Synopsis
We want to ``diagram'' the following sentence.
my $sentence = 'a panda eats shoots and leaves.';
Here's the result we are looking for. It is in Penntag form:
(S (NP (DT a) (NN panda))
(VP (VBZ eats) (NP (NNS shoots) (CC and) (NNS leaves)))
(. .))
(S (NP (DT a) (NN panda))
(VP (VP (VBZ eats) (NP (NNS shoots))) (CC and) (VP (VBZ leaves)))
(. .))
(S (NP (DT a) (NN panda))
(VP (VP (VBZ eats)) (VP (VBZ shoots)) (CC and) (VP (VBZ leaves)))
(. .))
Here is the grammar.
:default ::= action => [ values ] bless => ::lhs
lexeme default = action => [ value ] bless => ::name
S ::= NP VP period bless => S
NP ::= NN bless => NP
| NNS bless => NP
| DT NN bless => NP
| NN NNS bless => NP
| NNS CC NNS bless => NP
VP ::= VBZ NP bless => VP
| VP VBZ NNS bless => VP
| VP CC VP bless => VP
| VP VP CC VP bless => VP
| VBZ bless => VP
period ~ '.'
:discard ~ whitespace
whitespace ~ [\s]+
CC ~ 'and'
DT ~ 'a' | 'an'
NN ~ 'panda'
NNS ~ 'shoots' | 'leaves'
VBZ ~ 'eats' | 'shoots' | 'leaves'
Here's the code. It actually does two traversals, one that produces the full result as shown above, and another which ``prunes'' the forest down to a single tree.
my $panda_grammar = Marpa::R2::Scanless::G->new(
{ source => \$dsl, bless_package => 'PennTags', } );
my $panda_recce = Marpa::R2::Scanless::R->new( { grammar => $panda_grammar } );
$panda_recce->read( \$sentence );
my $asf = Marpa::R2::ASF->new( { slr=>$panda_recce } );
my $full_result = $asf->traverse( {}, \&full_traverser );
my $pruned_result = $asf->traverse( {}, \&pruning_traverser );
The code for the full traverser is in an appendix. The pruning code is simpler. Here it is:
sub penn_tag {
my ($symbol_name) = @_;
return q{.} if $symbol_name eq 'period';
return $symbol_name;
}
sub pruning_traverser {
# This routine converts the glade into a list of Penn-tagged elements. It is called recursively.
my ($glade, $scratch) = @_;
my $rule_id = $glade->rule_id();
my $symbol_id = $glade->symbol_id();
my $symbol_name = $panda_grammar->symbol_name($symbol_id);
# A token is a single choice, and we know enough to fully Penn-tag it
if ( not defined $rule_id ) {
my $literal = $glade->literal();
my $penn_tag = penn_tag($symbol_name);
return "($penn_tag $literal)";
}
my $length = $glade->rh_length();
my @return_value = map { $glade->rh_value($_) } 0 .. $length - 1;
# Special case for the start rule
return (join q{ }, @return_value) . "\n" if $symbol_name eq '[:start]' ;
my $join_ws = q{ };
$join_ws = qq{\n } if $symbol_name eq 'S';
my $penn_tag = penn_tag($symbol_name);
return "($penn_tag " . ( join $join_ws, @return_value ) . ')';
}
Here is the ``pruned'' output:
(S (NP (DT a) (NN panda))
(VP (VBZ eats) (NP (NNS shoots) (CC and) (NNS leaves)))
(. .))
THIS INTERFACE is ALPHA and EXPERIMENTAL
The interface described in this document is very much a work in progress. It is alpha and experimental --- subject to radical change without notice.About this document
This document describes the abstract syntax forests (ASF's) of Marpa's SLIF interface. An ASF is an efficient and practical way to represent multiple abstract syntax trees (AST's).Constructor
new()
my $asf = Marpa::R2::ASF->new( { slr => $slr } );
die 'No ASF' if not defined $asf;
Creates a new ASF object. Must be called with a list of one or more hashes of named arguments. Currently only one named argument is allowed, the "slr" argument, and that argument is required. The value of the "slr" argument must be a SLIF recognizer object.
Returns the new ASF object, or "undef" if there was a problem.
Accessor
grammar()
my $grammar = $asf->grammar();
Returns the SLIF grammar associated with the ASF. This can be convenient when using SLIF grammar methods while examining an ASF. All failures are thrown as exceptions.
The traverser method
traverse()
my $full_result = $asf->traverse( {}, \&full_traverser );
Performs a traversal of the ASF. Returns the value of the traversal, which is computed as described below. It requires two arguments. The first is a per-traversal object, which must be a Perl reference. The second argument must be a reference to a traverser function, Discussion of how to write a traverser follows. The "traverse()" method may be called repeatedly for an ASF, with the same traverser, or with different ones.
How to write a traverser
The process of writing a traverser will be familiar if you have experience with traversing trees. The traverser may be called at every node of the forest. (These nodes are called glades.) The traverser must return a value, which may not be an "undef". The value returned by the traverser becomes the value of the glade. The value of the topmost glade (called the peak) becomes the value of the traversal, and will be the value returned by the "traverse()" method.The traverser is called at most once for each glade --- subsequent attempts to determine the value of a glade will return a memoized value. The traverser is always invoked for the peak, and for any glade whose value is required. It may or may not be invoked for other glades.
The traverser is always invoked with two arguments. The first argument will be a glade object. Methods of the glade object are used to find information about the glade, and to move around in it.
The second of the two arguments to a traverser is the per-traversal object, which will be shared by all calls in the traversal. It may be used as a ``scratch pad'' for information which it is not convenient to pass via return values, as a means of avoiding the use of globals.
``Moving around'' in a glade means visiting its parse alternatives. (Parse alternatives are usually called alternatives, when the meaning is clear.) If a glade has exactly one alternative, it is called a trivial glade. When invoked, the traverser points at the first alternative. Alternatives after the first may be visited using the the "next()" glade method.
Parse alternatives may be either token alternatives or rule alternatives. Whether or not the current alternative of the glade is a rule can be determined using the the "rule_id()" glade method, which returns "undef" if and only if the glade is positioned at a token alternative.
As a special case, a glade representing a nulled symbol is always a trivial glade, containing only one token alternative. This means that a nulled symbol is always treated as a token in this context, even when it actually is the LHS symbol of a nulled rule.
At all alternatives, the "span()" and the "literal()" glade methods are of use. The "symbol_id()" glade method is also always of use although its meaning varies. At token alteratives, the "symbol_id()" method returns the token symbol. At rule alteratives, the "symbol_id()" method returns the ID of the LHS of the rule.
At rule alternatives, the "rh_length()" and the "rh_value()" glade methods are of use. The "rh_length()" method returns the length of the RHS, and the "rh_value()" method returns the value of one of the RHS children, as determined using its traverser.
At the peak of the ASF, the symbol will be named '"[:start]"'. This case often requires special treatment. Note that it is entirely possible for the peak glade to be non-trivial.
Glade methods
These are methods of the glade object. Glade objects are passed as arguments to the traversal routine, and are only valid within its scope.literal()
my $literal = $glade->literal();
Returns the glade literal, a string in the input which corresponds to this glade. The glade literal remains constant inside a glade. The "literal()" method accepts no arguments.
span()
my ( $start, $length ) = $glade->span(); my $end = $start + $length - 1;
Returns the glade span, two numbers which describe the location which corresponds to this glade. The first number will be the start of the span, as an offset in the input stream. The second number will be its length. The glade span remains constant within a glade. The "span()" method accepts no arguments.
Then ``end'' character of the span, when defined, may be calculated as its start plus its length, minus one. Applications should note that glades representing nulled symbols are special cases. They will have a length of zero and, properly speaking, their literals are zero length and do not have defined first (start) and last (end) characters.
symbol_id()
my $symbol_id = $glade->symbol_id();
Returns the glade symbol, which remains constant inside a glade. For a token alternative, the glade symbol is the token symbol. For a rule alternative, the glade symbol is the LHS symbol of the rule. The symbol ID remains constant within a glade. The "symbol_id()" method accepts no arguments.
rule_id()
my $rule_id = $glade->rule_id();
Returns the ID of the rule for the current alternative. The ID will be non-negative, but it may be zero. Returns "undef" if and only if the current alternative is a token alternative. The "rule_id()" method accepts no arguments.
rh_length()
my $length = $glade->rh_length();
Returns the number of RHS children of the current rule. On success, this will always be an integer greater than zero. The "rh_length()" method accepts no arguments. It is a fatal error to call "rh_length()" for a glade that currently points to a token alternative.
rh_value()
my $child_value = $glade->rh_value($rh_ix);
Requires exactly one argument, $rh_ix, which must be the zero-based index of a RHS child of the current rule instance. Returns the value of the $rh_ix'th child of the current rule instance. For convenient iteration, returns "undef" if the value of the $rh_ix is greater than or equal to the RHS length. It is a fatal error to call "rh_value()" for a glade that currently points to a token alternative.
next()
last CHOICE if not defined $glade->next();
Points the glade at the next alternative. If there is no next alternative, returns "undef". On success, returns a defined value. One of the values returned on success may be the integer zero, so applications checking for failure should be careful to check for a Perl defined value, and not for a Perl true value.
In addition, because the "rule_id()" method remains constant only within a symch, and the "next()" method may change the current symch, "rule_id()" method must always be called to obtain the current rule ID in a "while" loop where "next()" method is used as the exit condition.
Details
This section contains additional explanations, not essential to understanding the topic of this document. Often they are formal or mathematical. Some people find these helpful, but others do not, which is why they are segregated here.Symches and factorings
Symch and factoring are terms which are useful for some advanced applications. For the purposes of this document, the reader can consider the term ``factoring'' as a synonym for ``parse alternative''. A symch is either a rule symch or a token alternative. A rule symch is a series of rule alternatives (factorings) which share the same rule ID and the same glade. A glade's token alternative is a symch all by itself. The term symch is shorthand for ``symbolic choice''.For each glade accessor, its value can be classified as
- remaining constant inside a glade;
- remaining constant within a symch; or
- potentially varying with each factoring.
The values of the "literal()", "span()", and "symbol_id()" methods remain constant inside each glade. The "rule_id()" method remains constant within a symch --- in fact, the rule ID and the glade define a symch. (Recall that for this purpose, the token alternative's "undef" is considered a rule ID.) The values of the "rh_length()" method and the values of the "rh_value()" method method may vary with each alternative (factoring).
When moving through a glade using the "next()" method, alternatives within the same symch are visited as a group. More precisely, let the ``current rule ID'' be defined as the rule ID of the alternative at which the glade is currently pointing. The "next()" glade method guarantees that, before any alternative with a rule ID different from the current rule ID is visited, all of the so-far-unvisited alternatives that share the current rule ID will be visited.
Appendix: full traverser code
sub full_traverser {
# This routine converts the glade into a list of Penn-tagged elements. It is called recursively.
my ($glade, $scratch) = @_;
my $rule_id = $glade->rule_id();
my $symbol_id = $glade->symbol_id();
my $symbol_name = $panda_grammar->symbol_name($symbol_id);
# A token is a single choice, and we know enough to fully Penn-tag it
if ( not defined $rule_id ) {
my $literal = $glade->literal();
my $penn_tag = penn_tag($symbol_name);
return ["($penn_tag $literal)"];
} ## end if ( not defined $rule_id )
# Our result will be a list of choices
my @return_value = ();
CHOICE: while (1) {
# The results at each position are a list of choices, so
# to produce a new result list, we need to take a Cartesian
# product of all the choices
my $length = $glade->rh_length();
my @results = ( [] );
for my $rh_ix ( 0 .. $length - 1 ) {
my @new_results = ();
for my $old_result (@results) {
my $child_value = $glade->rh_value($rh_ix);
for my $new_value ( @{ $child_value } ) {
push @new_results, [ @{$old_result}, $new_value ];
}
}
@results = @new_results;
} ## end for my $rh_ix ( 0 .. $length - 1 )
# Special case for the start rule
if ( $symbol_name eq '[:start]' ) {
return [ map { join q{}, @{$_} } @results ];
}
# Now we have a list of choices, as a list of lists. Each sub list
# is a list of Penn-tagged elements, which we need to join into
# a single Penn-tagged element. The result will be to collapse
# one level of lists, and leave us with a list of Penn-tagged
# elements
my $join_ws = q{ };
$join_ws = qq{\n } if $symbol_name eq 'S';
push @return_value,
map { '(' . penn_tag($symbol_name) . q{ } . ( join $join_ws, @{$_} ) . ')' }
@results;
# Look at the next alternative in this glade, or end the
# loop if there is none
last CHOICE if not defined $glade->next();
} ## end CHOICE: while (1)
# Return the list of Penn-tagged elements for this glade
return \@return_value;
} ## end sub full_traverser
Copyright and License
Copyright 2014 Jeffrey Kegler This file is part of Marpa::R2. Marpa::R2 is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. Marpa::R2 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. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with Marpa::R2. If not, see http://www.gnu.org/licenses/.