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Parse-Eyapp-1.181
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/ Parse::Eyapp::translationschemestut

NAME

Parse::Eyapp::translationschemestut - Introduction to Translation Schemes in Eyapp

INTRODUCTION

A translation scheme scheme is a context free grammar where the right hand sides of the productions have been augmented with semantic actions (i.e. with chunks of Perl code):

                                A -> alpha { action(@_) } beta

The analyzer generated by Parse::Eyapp executes action() after all the semantic actions associated with alpha have been executed and before the execution of any of the semantic actions associated with beta.

In a translation scheme the embedded actions modify the attributes associated with the symbols of the grammar.

                                A -> alpha { action(@_) } beta

each symbol on the right hand side of a production rule has an associated scalar attribute. In ordinary eyapp programs the attributes of the symbol to the left of action are passed as arguments to action (in the example, those of alpha). These arguments are preceded by a reference to the syntax analyzer object. There is no way inside an ordinary eyapp program for an intermediate action to access the attributes of the symbols on its right, i.e. those associated with the symbols of beta. This restriction is lifted if you use the %metatree directive.

Eyapp allows through the %metatree directive the creation of Translation Schemes where the actions have access to almost any node of the syntax tree.

When using the %metatree directive semantic actions aren't immediately executed. Instead they are inserted as nodes of the syntax tree. The main difference with ordinary nodes being that the attribute of such a CODE node is a reference to the anonymous subroutine representing the semantic action. The tree is later traversed in depth-first order using the $t->translation_scheme method: each time a CODE node is visited the action is executed.

The following example parses a tiny subset of a typical typed language and decorates the syntax tree with a new attribute t holding the type of each declared variable:

 use strict; # File examples/trans_scheme_simple_decls4.pl
 use Data::Dumper;
 use Parse::Eyapp;
 our %s; # symbol table

 my $ts = q{ 
   %token FLOAT INTEGER NAME

   %{
   our %s;
   %}

   %metatree

   %%
   Dl:  D <* ';'>
   ;

   D : $T { $L->{t} = $T->{t} } $L
   ;

   T : FLOAT    { $lhs->{t} = "FLOAT" }
     | INTEGER  { $lhs->{t} = "INTEGER" }
   ;

   L : $NAME
         { $NAME->{t} = $lhs->{t}; $s{$NAME->{attr}} = $NAME }
     | $NAME { $NAME->{t} = $lhs->{t}; $L->{t} = $lhs->{t} } ',' $L
         { $s{$NAME->{attr}} = $NAME }
   ;
   %%
 }; # end $ts

 sub Error { die "Error sintáctico\n"; }

 { # Closure of $input, %reserved_words and $validchars
   my $input = "";
   my %reserved_words = ();
   my $validchars = "";

   sub parametrize__scanner {
     $input = shift;
     %reserved_words = %{shift()};
     $validchars = shift;
   }

   sub scanner {
     $input =~ m{\G\s+}gc;                     # skip whites
     if ($input =~ m{\G([a-z_A_Z]\w*)\b}gc) {
       my $w = uc($1);                 # upper case the word
       return ($w, $w) if exists $reserved_words{$w};
       return ('NAME', $1);            # not a reserved word
     }
     return ($1, $1) if ($input =~ m/\G([$validchars])/gc);
     die "Not valid token: $1\n" if ($input =~ m/\G(\S)/gc);
     return ('', undef); # end of file
   }
 } # end closure

 Parse::Eyapp->new_grammar(input=>$ts,classname=>'main',outputfile=>'Types.pm');
 my $parser = main->new(yylex => \&scanner, yyerror => \&Error); 

 parametrize__scanner(
   "float x,y;\ninteger a,b\n",
   { INTEGER => 'INTEGER', FLOAT => 'FLOAT'},
   ",;"
 );

 my $t = $parser->YYParse() or die "Syntax Error analyzing input";

 $t->translation_scheme;

 $Data::Dumper::Indent = 1;
 $Data::Dumper::Terse = 1;
 $Data::Dumper::Deepcopy  = 1;
 $Data::Dumper::Deparse = 1;
 print Dumper($t);
 print Dumper(\%s);

Inside a Translation Scheme the lexical variable $lhs refers to the attribute of the father.

EXECUTION STAGES OF A TRANSLATION SCHEME

The execution of a Translation Scheme can be divided in the following stages:

1. During the first stage the grammar is analyzed and the parser is built:
 Parse::Eyapp->new_grammar(input=>$ts,classname=>'main',outputfile=>'Types.pm');

This stage is called Class Construction Time

2. A parser conforming to the generated grammar is built
  my $parser = main->new(yylex => \&scanner, yyerror => \&Error);

This stage is called Parser Construction Time

3. The next phase is Tree construction time. The input is set and the tree is built:
 parametrize__scanner(
    "float x,y;\ninteger a,b\n",
    { INTEGER => 'INTEGER', FLOAT => 'FLOAT'},
    ",;"
  );

  my $t = $parser->YYParse() or die "Syntax Error analyzing input";
4. The last stage is Execution Time. The tree is traversed in depth first order and the CODE nodes are executed.
                           $t->translation_scheme;

This combination of bottom-up parsing with depth first traversing leads to a semantic behavior similar to recursive top-down parsers but with two advantages:

  • The grammar can be left-recursive

  • At the time of executing the action the syntax tree is already built, therefore we can refer to nodes on the right side of the action like in:

                          D : $T { $L->{t} = $T->{t} } $L

THE %begin DIRECTIVE

The %begin { code } directive can be used when building a translation scheme, i.e. when under the control of the %metatree directive. It indicates that such { code } will be executed at tree construction time. Therefore the code receives as arguments the references to the nodes of the branch than is being built. Usually begin code assist in the construction of the tree. Line 39 of the following code shows an example. The action { $exp } simplifies the syntax tree bypassing the parenthesis node. The example also illustrates the combined use of default actions and translation schemes.

  pl@nereida:~/LEyapp/examples$ 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  %defaultaction {
    15     $lhs->{n} = eval " $left->{n} $_[2]->{attr} $right->{n} "
    16  }
    17
    18  %metatree
    19
    20  %right   '='
    21  %left   '-' '+'
    22  %left   '*' '/'
    23
    24  %%
    25  line:       %name EXP
    26                exp <+ ';'> /* Expressions separated by semicolons */
    27                  { $lhs->{n} = $_[1]->Last_child->{n} }
    28  ;
    29
    30  exp:
    31              %name PLUS
    32                exp.left '+' exp.right
    33          |   %name MINUS
    34                exp.left '-' exp.right
    35          |   %name TIMES
    36                exp.left '*' exp.right
    37          |   %name DIV
    38                exp.left '/' exp.right
    39          |   %name NUM
    40                $NUM
    41                  { $lhs->{n} = $NUM->{attr} }
    42          |   '(' $exp ')'  %begin { $exp }
    43          |   %name VAR
    44                $VAR
    45                  { $lhs->{n} = $sym{$VAR->{attr}}->{n} }
    46          |   %name ASSIGN
    47                $VAR '=' $exp
    48                  { $lhs->{n} = $sym{$VAR->{attr}}->{n} = $exp->{n} }
    49
    50  ;
    51
    52  %%
    53  # tail code is available at tree construction time
    54  sub _Error {
    55    die "Syntax error.\n";
    56  }
    57
    58  sub _Lexer {
    59      my($parser)=shift;
    60
    61      for ($parser->YYData->{INPUT}) {
    62          s/^\s+//;
    63          $_ or  return('',undef);
    64          s/^([0-9]+(?:\.[0-9]+)?)// and return('NUM',$1);
    65          s/^([A-Za-z][A-Za-z0-9_]*)// and return('VAR',$1);
    66          s/^(.)// and return($1,$1);
    67      }
    68      return('',undef);
    69  }
    70
    71  sub Run {
    72      my($self)=shift;
    73      return $self->YYParse( yylex => \&_Lexer, yyerror => \&_Error );
    74  }
    75  }; # end translation scheme
    76
    77  sub TERMINAL::info { $_[0]->attr }
    78
    79  my $p = Parse::Eyapp->new_grammar(
    80    input=>$translationscheme,
    81    classname=>'main',
    82    firstline => 6,
    83    outputfile => 'main.pm');
    84  die $p->qtables() if $p->Warnings;
    85  my $parser = main->new();
    86  print "Write a sequence of arithmetic expressions: " if is_interactive();
    87  $parser->YYData->{INPUT} = <>;
    88  my $t = $parser->Run() or die "Syntax Error analyzing input";
    89  $t->translation_scheme;
    90
    91  $Parse::Eyapp::Node::INDENT = 2;
    92  my $treestring = $t->str;
    93
    94  $Data::Dumper::Indent = 1;
    95  $Data::Dumper::Terse = 1;
    96  $Data::Dumper::Deepcopy  = 1;
    97  our %sym;
    98  my $symboltable = Dumper(\%sym);
    99
   100  print <<"EOR";
   101  ***********Tree*************
   102  $treestring
   103  ******Symbol table**********
   104  $symboltable
   105  ************Result**********
   106  $t->{n}
   107
   108  EOR

When executed with input a=2*3;b=a*a the program produces an output similar to this:

  pl@nereida:~/LEyapp/examples$ trans_scheme_default_action.pl
  Write a sequence of arithmetic expressions: a=2*3;b=a*a
  ***********Tree*************

  EXP(
    _PLUS_LIST(
      ASSIGN(
        TERMINAL[a],
        TERMINAL[=],
        TIMES(
          NUM(TERMINAL[2], CODE),
          TERMINAL[*],
          NUM(TERMINAL[3], CODE),
          CODE
        ) # TIMES,
        CODE
      ) # ASSIGN,
      ASSIGN(
        TERMINAL[b],
        TERMINAL[=],
        TIMES(
          VAR(TERMINAL[a], CODE),
          TERMINAL[*],
          VAR(TERMINAL[a], CODE),
          CODE
        ) # TIMES,
        CODE
      ) # ASSIGN
    ) # _PLUS_LIST,
    CODE
  ) # EXP
  ******Symbol table**********
  {
    'a' => {
      'n' => 6
    },
    'b' => {
      'n' => 36
    }
  }

  ************Result**********
  36

SEE ALSO

REFERENCES

CONTRIBUTORS

AUTHOR

Casiano Rodriguez-Leon (casiano@ull.es)

ACKNOWLEDGMENTS

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.

LICENCE AND COPYRIGHT

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.

1 POD Error

The following errors were encountered while parsing the POD:

Around line 81:

Non-ASCII character seen before =encoding in 'sintáctico\n";'. Assuming UTF-8