Parse::Eyapp::Node - The nodes of the Syntax Trees

  use Parse::Eyapp;
  use Parse::Eyapp::Treeregexp;

  sub TERMINAL::info {
    $_[0]{attr}
  }

  my $grammar = q{
    %right  '='     # Lowest precedence
    %left   '-' '+' # + and - have more precedence than = Disambiguate a-b-c as (a-b)-c
    %left   '*' '/' # * and / have more precedence than + Disambiguate a/b/c as (a/b)/c
    %left   NEG     # Disambiguate -a-b as (-a)-b and not as -(a-b)
    %tree           # Let us build an abstract syntax tree ...

    %%
    line:
        exp <%name EXPRESSION_LIST + ';'>
          { $_[1] } /* list of expressions separated by ';' */
    ;

    /* The %name directive defines the name of the class to 
       which the node being built belongs */
    exp:
        %name NUM
        NUM
      | %name VAR
        VAR
      | %name ASSIGN
        VAR '=' exp
      | %name PLUS
        exp '+' exp
      | %name MINUS
        exp '-' exp
      | %name TIMES
        exp '*' exp
      | %name DIV
        exp '/' exp
      | %name UMINUS
        '-' exp %prec NEG
      | '(' exp ')'
          { $_[2] }  /* Let us simplify a bit the tree */
    ;

    %%
    sub _Error { die "Syntax error near ".($_[0]->YYCurval?$_[0]->YYCurval:"end of file")."\n" }

    sub _Lexer {
      my($parser)=shift; # The parser object

      for ($parser->YYData->{INPUT}) { # Topicalize
        m{\G\s+}gc;
        $_ eq '' and return('',undef);
        m{\G([0-9]+(?:\.[0-9]+)?)}gc and return('NUM',$1);
        m{\G([A-Za-z][A-Za-z0-9_]*)}gc and return('VAR',$1);
        m{\G(.)}gcs and return($1,$1);
      }
      return('',undef);
    }

    sub Run {
        my($self)=shift;
        $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error, );
    }
  }; # end grammar

  our (@all, $uminus);

  Parse::Eyapp->new_grammar( # Create the parser package/class
    input=>$grammar,
    classname=>'Calc', # The name of the package containing the parser
    firstline=>7       # String $grammar starts at line 7 (for error diagnostics)
  );
  my $parser = Calc->new();                # Create a parser
  $parser->YYData->{INPUT} = "2*-3+b*0;--2\n"; # Set the input
  my $t = $parser->Run;                    # Parse it!
  local $Parse::Eyapp::Node::INDENT=2;
  print "Syntax Tree:",$t->str;

  # Let us transform the tree. Define the tree-regular expressions ..
  my $p = Parse::Eyapp::Treeregexp->new( STRING => q{
      { #  Example of support code
        my %Op = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/');
      }
      constantfold: /TIMES|PLUS|DIV|MINUS/:bin(NUM($x), NUM($y))
        => {
          my $op = $Op{ref($bin)};
          $x->{attr} = eval  "$x->{attr} $op $y->{attr}";
          $_[0] = $NUM[0];
        }
      uminus: UMINUS(NUM($x)) => { $x->{attr} = -$x->{attr}; $_[0] = $NUM }
      zero_times_whatever: TIMES(NUM($x), .) and { $x->{attr} == 0 } => { $_[0] = $NUM }
      whatever_times_zero: TIMES(., NUM($x)) and { $x->{attr} == 0 } => { $_[0] = $NUM }
    },
    OUTPUTFILE=> 'main.pm'
  );
  $p->generate(); # Create the tranformations

  $t->s($uminus); # Transform UMINUS nodes
  $t->s(@all);    # constant folding and mult. by zero

  local $Parse::Eyapp::Node::INDENT=0;
  print "\nSyntax Tree after transformations:\n",$t->str,"\n";

The Parse::Eyapp::Node objects represent the nodes of the syntax tree. All the node classes build by %tree and %metatree directives inherit from Parse::Eyapp::Node and consequently have acces to the methods provided in such module.

The examples used in this document can be found in the directory examples/Node accompanying the distribution of Parse::Eyapp.

Nodes are usually created from a Eyapp grammar using the %tree or %metatree directives. The Parse::Eyapp::Node constructor new offers an alternative way to create forests.

This class method can be used to build multiple nodes on a row. It receives a string describing the tree and optionally a reference to a subroutine. Such subroutine (called the attribute handler) is in charge to initialize the attributes of the just created nodes. The attribute handler is called with the array of references to the nodes as they appear in the string from left to right.

Parse::Eyapp::Node->new returns an array of pointers to the nodes created as they appear in the input string from left to right. In scalar context returns a pointer to the first of these trees.

The following example (see file examples/Node/28foldwithnewwithvars.pl) of a treeregexp transformation creates a new NUM(TERMINAL) node using Parse::Eyapp::Node->new:

 my $p = Parse::Eyapp::Treeregexp->new( STRING => q{
   {
     my %Op = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/');
   }
   constantfold: /TIMES|PLUS|MINUS|DIV/(NUM($x), NUM($y))
      => {
     my $op = $Op{ref($_[0])};

     my $res = Parse::Eyapp::Node->new(
       q{NUM(TERMINAL)},
       sub {
         my ($NUM, $TERMINAL) = @_;
         $TERMINAL->{attr} = eval "$x->{attr} $op $y->{attr}";
         $TERMINAL->{token} = 'NUM';
       },
     );
     $_[0] = $res;
   }
   },
 );

The call to Parse::Eyapp::Node->new creates a tree NUM(TERMINAL) and decorates the TERMINAL leaf with attributes attr and token. The constantfold transformation substitutes all the binary operation trees whose children are numbers for a NUM(TERMINAL) tree holding as attribute the number resulting of operating the two numbers.

The input string can describe more than one tree. Different trees are separated by white spaces. Consider the following example (in examples/Node/builder.pl):

  $ cat -n builder.pl
     1  #!/usr/bin/perl -w
     2  use strict;
     3  use Parse::Eyapp::Node;
     4
     5  use Data::Dumper;
     6  $Data::Dumper::Indent = 1;
     7  $Data::Dumper::Purity = 1;
     8
     9  my $string = shift || 'ASSIGN(VAR(TERMINAL), TIMES(NUM(TERMINAL),NUM(TERMINAL)))  ';
    10  my @t = Parse::Eyapp::Node->new(
    11             $string,
    12             sub { my $i = 0; $_->{n} = $i++ for @_ }
    13          );
    14
    15  print "****************\n";
    16  print Dumper(\@t);

When feed with input 'A(C,D) E(F)' the following forest is built:

  $ builder.pl 'A(C,D) E(F)'
  ****************
  $VAR1 = [
    bless( {
      'n' => 0,
      'children' => [
        bless( { 'n' => 1, 'children' => [] }, 'C' ),
        bless( { 'n' => 2, 'children' => [] }, 'D' )
      ]
    }, 'A' ),
    {},
    {},
    bless( {
      'n' => 3,
      'children' => [
        bless( { 'n' => 4, 'children' => [] }, 'F' )
      ]
    }, 'E' ),
    {}
  ];
  $VAR1->[1] = $VAR1->[0]{'children'}[0];
  $VAR1->[2] = $VAR1->[0]{'children'}[1];
  $VAR1->[4] = $VAR1->[3]{'children'}[0];

Thusm, the forest @t contains 5 subtrees A(C,D), C, D, E(F) and F.

Parse::Eyapp provides the method Parse::Eyapp::Node->hnew to build Directed Acyclic Graphs (DAGs) instead of trees. They are built using hashed consing, i.e. memoizing the creation of nodes.

The method Parse::Eyapp::Node->hnew works very much like Parse::Eyapp::Node->new but if one of the implied trees was previously built, hnew returns a reference to the existing one.

See the following debugger session where several DAGs describing type expressions are built:

  DB<2> x $a = Parse::Eyapp::Node->hnew('F(X_3(A_3(A_5(INT)), CHAR, A_5(INT)),CHAR)')
 0  F=HASH(0x85f6a20)
    'children' => ARRAY(0x85e92e4)
    |- 0  X_3=HASH(0x83f55fc)
    |     'children' => ARRAY(0x83f5608)
    |     |- 0  A_3=HASH(0x85a0488)
    |     |     'children' => ARRAY(0x859fad4)
    |     |        0  A_5=HASH(0x85e5d3c)
    |     |           'children' => ARRAY(0x83f4120)
    |     |              0  INT=HASH(0x83f5200)
    |     |                 'children' => ARRAY(0x852ccb4)
    |     |                      empty array
    |     |- 1  CHAR=HASH(0x8513564)
    |     |     'children' => ARRAY(0x852cad4)
    |     |          empty array
    |     `- 2  A_5=HASH(0x85e5d3c)
    |           -> REUSED_ADDRESS
    `- 1  CHAR=HASH(0x8513564)
          -> REUSED_ADDRESS
  DB<3> x $a->str
 0  'F(X_3(A_3(A_5(INT)),CHAR,A_5(INT)),CHAR)'

The second occurrence of A_5(INT) is labelled REUSED_ADDRESS. The same occurs with the second instance of CHAR.

Parse::Eyapp::Node->hnew can be more convenient than new in some compiler phases and tasks like detecting common subexpressions or during type checking. See file Types.eyp in examples/typechecking/Simple-Types-XXX.tar.gz for a more comprehensive example.

Calls to Parse::Eyapp::Node->hexpand have the syntax

    $z = Parse::Eyapp::Node->hexpand('CLASS', @children, \&handler)

Creates a dag of type 'CLASS' with children @children in a way compatible with hnew. The last optional argument can be a reference to a sub. Such sub will be called after the creation of the DAG with a reference to the root of the DAG as single argument. The following session with the debugger illustrates the use ofParse::Eyapp::Node->hexpand. First we create a DAG using hnew:

  pl@nereida:~/Lbook/code/Simple-Types/script$ perl -MParse::Eyapp::Node -wde 0
  main::(-e:1):   0
    DB<1> $x = Parse::Eyapp::Node->hnew('A(C(B),C(B))')
    DB<2> x $x
  0  A=HASH(0x850c850)
     'children' => ARRAY(0x850ca30)
        0  C=HASH(0x850c928)
           'children' => ARRAY(0x850c9e8)
              0  B=HASH(0x850c9a0)
                 'children' => ARRAY(0x83268c8)
                      empty array
        1  C=HASH(0x850c928)
           -> REUSED_ADDRESS

We obtain the REUSED_ADDRESS for the second child since the C(B) subtree appears twice. Now, suppose we want to expand the exsting tree/DAG C(B) to A(C(B)). We can do that using hexpand:

    DB<3> $y = Parse::Eyapp::Node->hexpand('A', $x->child(0))
    DB<4> x $y
  0  A=HASH(0x8592558)
     'children' => ARRAY(0x832613c)
        0  C=HASH(0x850c928)
           'children' => ARRAY(0x850c9e8)
              0  B=HASH(0x850c9a0)
                 'children' => ARRAY(0x83268c8)
                      empty array
We get new memory for C<$y>: C<HASH(0x8592558)> is anew address.
Assume we want to expand the tree/DAG C<C(B)> to C<A(C(B),C(B))>.
We can do it this way:

    DB<5> $z = Parse::Eyapp::Node->hexpand('A', $x->children)
    DB<6> x $z
  0  A=HASH(0x850c850)
     'children' => ARRAY(0x850ca30)
        0  C=HASH(0x850c928)
           'children' => ARRAY(0x850c9e8)
              0  B=HASH(0x850c9a0)
                 'children' => ARRAY(0x83268c8)
                      empty array
        1  C=HASH(0x850c928)
           -> REUSED_ADDRESS

Notice that the address c<0x850c850> for $z is the same than the address for $x. No new memory has been allocated for $z.

The following command illustrates the use of hexpand with a handler:

    DB<7> $z = Parse::Eyapp::Node->hexpand('A', $x->children, sub { $_[0]->{t} = "X" })
    DB<8> x $z
  0  A=HASH(0x850c850)
     'children' => ARRAY(0x850ca30)
        0  C=HASH(0x850c928)
           'children' => ARRAY(0x850c9e8)
              0  B=HASH(0x850c9a0)
                 'children' => ARRAY(0x83268c8)
                      empty array
        1  C=HASH(0x850c928)
           -> REUSED_ADDRESS
     't' => 'X'

Returns (or sets) the type (class) of the node. It can be called as a subroutine when $node is not a Parse::Eyapp::Node like this:

                     Parse::Eyapp::Node::type($scalar)

This is the case when visiting CODE nodes.

The following session with the debugger illustrates how it works:

  > perl -MParse::Eyapp::Node -de0
  DB<1> @t = Parse::Eyapp::Node->new("A(B,C)") # Creates a tree
  DB<2> x map { $_->type } @t # Get the types of the three nodes
  0  'A'
  1  'B'
  2  'C'
  DB<3> x Parse::Eyapp::Node::type(sub {})
  0  'CODE'
  DB<4> x Parse::Eyapp::Node::type("hola")
  0  'Parse::Eyapp::Node::STRING'
  DB<5> x Parse::Eyapp::Node::type({ a=> 1})
  0  'HASH'
  DB<6> x Parse::Eyapp::Node::type([ a, 1 ])
  0  'ARRAY'

As it is shown in the example it can be called as a subroutine with a (CODE/HASH/ARRAY) reference or an ordinary scalar.

The words HASH, CODE, ARRAY and STRING are reserved for ordinary Perl references. Avoid naming a AST node with one of those words.

To be used as a setter, be sure Parse::Eyapp::Driver is loaded:

  $ perl -MParse::Eyapp::Driver -MParse::Eyapp::Node -wde0
  main::(-e:1):   0
    DB<1> x $t = Parse::Eyapp::Node->new("A(B,C)") # Creates a tree
  0  A=HASH(0x8557bdc)
     'children' => ARRAY(0x8557c90)
        0  B=HASH(0x8557cf0)
           'children' => ARRAY(0x8325804)
                empty array
        1  C=HASH(0x8557c6c)
           'children' => ARRAY(0x8557d5c)
                empty array
    DB<2> x $t->type('FUN') # Change the type of $t to 'FUN'
  0  'FUN'
    DB<3> x $t
  0  FUN=HASH(0x8557bdc)
     'children' => ARRAY(0x8557c90)
        0  B=HASH(0x8557cf0)
           'children' => ARRAY(0x8325804)
                empty array
        1  C=HASH(0x8557c6c)
           'children' => ARRAY(0x8557d5c)
                empty array
    DB<4> x $t->isa('Parse::Eyapp::Node')
  0  1

Setter-getter to modify a specific child of a node. It is called like:

                   $node->child($i)

Returns the child with index $i. Returns undef if the child does not exists. It has two obligatory parameters: the node (since it is a method) and the index of the child. Sets the new value if called

                    $node->child($i, $tree)

The method will croak if the obligatory parameters are not provided.

In the files examples/Node/TSwithtreetransformations2.eyp and examples/node/usetswithtreetransformations2.pl) you can find a somewhat complicated example of call to child as a setter. It is inside a transformation that swaps the children of a PLUS node (remember that the tree is a concrete tree including code since it is a translation scheme built under the directive %metatree):

  my $transform = Parse::Eyapp::Treeregexp->new( STRING => q{
     ........................................................

     commutative_add: PLUS($x, ., $y, .) # 1st dot correspond to '+' 2nd dot to CODE
       => { my $t = $x; $_[0]->child(0, $y); $_[0]->child(2, $t)}

     ........................................................
  }

Remember that when the Eyapp program runs under the %tree alias directive The dot and dollar notations can be used to generate named getter-setters to access the children:

  examples/Node$ cat -n alias_and_yyprefix.pl
     1  #!/usr/local/bin/perl
     2  use warnings;
     3  use strict;
     4  use Parse::Eyapp;
     5
     6  my $grammar = q{
     7    %prefix R::S::
     8
     9    %right  '='
    10    %left   '-' '+'
    11    %left   '*' '/'
    12    %left   NEG
    13    %tree bypass alias
    14
    15    %%
    16    line: $exp  { $_[1] }
    17    ;
    18
    19    exp:
    20        %name NUM
    21              $NUM
    22      | %name VAR
    23              $VAR
    24      | %name ASSIGN
    25              $VAR '=' $exp
    26      | %name PLUS
    27              exp.left '+' exp.right
    28      | %name MINUS
    29              exp.left '-' exp.right
    30      | %name TIMES
    31              exp.left '*' exp.right
    32      | %name DIV
    33              exp.left '/' exp.right
    34      | %no bypass UMINUS
    35              '-' $exp %prec NEG
    36      |   '(' exp ')'  { $_[2] } /* Let us simplify a bit the tree */
    37    ;
    38
    39    %%
    40
    .............................
    76  }; # end grammar
    77
    78
    79  Parse::Eyapp->new_grammar(
    80    input=>$grammar,
    81    classname=>'Alias',
    82    firstline =>7,
    83    outputfile => 'main',
    84  );
    85  my $parser = Alias->new();
    86  $parser->YYData->{INPUT} = "a = -(2*3+5-1)\n";
    87  my $t = $parser->Run;
    88  $Parse::Eyapp::Node::INDENT=0;
    89  print $t->VAR->str."\n";             # a
    90  print "***************\n";
    91  print $t->exp->exp->left->str."\n";  # 2*3+5
    92  print "***************\n";
    93  print $t->exp->exp->right->str."\n"; # 1

Here methods with names left and right will be created inside the class R::S (see the use of the %prefix directive in line 7) to access the corresponding children associated with the two instances of exp in the right hand side of the production rule. when executed, teh former program produces this output:

  examples/Node$ alias_and_yyprefix.pl
  R::S::TERMINAL
  ***************
  R::S::PLUS(R::S::TIMES(R::S::NUM,R::S::NUM),R::S::NUM)
  ***************
  R::S::NUM

Returns the array of children of the node. When the tree is a translation scheme the CODE references are also included. See examples/Node/TSPostfix3.eyp for an example of use inside a Translation Scheme:

  examples/Node$ cat TSPostfix3.eyp
  ...................... # precedence declarations

  %metatree

  %defaultaction {
    if (@_==2) {  # NUM and VAR
      $lhs->{t} = $_[1]->{attr};
      return
    }
    if (@_==4) { # binary operations
      $lhs->{t} = "$_[1]->{t} $_[3]->{t} $_[2]->{attr}";
      return
    }
    die "Fatal Error. Unexpected input. Numargs = ".scalar(@_)."\n".Parse::Eyapp::Node->str(@_);
  }

  %%
  line: %name PROG
         exp <%name EXP + ';'>
           { @{$lhs->{t}} = map { $_->{t}} ($_[1]->children()); }

  ;

  exp:        %name NUM NUM
          |   %name VAR VAR
          |   %name ASSIGN VAR '=' exp  {  $lhs->{t} = "$_[1]->{attr} $_[3]->{t} ="; }
          |   %name PLUS   exp '+' exp
          |   %name MINUS  exp '-' exp
          |   %name TIMES  exp '*' exp
          |   %name DIV    exp '/' exp
          |   %name NEG    '-' exp %prec NEG { $_[0]->{t} = "$_[2]->{t} NEG" }
          |   '(' exp ')' %begin { $_[2] }
  ;

  %%

  ........................

The tree in a Translation Scheme contains the references to the CODE implementing the semantic actions. For example, the syntax tree built by the parser for the input a=-b*3 in TSPostfix3.eyp is:

 PROG(EXP(
     ASSIGN(
       TERMINAL[a],
       TERMINAL[=],
       TIMES(
         NEG(TERMINAL[-], VAR(TERMINAL[b], CODE), CODE),
         TERMINAL[*],
         NUM(TERMINAL[3], CODE),
         CODE
       ) # TIMES,
       CODE
     ) # ASSIGN
   ) # EXP,
   CODE
 ) # PROG

$node->children can also be used as a setter.

Returns the array of children of the node. When dealing with a translation scheme, the $node->Children method (Notice the case difference with $node->children, first in uppercase) returns the non CODE children of the node. The following execution with the debugger of the example in examples/Node/ts_with_ast.pl illustrates the difference:

  examples/Node$ perl -wd ts_with_ast.pl
  main::(ts_with_ast.pl:6):       my $translationscheme = q{
  main::(ts_with_ast.pl:7):       %{

The $translationscheme variable contains the code of a small calculator:

  %metatree

  %left   '-' '+'
  %left   '*'
  %left   NEG

  %%
  line:       %name EXP
                $exp  { $lhs->{n} = $exp->{n} }
  ;

  exp:
              %name PLUS
                exp.left '+'  exp.right
                  { $lhs->{n} .= $left->{n} + $right->{n} }
          |   %name TIMES
                exp.left '*' exp.right
                  { $lhs->{n} = $left->{n} * $right->{n} }
          |   %name NUM   $NUM
                  { $lhs->{n} = $NUM->{attr} }
          |   '(' $exp ')'  %begin { $exp }
          |   exp.left '-' exp.right
                  { $lhs->{n} = $left->{n} - $right->{n} }

          |   '-' $exp %prec NEG
                  { $lhs->{n} = -$exp->{n} }
  ;

We run the program with input 2+(3) and stop it at line 88, just after the augmented AST (CODE node included) has been built:

    DB<1> c 88
  main::(ts_with_ast.pl:88):      $t->translation_scheme;

Now, let us see the difference between the methods children and Children:

    DB<2> @a = $t->children; @b = $t->Children
    DB<3> print Parse::Eyapp::Node::str($_)."\n" for @a
  PLUS(NUM(TERMINAL,CODE),TERMINAL,NUM(TERMINAL,CODE),CODE)
  CODE
    DB<4> print $_->str."\n" for @b
  PLUS(NUM(TERMINAL,CODE),TERMINAL,NUM(TERMINAL,CODE),CODE)
    DB<5>             

Return the last child of the node. When dealing with translation schemes, the last can be a CODE node.

The $node->Last_child method returns the last non CODE child of the node. See an example:

  examples/Node$ cat -n trans_scheme_default_action.pl
     1  #!/usr/bin/perl -w
     2  use strict;
     3  use Data::Dumper;
     4  use Parse::Eyapp;
     5  use IO::Interactive qw(is_interactive);
     6
     7  my $translationscheme = q{
     8  %{
     9  # head code is available at tree construction time
    10  use Data::Dumper;
    11  our %sym; # symbol table
    12  %}
    13
    14  %prefix Calc::
    15
    16  %defaultaction {
    17     $lhs->{n} = eval " $left->{n} $_[2]->{attr} $right->{n} "
    18  }
    19
    20  %metatree
    21
    22  %right   '='
    23  %left   '-' '+'
    24  %left   '*' '/'
    25
    26  %%
    27  line:       %name EXP
    28                exp <+ ';'> /* Expressions separated by semicolons */
    29                  { $lhs->{n} = $_[1]->Last_child->{n} }
    30  ;
    31
    32  exp:
    33              %name PLUS
    34                exp.left '+' exp.right
    35          |   %name MINUS
    36                exp.left '-' exp.right
    37          |   %name TIMES
    38                exp.left '*' exp.right
    39          |   %name DIV
    40                exp.left '/' exp.right
    41          |   %name NUM
    42                $NUM
    43                  { $lhs->{n} = $NUM->{attr} }
    44          |   '(' $exp ')'  %begin { $exp }
    45          |   %name VAR
    46                $VAR
    47                  { $lhs->{n} = $sym{$VAR->{attr}}->{n} }
    48          |   %name ASSIGN
    49                $VAR '=' $exp
    50                  { $lhs->{n} = $sym{$VAR->{attr}}->{n} = $exp->{n} }
    51
    52  ;
    53
    54  %%
    55  # tail code is available at tree construction time
    ......................................................
    77  }; # end translation scheme
    78
    ......................................................

The node associated with $_[1] in

    27  line:       %name EXP
    28                exp <+ ';'> /* Expressions separated by semicolons */
    29                  { $lhs->{n} = $_[1]->Last_child->{n} }

is associated with the whole expression

                               exp <+ ';'>

and is a Calc::_PLUS_LIST node. When feed with input a=3;b=4 the children are the two Calc::ASSIGN subtrees associated with a=3 and b=4 and the CODE associated with the semantic action:

            { $lhs->{n} = $_[1]->Last_child->{n} }

Using Last_child we are avoiding the last CODE child and setting the n(umeric) attribute of the EXP node to the one associated with b=4 (i.e. 4).

  examples/Node$ trans_scheme_default_action.pl
  Write a sequence of arithmetic expressions: a=3;b=4
  ***********Tree*************

  Calc::EXP(
    Calc::_PLUS_LIST(
      Calc::ASSIGN(
        Calc::TERMINAL,
        Calc::TERMINAL,
        Calc::NUM(
          Calc::TERMINAL,
          CODE
        ),
        CODE
      ) # Calc::ASSIGN,
      Calc::ASSIGN(
        Calc::TERMINAL,
        Calc::TERMINAL,
        Calc::NUM(
          Calc::TERMINAL,
          CODE
        ),
        CODE
      ) # Calc::ASSIGN
    ) # Calc::_PLUS_LIST,
    CODE
  ) # Calc::EXP
  ******Symbol table**********
  {
    'a' => { 'n' => '3' },
    'b' => { 'n' => '4' }
  }

  ************Result**********
  4

The descendant method returns the descendant of a node given its coordinates. The coordinates of a node $s relative to a tree $t to which it belongs is a string of numbers separated by dots like ".1.3.2" which denotes the child path from $t to $s, i.e. $s == $t->child(1)->child(3)->child(2).

See a session with the debugger:

   DB<7> x $t->child(0)->child(0)->child(1)->child(0)->child(2)->child(1)->str
 0  '
 BLOCK[8:4:test]^{0}(
   CONTINUE[10,10]
 )
   DB<8> x $t->descendant('.0.0.1.0.2.1')->str
 0  '
 BLOCK[8:4:test]^{0}(
   CONTINUE[10,10]

The str method returns a string representation of the tree. The str method traverses the syntax tree dumping the type of the node being visited in a string. To be specific the value returned by the function referenced by $CLASS_HANDLER will be dumped. The default value fo such function is to return the type of the node. If the node being visited has a method info it will be executed and its result inserted between $DELIMITERs into the string. Thus, in the SYNOPSIS example, by adding the info method to the class TERMINAL:

 sub TERMINAL::info {
   $_[0]{attr}
 }

we achieve the insertion of attributes in the string being built by str.

The existence of some methods (like footnote) and the values of some package variables influence the behavior of str. Among the most important are:

  @PREFIXES = qw(Parse::Eyapp::Node::);                                # Prefixes to suppress 
  $INDENT = 0; # -1 compact, no info, no footnotes 
               # 0 = compact, 1 = indent, 2 = indent and include Types in closing parenthesis
  $STRSEP = ',';                                # Separator between nodes, by default a comma
  $DELIMITER = '[';                         # The string returned by C<info> will be enclosed 
  $FOOTNOTE_HEADER = "\n---------------------------\n"; 
  $FOOTNOTE_SEP = ")\n"; 
  $FOOTNOTE_LEFT = '^{';                               # Left delimiter for a footnote number
  $FOOTNOTE_RIGHT = '}';                              # Right delimiter for a footnote number
  $LINESEP = 4;                             # When indent=2 the enclosing parenthesis will be
                                            # commented if more than $LINESEP apart
  $CLASS_HANDLER = sub { type($_[0]) }; # What to print to identify the node

Footnotes and attribute info will not be inserted when $INDENT is -1. A compact representation will be obtained. Such representation can be feed to new or hnew to obtain a copy of the tree. See the following session with the debugger:

  pl@nereida:~/LEyapp$ perl -MParse::Eyapp::Node -wde 0
  main::(-e:1):   0
    DB<1> $x = Parse::Eyapp::Node->new('A(B(C,D),D)', sub { $_->{order} = $i++ for @_; })
    DB<2> *A::info = *B::info = *C::info = *D::info = sub { shift()->{order} }
    DB<3> p $x->str
  A[0](B[1](C[2],D[3]),D[4])
    DB<4> $Parse::Eyapp::Node::INDENT=-1
    DB<5> p $x->str
  A(B(C,D),D)
    DB<6> x Parse::Eyapp::Node->hnew($x->str)
  0  A=HASH(0x8574704)
     'children' => ARRAY(0x85745d8)
        0  B=HASH(0x857468c)
           'children' => ARRAY(0x8574608)
              0  C=HASH(0x85745b4)
                 'children' => ARRAY(0x8509670)
                      empty array
              1  D=HASH(0x8574638)
                 'children' => ARRAY(0x857450c)
                      empty array
        1  D=HASH(0x8574638)
           -> REUSED_ADDRESS
  1  B=HASH(0x857468c)
     -> REUSED_ADDRESS
  2  C=HASH(0x85745b4)
     -> REUSED_ADDRESS
  3  D=HASH(0x8574638)
     -> REUSED_ADDRESS
  4  D=HASH(0x8574638)
     -> REUSED_ADDRESS

The following list defines the $DELIMITERs you can choose for attribute representation:

          '[' => ']', '{' => '}', '(' => ')', '<' => '>'

If the node being visited has a method footnote, the string returned by the method will be concatenated at the end of the string as a footnote. The variables $FOOTNOTE_LEFT and $FOOTNOTE_RIGHT govern the displaying of footnote numbers.

Follows an example of output using footnotes.

 nereida:~/doc/casiano/PLBOOK/PLBOOK/code/Simple-Types/script> \
                                          usetypes.pl prueba24.c
 PROGRAM^{0}(FUNCTION[f]^{1}(RETURNINT(TIMES(INUM(TERMINAL[2:2]),VAR(TERMINAL[a:2])))))
 ---------------------------
 0)
 Types:
 $VAR1 = {
   'CHAR' => bless( {
     'children' => []
   }, 'CHAR' ),
   'VOID' => bless( {
     'children' => []
   }, 'VOID' ),
   'INT' => bless( {
     'children' => []
   }, 'INT' ),
   'F(X_1(INT),INT)' => bless( {
     'children' => [
       bless( {
         'children' => [
           $VAR1->{'INT'}
         ]
       }, 'X_1' ),
       $VAR1->{'INT'}
     ]
   }, 'F' )
 };
 Symbol Table:
 $VAR1 = {
   'f' => {
     'type' => 'F(X_1(INT),INT)',
     'line' => 1
   }
 };

 ---------------------------
 1)
 $VAR1 = {
   'a' => {
     'type' => 'INT',
     'param' => 1,
     'line' => 1
   }
 };

The first footnote was due to a call to PROGRAM:footnote. The footnote method for the PROGRAM node was defined as:

 nereida:~/doc/casiano/PLBOOK/PLBOOK/code/Simple-Types/lib/Simple> \
                             sed -n -e '691,696p' Types.eyp | cat -n
     1  sub PROGRAM::footnote {
     2    return "Types:\n"
     3           .Dumper($_[0]->{types}).
     4           "Symbol Table:\n"
     5           .Dumper($_[0]->{symboltable})
     6  }

The second footnote was produced by the existence of a FUNCTION::footnote method:

 nereida:~/doc/casiano/PLBOOK/PLBOOK/code/Simple-Types/lib/Simple> \
                            sed -n -e '702,704p' Types.eyp | cat -n
 1  sub FUNCTION::footnote {
 2    return Dumper($_[0]->{symboltable})
 3  }

The source program for the example was:

     1  int f(int a) {
     2    return 2*a;
     3  }

A call $tree1->equal($tree2) compare the two trees $tree1 and $tree2. Two trees are considered equal if their root nodes belong to the same class, they have the same number of children and the children are (recursively) equal.

In Addition to the two trees the programmer can specify pairs attribute_key => equality_handler:

  $tree1->equal($tree2, attr1 => \&handler1, attr2 => \&handler2, ...)

In such case the definition of equality is more restrictive: Two trees are considered equal if

* Their root nodes belong to the same class,

* They have the same number of children

* For each of the specified attributes occur that for both nodes the existence and definition of the key is the same

* Assuming the key exists and is defined for both nodes, the equality handlers return true for each of its attributes and

* The respective children are (recursively) equal.

An attribute handler receives as arguments the values of the attributes of the two nodes being compared and must return true if, and only if, these two attributes are considered equal. Follows an example:

  examples/Node$ cat -n equal.pl
     1  #!/usr/bin/perl -w
     2  use strict;
     3  use Parse::Eyapp::Node;
     4
     5  my $string1 = shift || 'ASSIGN(VAR(TERMINAL))';
     6  my $string2 = shift || 'ASSIGN(VAR(TERMINAL))';
     7  my $t1 = Parse::Eyapp::Node->new($string1, sub { my $i = 0; $_->{n} = $i++ for @_ });
     8  my $t2 = Parse::Eyapp::Node->new($string2);
     9
    10  # Without attributes
    11  if ($t1->equal($t2)) {
    12    print "\nNot considering attributes: Equal\n";
    13  }
    14  else {
    15    print "\nNot considering attributes: Not Equal\n";
    16  }
    17
    18  # Equality with attributes
    19  if ($t1->equal($t2, n => sub { return $_[0] == $_[1] })) {
    20    print "\nConsidering attributes: Equal\n";
    21  }
    22  else {
    23    print "\nConsidering attributes: Not Equal\n";
    24  }

When the former program is run without arguments produces the following output:

  examples/Node$ equal.pl

  Not considering attributes: Equal

  Considering attributes: Not Equal

During the development of your compiler you add new stages to the existing ones. The consequence is that the AST is decorated with new attributes. Unfortunately, this implies that tests you wrote using is_deeply and comparisons against formerly correct abstract syntax trees are no longer valid. This is due to the fact that is_deeply requires both tree structures to be equivalent in every detail and that our new code produces a tree with new attributes.

Instead of is_deeply use the equal method to check for partial equivalence between abstract syntax trees. You can follow these steps:

* Dump the tree for the source inserting Data::Dumper statements

* Carefully check that the tree is really correct

* Decide which attributes will be used for comparison

* Write the code for the expected value editing the output produced by Data::Dumper

* Write the handlers for the attributes you decided. Write the comparison using equal.

Tests using this methodology will not fail even if later code decorating the AST with new attributes is introduced.

See an example that checks an abstract syntax tree produced by the simple compiler (see examples/typechecking/Simple-Types-XXX.tar.gz) for a really simple source:

  Simple-Types/script$ cat prueba27.c
  int f() {
  }

The first thing is to obtain a description of the tree, that can be done executing the compiler under the control of the Perl debugger, stopping just after the tree has been built and dumping the tree with Data::Dumper:

  pl@nereida:~/Lbook/code/Simple-Types/script$ perl -wd usetypes.pl prueba27.c
  main::(usetypes.pl:5):  my $filename = shift || die "Usage:\n$0 file.c\n";
    DB<1> c 12
  main::(usetypes.pl:12): Simple::Types::show_trees($t, $debug);
    DB<2> use Data::Dumper
    DB<3> $Data::Dumper::Purity = 1
    DB<4> p Dumper($t)
  $VAR1 = bless( {
                   ..............................................
                 }, 'PROGRAM' );
  ...............................................................

Once we have the shape of a correct tree we can write our tests:

  examples/Node$ cat -n testequal.pl
     1  #!/usr/bin/perl -w
     2  use strict;
     3  use Parse::Eyapp::Node;
     4  use Data::Dumper;
     5  use Data::Compare;
     6
     7  my $debugging = 0;
     8
     9  my $handler = sub {
    10    print Dumper($_[0], $_[1]) if $debugging;
    11    Compare($_[0], $_[1])
    12  };
    13
    14  my $t1 = bless( {
    15                   'types' => {
    16                                'CHAR' => bless( { 'children' => [] }, 'CHAR' ),
    17                                'VOID' => bless( { 'children' => [] }, 'VOID' ),
    18                                'INT' => bless( { 'children' => [] }, 'INT' ),
    19                                'F(X_0(),INT)' => bless( {
    20                                   'children' => [
    21                                      bless( { 'children' => [] }, 'X_0' ),
    22                                      bless( { 'children' => [] }, 'INT' ) ]
    23                                 }, 'F' )
    24                              },
    25                   'symboltable' => { 'f' => { 'type' => 'F(X_0(),INT)', 'line' => 1 } },
    26                   'lines' => 2,
    27                   'children' => [
    28                                   bless( {
    29                                            'symboltable' => {},
    30                                            'fatherblock' => {},
    31                                            'children' => [],
    32                                            'depth' => 1,
    33                                            'parameters' => [],
    34                                            'function_name' => [ 'f', 1 ],
    35                                            'symboltableLabel' => {},
    36                                            'line' => 1
    37                                          }, 'FUNCTION' )
    38                                 ],
    39                   'depth' => 0,
    40                   'line' => 1
    41                 }, 'PROGRAM' );
    42  $t1->{'children'}[0]{'fatherblock'} = $t1;
    43
    44  # Tree similar to $t1 but without some attributes (line, depth, etc.)
    45  my $t2 = bless( {
    46                   'types' => {
    47                                'CHAR' => bless( { 'children' => [] }, 'CHAR' ),
    48                                'VOID' => bless( { 'children' => [] }, 'VOID' ),
    49                                'INT' => bless( { 'children' => [] }, 'INT' ),
    50                                'F(X_0(),INT)' => bless( {
    51                                   'children' => [
    52                                      bless( { 'children' => [] }, 'X_0' ),
    53                                      bless( { 'children' => [] }, 'INT' ) ]
    54                                 }, 'F' )
    55                              },
    56                   'symboltable' => { 'f' => { 'type' => 'F(X_0(),INT)', 'line' => 1 } },
    57                   'children' => [
    58                                   bless( {
    59                                            'symboltable' => {},
    60                                            'fatherblock' => {},
    61                                            'children' => [],
    62                                            'parameters' => [],
    63                                            'function_name' => [ 'f', 1 ],
    64                                          }, 'FUNCTION' )
    65                                 ],
    66                 }, 'PROGRAM' );
    67  $t2->{'children'}[0]{'fatherblock'} = $t2;
    68
    69  # Tree similar to $t1 but without some attributes (line, depth, etc.)
    70  # and without the symboltable and types attributes used in the comparison
    71  my $t3 = bless( {
    72                   'types' => {
    73                                'CHAR' => bless( { 'children' => [] }, 'CHAR' ),
    74                                'VOID' => bless( { 'children' => [] }, 'VOID' ),
    75                                'INT' => bless( { 'children' => [] }, 'INT' ),
    76                                'F(X_0(),INT)' => bless( {
    77                                   'children' => [
    78                                      bless( { 'children' => [] }, 'X_0' ),
    79                                      bless( { 'children' => [] }, 'INT' ) ]
    80                                 }, 'F' )
    81                              },
    82                   'children' => [
    83                                   bless( {
    84                                            'symboltable' => {},
    85                                            'fatherblock' => {},
    86                                            'children' => [],
    87                                            'parameters' => [],
    88                                            'function_name' => [ 'f', 1 ],
    89                                          }, 'FUNCTION' )
    90                                 ],
    91                 }, 'PROGRAM' );
    92
    93  $t3->{'children'}[0]{'fatherblock'} = $t2;
    94
    95  # Without attributes
    96  if (Parse::Eyapp::Node::equal($t1, $t2)) {
    97    print "\nNot considering attributes: Equal\n";
    98  }
    99  else {
   100    print "\nNot considering attributes: Not Equal\n";
   101  }
   102
   103  # Equality with attributes
   104  if (Parse::Eyapp::Node::equal(
   105        $t1, $t2,
   106        symboltable => $handler,
   107        types => $handler,
   108      )
   109     ) {
   110        print "\nConsidering attributes: Equal\n";
   111  }
   112  else {
   113    print "\nConsidering attributes: Not Equal\n";
   114  }
   115
   116  # Equality with attributes
   117  if (Parse::Eyapp::Node::equal(
   118        $t1, $t3,
   119        symboltable => $handler,
   120        types => $handler,
   121      )
   122     ) {
   123        print "\nConsidering attributes: Equal\n";
   124  }
   125  else {
   126    print "\nConsidering attributes: Not Equal\n";
   127  }

The code defining tree $t1 was obtained from an output using Data::Dumper. The code for trees $t2 and $t3 was written using cut-and-paste from $t1. They have the same shape than $t1 but differ in their attributes. Tree $t2 shares with $t1 the attributes symboltable and types used in the comparison and so equal returns true when compared. Since $t3 differs from $t1 in the attributes symboltable and types the call to equal returns false.

The $node->delete($child) method is used to delete the specified child of $node. The child to delete can be specified using the index or a reference. It returns the deleted child.

Throws an exception if the object can't do children or has no children. See also the delete method of treeregexes (Parse::Eyapp:YATW objects) to delete the node being visited.

The following example moves out of a loop an assignment statement assuming is an invariant of the loop. To do it, it uses the delete and insert_before methods:

  nereida:~/src/perl/YappWithDefaultAction/examples> \
              sed -ne '98,113p' moveinvariantoutofloopcomplexformula.pl
  my $p = Parse::Eyapp::Treeregexp->new( STRING => q{
    moveinvariant: BLOCK(
                     @prests,
                     WHILE(VAR($b), BLOCK(@a, ASSIGN($x, NUM($e)), @c)),
                     @possts
                   )
      => {
           my $assign = $ASSIGN;
           $BLOCK[1]->delete($ASSIGN);
           $BLOCK[0]->insert_before($WHILE, $assign);
         }
    },
    FIRSTLINE => 99,
  );
  $p->generate();
  $moveinvariant->s($t);

The example below deletes CODE nodes from the tree build for a translation scheme:

  my $transform = Parse::Eyapp::Treeregexp->new( 
    STRING=>q{
      delete_code: CODE => { Parse::Eyapp::Node::delete($CODE) }
    },
  )

Observe how delete is called as a subroutine.

Inserts $newchild at the beginning of the list of children of $node. See also the unshift method for Parse::Eyapp:YATW treeregexp transformation objects

Inserts $newchild at the end of the list of children of $node.

Inserts $newchild before $position in the list of children of $node. Variable $position can be an index or a reference.

The method throws an exception if $position is an index and is not in range. Also if $node has no children.

The method throws a warning if $position is a reference and does not define an actual child. In such case $new_child is not inserted.

See also the insert_before method for Parse::Eyapp:YATW treeregexp transformation objects

Inserts $newchild after $position in the list of children of $node. Variable $position can be an index or a reference.

The method throws an exception if $position is an index and is not in the range of $node-children>.

The method throws a warning if $position is a reference and does not exists in the list of children. In such case $new_child is not inserted.

Traverses $node. Each time a CODE node is visited the subroutine referenced is called with arguments the node and its children. Usually the code will decorate the nodes with new attributes or will update existing ones. Obviously this method does nothing for an ordinary AST. It is used after compiling an Eyapp program that makes use of the %metatree directive. (See examples/Node/TSPostfix3.eyp for an example).

Bottom-up decorator. The tree is traversed bottom-up. The set of transformations in @transformations is applied to each node in the tree referenced by $node in the order supplied by the user. As soon as one succeeds no more transformations are applied.

For an example see the files lib/Simple/Types.eyp and lib/Simple/Trans.trg in examples/typechecking/Simple-Types-XXX.tar.gz shows an extract of the type-checking phase of a toy-example compiler:

  examples/typechecking/Simple-Types-0.4/lib/Simple$  sed -ne '600,613p' Types.eyp
   my @typecheck = (     # Check typing transformations for
     our $inum,          # - Numerical constantss
     our $charconstant,  # - Character constants
     our $bin,           # - Binary Operations
     our $arrays,        # - Arrays
     our $assign,        # - Assignments
     our $control,       # - Flow control sentences
     our $functioncall,  # - Function calls
     our $statements,    # - Those nodes with void type
                         #   (STATEMENTS, PROGRAM, etc.)
     our $returntype,    # - Return
   );

   $t->bud(@typecheck);

You can find another example of use of bud in the file examples/ParsingStringsAndTrees/infix2pir.pl

Parse::Eyapp:YATW objects represent tree transformations. They carry the information of what nodes match and how to modify them.

Builds a treeregexp transformation object. Though usually you build a transformation by means of Treeregexp programs you can directly invoke the method to build a tree transformation. A transformation object can be built from a function that conforms to the YATW tree transformation call protocol (see the section The YATW Tree Transformation Call Protocol). Follows an example (file examples/12ts_simplify_with_s.pl):

 nereida:~/src/perl/YappWithDefaultAction/examples> \
        sed -ne '68,$p' 12ts_simplify_with_s.pl | cat -n
  1  sub is_code {
  2    my $self = shift; # tree
  3
  4    # After the shift $_[0] is the father, $_[1] the index
  5    if ((ref($self) eq 'CODE')) {
  6      splice(@{$_[0]->{children}}, $_[1], 1);
  7      return 1;
  8    }
  9    return 0;
 10  }
 11
 12  Parse::Eyapp->new_grammar(
 13    input=>$translationscheme,
 14    classname=>'Calc',
 15    firstline =>7,
 16  );
 17  my $parser = Calc->new();                # Create the parser
 18
 19  $parser->YYData->{INPUT} = "2*-3\n";  print "2*-3\n"; # Set the input
 20  my $t = $parser->Run;                    # Parse it
 21  print $t->str."\n";
 22  my $p = Parse::Eyapp::YATW->new(PATTERN => \&is_code);
 23  $p->s($t);
 24  { no warnings; # make attr info available only for this display
 25    local *TERMINAL::info = sub { $_[0]{attr} };
 26    print $t->str."\n";
 27  }

After the Parse::Eyapp::YATW object $p is built at line 22 the call to method $p->s($t) applies the transformation is_code using a bottom-up traversing of the tree $t. The achieved effect is the elimination of CODE references in the translation scheme tree. When executed the former code produces:

 nereida:~/src/perl/YappWithDefaultAction/examples> 12ts_simplify_with_s.pl
 2*-3
 EXP(TIMES(NUM(TERMINAL,CODE),TERMINAL,UMINUS(TERMINAL,NUM(TERMINAL,CODE),CODE),CODE),CODE)
 EXP(TIMES(NUM(TERMINAL[2]),TERMINAL[*],UMINUS(TERMINAL[-],NUM(TERMINAL[3]))))

The file foldrule6.pl in the examples/ distribution directory gives you another example:

 nereida:~/src/perl/YappWithDefaultAction/examples> cat -n foldrule6.pl
   1  #!/usr/bin/perl -w
   2  use strict;
   3  use Rule6;
   4  use Parse::Eyapp::YATW;
   5
   6  my %BinaryOperation = (PLUS=>'+', MINUS => '-', TIMES=>'*', DIV => '/');
   7
   8  sub set_terminfo {
   9    no warnings;
  10    *TERMINAL::info = sub { $_[0]{attr} };
  11  }
  12  sub is_foldable {
  13    my ($op, $left, $right);
  14    return 0 unless defined($op = $BinaryOperation{ref($_[0])});
  15    return 0 unless ($left = $_[0]->child(0), $left->isa('NUM'));
  16    return 0 unless ($right = $_[0]->child(1), $right->isa('NUM'));
  17
  18    my $leftnum = $left->child(0)->{attr};
  19    my $rightnum = $right->child(0)->{attr};
  20    $left->child(0)->{attr} = eval "$leftnum $op $rightnum";
  21    $_[0] = $left;
  22  }
  23
  24  my $parser = new Rule6();
  25  $parser->YYData->{INPUT} = "2*3";
  26  my $t = $parser->Run;
  27  &set_terminfo;
  28  print "\n***** Before ******\n";
  29  print $t->str;
  30  my $p = Parse::Eyapp::YATW->new(PATTERN => \&is_foldable);
  31  $p->s($t);
  32  print "\n***** After ******\n";
  33  print $t->str."\n";

when executed produces:

 nereida:~/src/perl/YappWithDefaultAction/examples> foldrule6.pl

 ***** Before ******
 TIMES(NUM(TERMINAL[2]),NUM(TERMINAL[3]))
 ***** After ******
 NUM(TERMINAL[6])

For a subroutine pattern_sub to work as a YATW tree transformation - as subroutines is_foldable and is_code above - has to conform to the following call description:

  pattern_sub(
      $_[0],  # Node being visited
      $_[1],  # Father of this node
      $index, # Index of this node in @Father->children
      $self,  # The YATW pattern object
  );

The pattern_sub must return TRUE if matched and FALSE otherwise.

The protocol may change in the near future. Avoid using other information than the fact that the first argument is the node being visited.

Works as Parse::Eyapp->new but receives an array of subs conforming to the YATW Tree Transformation Call Protocol.

  our @all = Parse::Eyapp::YATW->buildpatt(\&delete_code, \&delete_tokens);

The root of the tree that is currently matched by the YATW transformation $yatw will be deleted from the tree as soon as is safe. That usually means when the processing of their siblings is finished. The following example (taken from file examples/13ts_simplify_with_delete.pl in the Parse::Eyapp distribution) illustrates how to eliminate CODE and syntactic terminals from the syntax tree:

 pl@nereida:~/src/perl/YappWithDefaultAction/examples$ \
        sed -ne '62,$p' 13ts_simplify_with_delete.pl | cat -n
  1  sub not_useful {
  2    my $self = shift; # node
  3    my $pat = $_[2];  # get the YATW object
  4
  5    (ref($self) eq 'CODE') or ((ref($self) eq 'TERMINAL') and ($self->{token} eq $self->{attr}))
  6      or do { return 0 };
  7    $pat->delete();
  8    return 1;
  9  }
 10
 11  Parse::Eyapp->new_grammar(
 12    input=>$translationscheme,
 13    classname=>'Calc',
 14    firstline =>7,
 15  );
 16  my $parser = Calc->new();                # Create the parser
 17
 18  $parser->YYData->{INPUT} = "2*3\n"; print $parser->YYData->{INPUT};
 19  my $t = $parser->Run;                    # Parse it
 20  print $t->str."\n";                      # Show the tree
 21  my $p = Parse::Eyapp::YATW->new(PATTERN => \&not_useful); 
 22  $p->s($t);                               # Delete nodes
 23  print $t->str."\n";                      # Show the tree

when executed we get the following output:

 pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 13ts_simplify_with_delete.pl
 2*3
 EXP(TIMES(NUM(TERMINAL[2],CODE),TERMINAL[*],NUM(TERMINAL[3],CODE),CODE))
 EXP(TIMES(NUM(TERMINAL[2]),NUM(TERMINAL[3])))

The call $yatw->unshift($b) safely unshifts (inserts at the beginning) the node $b in the list of its siblings of the node that matched (i.e in the list of siblings of $_[0]). The following example shows a YATW transformation insert_child that illustrates the use of unshift (file examples/26delete_with_trreereg.pl):

 pl@nereida:~/src/perl/YappWithDefaultAction/examples$ \
         sed -ne '70,$p' 26delete_with_trreereg.pl | cat -n
  1  my $transform = Parse::Eyapp::Treeregexp->new( STRING => q{
  2
  3      delete_code : CODE => { $delete_code->delete() }
  4
  5      {
  6        sub not_semantic {
  7          my $self = shift;
  8          return  1 if ((ref($self) eq 'TERMINAL') and ($self->{token} eq $self->{attr}));
  9          return 0;
 10        }
 11      }
 12
 13      delete_tokens : TERMINAL and { not_semantic($TERMINAL) } => {
 14        $delete_tokens->delete();
 15      }
 16
 17      insert_child : TIMES(NUM(TERMINAL), NUM(TERMINAL)) => {
 18        my $b = Parse::Eyapp::Node->new( 'UMINUS(TERMINAL)',
 19          sub { $_[1]->{attr} = '4.5' }); # The new node will be a sibling of TIMES
 20
 21        $insert_child->unshift($b); 
 22      }
 23    },
 24  )->generate();
 25
 26  Parse::Eyapp->new_grammar(
 27    input=>$translationscheme,
 28    classname=>'Calc',
 29    firstline =>7,
 30  );
 31  my $parser = Calc->new();                # Create the parser
 32
 33  $parser->YYData->{INPUT} = "2*3\n"; print $parser->YYData->{INPUT}; # Set the input
 34  my $t = $parser->Run;                # Parse it
 35  print $t->str."\n";                        # Show the tree
 36  # Get the AST
 37  our ($delete_tokens, $delete_code);
 38  $t->s($delete_tokens, $delete_code);
 39  print $t->str."\n";                        # Show the tree
 40  our $insert_child;
 41  $insert_child->s($t);
 42  print $t->str."\n";                        # Show the tree

When is executed the program produces the following output:

 pl@nereida:~/src/perl/YappWithDefaultAction/examples$ 26delete_with_trreereg.pl
 2*3
 EXP(TIMES(NUM(TERMINAL[2],CODE),TERMINAL[*],NUM(TERMINAL[3],CODE),CODE))
 EXP(TIMES(NUM(TERMINAL[2]),NUM(TERMINAL[3])))
 EXP(UMINUS(TERMINAL[4.5]),TIMES(NUM(TERMINAL[2]),NUM(TERMINAL[3])))

Don't try to take advantage that the transformation sub receives in $_[1] a reference to the father (see the section The YATW Tree Transformation Call Protocol) and do something like:

  unshift $_[1]->{children}, $b

it is unsafe.

A call to $yatw->insert_before($node) safely inserts $node in the list of siblings of $_[0] just before $_[0] (i.e. the node that matched with $yatw). The following example (see file examples/YATW/moveinvariantoutofloopcomplexformula.pl) illustrates its use:

  my $p = Parse::Eyapp::Treeregexp->new( STRING => q{
    moveinvariant: WHILE(VAR($b), BLOCK(@a, ASSIGN($x, $e), @c)) 
         and { is_invariant($ASSIGN, $WHILE) } => {
           my $assign = $ASSIGN;
           $BLOCK->delete($ASSIGN);
           $moveinvariant->insert_before($assign);
         }
    },
  );

Here the ASSIGN($x, $e) subtree - if is loop invariant - will be moved to the list of siblings of $WHILE just before the $WHILE. Thus a program like

  "a =1000; c = 1; while (a) { c = c*a; b = 5; a = a-1 }\n"

is transformed in s.t. like:

  "a =1000; c = 1; b = 5; while (a) { c = c*a; a = a-1 }\n"

See the documentation in Parse::Eyapp::treematchingtut

* The project home is at http://code.google.com/p/parse-eyapp/. Use a subversion client to anonymously check out the latest project source code:

   svn checkout http://parse-eyapp.googlecode.com/svn/trunk/ parse-eyapp-read-only

* The tutorial Parsing Strings and Trees with Parse::Eyapp (An Introduction to Compiler Construction in seven pages) in http://nereida.deioc.ull.es/~pl/eyapsimple/

* Parse::Eyapp, Parse::Eyapp::eyapplanguageref, Parse::Eyapp::debuggingtut, Parse::Eyapp::defaultactionsintro, Parse::Eyapp::translationschemestut, Parse::Eyapp::Driver, Parse::Eyapp::Node, Parse::Eyapp::YATW, Parse::Eyapp::Treeregexp, Parse::Eyapp::Scope, Parse::Eyapp::Base, Parse::Eyapp::datagenerationtut

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/languageintro.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/debuggingtut.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/eyapplanguageref.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Treeregexp.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Node.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/YATW.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Eyapp.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/Base.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/translationschemestut.pdf

* The pdf file in http://nereida.deioc.ull.es/~pl/perlexamples/treematchingtut.pdf

* perldoc eyapp,

* perldoc treereg,

* perldoc vgg,

* The Syntax Highlight file for vim at http://www.vim.org/scripts/script.php?script_id=2453 and http://nereida.deioc.ull.es/~vim/

* Analisis Lexico y Sintactico, (Notes for a course in compiler construction) by Casiano Rodriguez-Leon. Available at http://nereida.deioc.ull.es/~pl/perlexamples/ Is the more complete and reliable source for Parse::Eyapp. However is in Spanish.

* Parse::Yapp,

* Man pages of yacc(1) and bison(1), http://www.delorie.com/gnu/docs/bison/bison.html

* Language::AttributeGrammar

* Parse::RecDescent.

* HOP::Parser

* HOP::Lexer

* ocamlyacc tutorial at http://plus.kaist.ac.kr/~shoh/ocaml/ocamllex-ocamlyacc/ocamlyacc-tutorial/ocamlyacc-tutorial.html

* The classic Dragon's book Compilers: Principles, Techniques, and Tools by Alfred V. Aho, Ravi Sethi and Jeffrey D. Ullman (Addison-Wesley 1986)

* CS2121: The Implementation and Power of Programming Languages (See http://www.cs.man.ac.uk/~pjj, http://www.cs.man.ac.uk/~pjj/complang/g2lr.html and http://www.cs.man.ac.uk/~pjj/cs2121/ho/ho.html) by Pete Jinks

* 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.