Marpa::R2::ASF - Marpa's abstract syntax forests (ASF's)
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)"; } 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 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).
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.
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.
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.
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 always invoked once for the peak. The traverser is also invoked once for any glade whose value is required. It may or may not be invoked for other glades. The traverser is never invoked twice for the same glade. If more than one attempt to made to retrieve the value of a glade, the traverser will only be invoked for the first one -- all subsequent attempts will return a memoized value.
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. The per-traversal object may be used as a "scratch pad" for information that it is not convenient to pass via return values. Prefer the per-traversal object to 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
next() glade method.
Parse alternatives may be either token alternatives or rule alternatives. Rule and token alternatives may be distnguished with 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 the 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.
These are methods of the glade object. Glade objects are passed as arguments to the traversal routine, and are only valid within its scope.
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.
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.
The "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.
my $symbol_id = $glade->symbol_id();
Returns the glade symbol. 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.
my $rule_id = $glade->rule_id();
Returns the ID of the rule for the current alternative. The ID will be a non-negative number, or
undef. (Note that, for alternatives in this interface, both zero and
undef are considered valid rule IDs.) Returns
undef if and only if the current alternative is a token alternative. The
rule_id() method accepts no arguments.
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.
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 child at index
$rh_ix 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.
my @return_value = $glade->rh_values();
Returns the RHS children of the current rule. The
rh_values() method accepts no arguments. It is a fatal error to call
rh_values() for a glade that currently points to a token alternative.
my @results = $glade->all_choices();
Returns a new result list produced by taking a Cartesian product of the parse results (which are a list of choices) at the current position.
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 using this method to control an interation 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 that is controlled by the
next() method.
This section contains additional explanations, not essential to understanding the rest of this document. Often they are formal or mathematical. While some people find these helpful, others find them distracting, which is why they are segregated here.
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".
The value that each glade accessor returns can be classified as
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 alternatives in the ASF interface,
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.
sub full)"]; } ## 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_travers.