples/debuggintut.

This method has been designed to solve shift-reduce and reduce-reduce conflicts at parsing-time using the postponed conflict strategy. See the corresponding section in Parse::Eyapp::debuggintut. It has to be called inside the semantic action associated with the postponed conflict rule conflictname. The LALR table is changed so that the action in the presence of the token $token will be to reduce by $productionname. The current shift-reduce action is saved to be restored using

  $parser->YYRestoreLRAction('conflictname', $token)

See the examples in examples/debuggingtut/ in files

* DynamicallyChangingTheParser2.eyp

* confusingsolveddynamic.eyp

* DebugDynamicResolution.eyp

* DynamicallyChangingTheParser2.eyp

* DynamicallyChangingTheParser3.eyp

* DynamicallyChangingTheParser.eyp

* DynamicvsTieIns.eyp

* nolr_k_grammarsolveddynamic.eyp

* pascalenumeratedvsrangesolvedviadyn.eyp

* Cplusplus.eyp.

Also:

      $parser->YYSetShift([$token1, $token2, ... ])

This method has been designed to solve shift-reduce at parsing-time using the postponed conflict strategy. See the corresponding section in Parse::Eyapp::debuggintut. It has to be called inside the semantic action associated with the postponed conflict rule conflictname. The LALR table is changed so that the action in the presence of the token $token will be to shift.

See the examples in examples/debuggingtut/ in files

* DebugDynamicResolution.eyp

* DynamicallyChangingTheParser.eyp

alias:

 $parser->slurp_file($filename[,$prompt[,$mode]])

Receives the name of the file, reads its contents and stores it in $parser->input.

If the file does not exists, it proceeds to read from STDIN. If a prompt was set with $parser->YYPrompt, it will be shown. The additional optional parameter $mode is used in such case to set $/. It can also be used as a class method.

YYState returns a reference to the list of states containing the LALR(1) tables: the action and GOTO tables. Each state is an anonymous hash:

  DB<4> x $parser->YYState(2)
  0  HASH(0xfa7120)
     'ACTIONS' => HASH(0xfa70f0) # token => state
           ':' => '-7'
     'DEFAULT' => '-6'

A negative number means reduction using the corresponding production rule (opposite) number. The former example tells to reduce by rule 7 when in state 2 and seeing token ':'. By default, the action when in state 2 is to reduce by rule number 6.

There are three keys: ACTIONS, GOTOS and DEFAULT

  DB<7> x $parser->YYState(13)
 0  HASH(0xfa8b50)
    'ACTIONS' => HASH(0xfa7530)
       'VAR' => 17
    'GOTOS' => HASH(0xfa8b20)
       'type' => 19

The GOTOS tables contains the DFA transition tables for the syntactic variables. The former example tells to move to state 19 when in state 13 after seeing the syntactic variable type (i.e. if after reducing by a rule of type we are in state 13).

If $length is zero or not provided it returns the state on top of the stack. Otherwise, returns the state $length units deep in the stack.

* Hal Finkel http://www.halssoftware.com/

* G. Williams http://kasei.us/

* Thomas L. Shinnick http://search.cpan.org/~tshinnic/

* Frank Leray

Casiano Rodriguez-Leon (casiano@ull.es)

This work has been supported by CEE (FEDER) and the Spanish Ministry of Educacion y Ciencia through Plan Nacional I+D+I number TIN2005-08818-C04-04 (ULL::OPLINK project http://www.oplink.ull.es/). Support from Gobierno de Canarias was through GC02210601 (Grupos Consolidados). The University of La Laguna has also supported my work in many ways and for many years.

A large percentage of code is verbatim taken from Parse::Yapp 1.05. The author of Parse::Yapp is Francois Desarmenien.

I wish to thank Francois Desarmenien for his Parse::Yapp module, to my students at La Laguna and to the Perl Community. Thanks to the people who have contributed to improve the module (see CONTRIBUTORS in Parse::Eyapp). Thanks to Larry Wall for giving us Perl. Special thanks to Juana.

Copyright (c) 2006-2008 Casiano Rodriguez-Leon (casiano@ull.es). All rights reserved.

Parse::Yapp copyright is of Francois Desarmenien, all rights reserved. 1998-2001

These modules are free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.

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.