Lingua::Treebank::Const - Object modeling constituent from a treebank


  use Lingua::Treebank::Const;

  my $text = <<EOTREE
    (NP-SBJ (DT this) )
    (VP (VBZ is)
      (NP-PRD (NNP Lisa) ))
    (. .) )

  my $utt = Lingua::Treebank::Const->new->from_penn_string($text)

  print $utt->as_penn_text(), "\n";;


          (DT this))
          (VBZ is)
              (NNP Lisa) ))
      (. .))

This is configurable (TO DO: document how so).


  module defines methods for accessing syntactic constituents; it
  identifies its parents and its children, and can write itself out in
  a variety of formats (currently Penn treebank style).


Module for describing simple constituents of the Penn Treebank. Recursive behaviors are implied.

Note assumption that terminal nodes (those with defined word values) will not have children, and vice versa. This assumption is currently unchecked by the code.

For a number of these methods, the jargonish notion of domination plays a large role, so for those who might not know:

a node A dominates another node B if B is a descendant of A.

Class methods


Constructs a new (uninitialized) token. If starting from text, can be used together with the from_penn_string initialization method, as below:

  my $text = <<EOTREE
    (NP-SBJ (DT this) )
    (VP (VBZ is)
      (NP-PRD (NNP Lisa) ))
    (. .) )

  my $utt = Lingua::Treebank::Const->new->from_penn_string($text)

Otherwise, resulting new unit will have no values (parent, children, tag or word set by default.

Instance methods

creation methods

These methods help to populate the fields of these objects from external data.


given a string of the Penn format, e.g.,

    (NP-SBJ (DT this) )
    (VP (VBZ is)
      (NP-PRD (NNP Lisa) ))
    (. .) )

populates the current node with tag S and the children field with new objects (tag NP, tag VP, and tag .). This method recurses on new and from_penn_string to do its job.

simple attributes


Records the tag of this constituent (0-domination). (This is the part of the constituent-label before the hyphen -- after the hyphen is the annot, not the tag).

TO DO: example here.


Returns whatever comes after the hyphen in the constituent label.


If this constituent is terminal, then word should contain the lexical item that is represented.


A string containing the word values of the terminal nodes donminated by this constituent. For example, calling text on a node created from the Penn text given in the description of the new function returns the string "this is lisa .".


Returns the parent of the current node.


Returns a reference to an array of Lingua::Treebank::Const objects that are the children of the current node.

Currently does not check whether word is populated.

methods about parentage

These methods ask questions about the dominating ancestors and direct children of the current node. Think of them as navigating up-and-down the tree.


No arguments

Returns whether self is a leaf. Does not check whether children are populated; if automatically generated from the from_penn_string method then this will always be correct.


No arguments. Boolean. Returns whether the instance is a root node (has no parents).


No arguments.

Returns the root node for the instance in question (might be itself)


Takes an ancestor node as argument.

Returns a list of all the nodes (distal first) between the instance and the root.

Returns undefined and carps when the given node is not an ancestor of the instance.


Takes presumed ancestor as argument.

Returns whether the ancestor is indeed an ancestor of the current instance.


Takes presumed descendant as argument.

Returns whether current instance is an ancestor of the presumed descendant.


Takes presumed sibling as argument.

Returns whether current instance shares an immediate parent with the presumed sibling.


No arguments.

Returns the farthest distance from the current node to a terminal node.


No arguments.

Returns the distance from the instance to the root.


what's the distance from the current node up to the node given as argument? (return undefined if the node given is not the ancestor of the instance node)

methods about siblings

These methods ask questions about siblings, and left-right movement in the tree. Think of them as moving left-and-right around in the tree.


One argument (daughter).

Returns the index of the daughter in the instance's children list. Zero-based, of course.


No arguments. Returns next (or previous) sibling at the same level (dependent on the same parent), or the empty list if no such leaf exists.


No arguments. Returns the leaf just before (or after) any of the leaves of this node, or the empty list if no such leaf exists.


No arguments. Returns leftmost (rightmost) leaf dominated by the instance.


No arguments. Returns left-to-right list of all terminal nodes at or below the current node.


One argument: a presumed cousin.

returns the lowest ancestor the instance and the cousin share (or undefined if they do not share an ancestor)


Both these methods take a subroutine as an argument and return those [child/ancestor] nodes that return true when the sub is called with the node as an argument.

The expectation is that the sub will not modify the node.

methods about structural comparison

These methods are ways of exposing and comparing regions of local structure.


Tests whether the argument has the same structure (and words) as the instance. These need not be the same object, just the same tags and words in the same structure.


Handy -- and unimplemented -- shorthands for checking certain kinds of matching structure.

methods about tree structure modification


Argument is DAUGHTER node.

Removes the DAUGHTER from the children list of the current instance. DAUGHTER node will still be a valid node, but it will no longer have a parent; it will be a root.

Note that detach may leave a degenerate tree: it may have no terminal node (one with words) at the end of a branch. To avoid this, use wither instead.


No arguments.

Detaches self from parent. self will become an independent root. If the parent has no other children, will recursively call parent-wither>, making a possibly zero-length list of degenerate roots above it until an ancestor has a different child than the one in this line of descent.

         A                   A
        / \          C   B   |
       B   X        / \      X
      /     \   => D   E     |
     C       Y               Y
    / \
   D   E
           Before    After

        calling C->wither()

Arguments are a LIST of new daughters to be inserted at the beginning/end of the children list.


Arguments are (DAUGHTER, LIST). Replaces DAUGHTER with the elements of LIST in the children of the current instance.

DAUGHTER is now its own root; see detach.


pull up all terminals to be children of the instance, regardless of how deep they are. Re-attach them to the current node, preserving leaf order.


       /        /
      A   ==>  A__
     / \      /|\ \
    X   B    C F D G
   /|\   \
  C F D   E

pulls in and removes one non-terminal node (which node is specified by argument), attaching its children directly to the current node, retaining what surface order the children originally had, e.g.:


       /        /
      A   ==>  A
     / \      /|\
    X   B    C D B
   / \   \    / \ \
  C   D   E  F   G E
     / \
    F   G

Argument is INDEX. Removes the daughter at INDEX. Will carp if there is no daughter at INDEX.

The daughter at INDEX remains well-formed, though if you do not maintain your own pointer to it, it will probably be collected by the garbage collector.


Arguments are INDEX, LIST of daughters. LIST daughters will be inserted beginning at position INDEX of the current instances children.

utility methods

These methods are methods that may (or not) be useful in programming with these objects. These methods are used internally, but are exposed for the programmer who might need them.

stringify overloading is certainly helpful in debugging, since the perl debugger representation of these objects is complicated by their up-reference to parents.


Returns a text string representing this constituent.

To do: document additional parameters to this, and the possible effects of changing them


This is the method called by default when the object handle is used in a string (see perldoc overload).

Depending on the value of $Lingua::Treebank::Const::STRINGIFY (see below), the string representation of the object varies. The default behavior is as_penn_text, above.

Note that like any object-ref, copying its stringification does NOT work to retain all its behaviors. Nor does an identical string representation necessarily mean the two objects are the same object; merely, that they have the same structure. (see equiv_to).


This is the mthod called by default when the object handle is used in a numeric context (usually == or !=).

Returns an integer representing the unique object. Identity on this method does indicate identity of the objects.

Rarely used in client code. The numeric inequality operators are unlikely to have any useful meaning on these objects, though they should behave consistently (you should get consistent answers given any two objects, regardless of methods called on those objects).

power user methods

walk ( &action, &stop_crit, $state, $bf_traversal )

An instance method. &action argument is required, others are optional.

Calls &action (a subroutine ref) as a method on node and its children, recursively, passing the node under consideration and the $state value (if provided).

If &stop_crit is defined, calls it on each node; when &stop_crit returns true, children of that node are not pursued.

For both action and stop_crit commands, if a string is passed, it will be called if a method by that name can be found in the object.

$state is passed into each of the child method calls. This is convenient for things like pushing interesting elements onto a list, or updating a counter. It must be a scalar, but can be a reference.

Passing a true value as $bf_traversal tells walk() to explore the tree breadth-first rather than depth-first. passing a false (but defined) value forces depth-first. Undefined values default to the value of $Lingua::Treebank::Const::BF_TRAVERSAL, which is undef (false) -- and thus depth-first by default.

  # find out how many children each NP has, but don't count anything
  # inside an EDITED node
  my $action = sub {
      my ($self, $state) = @_;
      return unless $self->tag() eq 'NP';

      # just print it
      print scalar @{$self->children}, "\n";

      # or store it in the state variable
      push @{$state}, scalar @{$self->children()};

  my $stop_crit = sub {$_[0]->tag() eq 'EDITED'};

  $tree->walk( $action, $stop_crit, \@counts );

  use List::Util 'sum';
  print "there were ", sum (@counts),
        " total children of NP nodes\n";

Class variables


Defaults to undefined. If true, changes the default behavior of the walk() method to be breadth-first rather than depth-first.


Changes the default stringification of the objects. Can be set to any of the following three values:


default value.


      (NNP Joe)
      (VB likes)
        (NNP Bach)
    (. .)


  Joe likes Bach .



To Do

check that destroy doesn't leak (undo parent links?)

dump as latex tree

read in other treebank formats (latex trees?)


None by default.



Original version; created by h2xs 1.22 with options

Improved comparison code by caching numerify results.

Should give minor speed improvements for data that works with the same tree over more than one operation. Little if any degradation (tiny increase in size) for those who only use each tree once.

Improved documentation.

Now lists all instance methods. Instance method documentation also organized better -- now falls into categories.

new interface variable

added $VERBOSE variable for suppressing non-fatal errors.

improved parsing

now copes with examples like (e.g.):

  ((FRAG (FOO bar))

critically, earlier versions failed when the tag was empty and not followed by whitespace

added new methods
new interface variable

added $BF_TRAVERSAL for changing walk() method defaults

added new methods

TODO: document these, add test cases, update version number


now with new ignore feature!


Version number jump to keep up with Lingua::Treebank


Documentation for Penn treebank


Jeremy Gillmor Kahn, <>


Copyright 2003 by Jeremy Gillmor Kahn

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.