# Natural Language Toolkit: Recursive Descent Parser
#
# Copyright (C) 2001-2023 NLTK Project
# Author: Edward Loper <edloper@gmail.com>
# Steven Bird <stevenbird1@gmail.com>
# URL: <https://www.nltk.org/>
# For license information, see LICENSE.TXT
from nltk.grammar import Nonterminal
from nltk.parse.api import ParserI
from nltk.tree import ImmutableTree, Tree
##//////////////////////////////////////////////////////
## Recursive Descent Parser
##//////////////////////////////////////////////////////
[docs]class RecursiveDescentParser(ParserI):
"""
A simple top-down CFG parser that parses texts by recursively
expanding the fringe of a Tree, and matching it against a
text.
``RecursiveDescentParser`` uses a list of tree locations called a
"frontier" to remember which subtrees have not yet been expanded
and which leaves have not yet been matched against the text. Each
tree location consists of a list of child indices specifying the
path from the root of the tree to a subtree or a leaf; see the
reference documentation for Tree for more information
about tree locations.
When the parser begins parsing a text, it constructs a tree
containing only the start symbol, and a frontier containing the
location of the tree's root node. It then extends the tree to
cover the text, using the following recursive procedure:
- If the frontier is empty, and the text is covered by the tree,
then return the tree as a possible parse.
- If the frontier is empty, and the text is not covered by the
tree, then return no parses.
- If the first element of the frontier is a subtree, then
use CFG productions to "expand" it. For each applicable
production, add the expanded subtree's children to the
frontier, and recursively find all parses that can be
generated by the new tree and frontier.
- If the first element of the frontier is a token, then "match"
it against the next token from the text. Remove the token
from the frontier, and recursively find all parses that can be
generated by the new tree and frontier.
:see: ``nltk.grammar``
"""
[docs] def __init__(self, grammar, trace=0):
"""
Create a new ``RecursiveDescentParser``, that uses ``grammar``
to parse texts.
:type grammar: CFG
:param grammar: The grammar used to parse texts.
:type trace: int
:param trace: The level of tracing that should be used when
parsing a text. ``0`` will generate no tracing output;
and higher numbers will produce more verbose tracing
output.
"""
self._grammar = grammar
self._trace = trace
[docs] def grammar(self):
return self._grammar
[docs] def parse(self, tokens):
# Inherit docs from ParserI
tokens = list(tokens)
self._grammar.check_coverage(tokens)
# Start a recursive descent parse, with an initial tree
# containing just the start symbol.
start = self._grammar.start().symbol()
initial_tree = Tree(start, [])
frontier = [()]
if self._trace:
self._trace_start(initial_tree, frontier, tokens)
return self._parse(tokens, initial_tree, frontier)
def _parse(self, remaining_text, tree, frontier):
"""
Recursively expand and match each elements of ``tree``
specified by ``frontier``, to cover ``remaining_text``. Return
a list of all parses found.
:return: An iterator of all parses that can be generated by
matching and expanding the elements of ``tree``
specified by ``frontier``.
:rtype: iter(Tree)
:type tree: Tree
:param tree: A partial structure for the text that is
currently being parsed. The elements of ``tree``
that are specified by ``frontier`` have not yet been
expanded or matched.
:type remaining_text: list(str)
:param remaining_text: The portion of the text that is not yet
covered by ``tree``.
:type frontier: list(tuple(int))
:param frontier: A list of the locations within ``tree`` of
all subtrees that have not yet been expanded, and all
leaves that have not yet been matched. This list sorted
in left-to-right order of location within the tree.
"""
# If the tree covers the text, and there's nothing left to
# expand, then we've found a complete parse; return it.
if len(remaining_text) == 0 and len(frontier) == 0:
if self._trace:
self._trace_succeed(tree, frontier)
yield tree
# If there's still text, but nothing left to expand, we failed.
elif len(frontier) == 0:
if self._trace:
self._trace_backtrack(tree, frontier)
# If the next element on the frontier is a tree, expand it.
elif isinstance(tree[frontier[0]], Tree):
yield from self._expand(remaining_text, tree, frontier)
# If the next element on the frontier is a token, match it.
else:
yield from self._match(remaining_text, tree, frontier)
def _match(self, rtext, tree, frontier):
"""
:rtype: iter(Tree)
:return: an iterator of all parses that can be generated by
matching the first element of ``frontier`` against the
first token in ``rtext``. In particular, if the first
element of ``frontier`` has the same type as the first
token in ``rtext``, then substitute the token into
``tree``; and return all parses that can be generated by
matching and expanding the remaining elements of
``frontier``. If the first element of ``frontier`` does not
have the same type as the first token in ``rtext``, then
return empty list.
:type tree: Tree
:param tree: A partial structure for the text that is
currently being parsed. The elements of ``tree``
that are specified by ``frontier`` have not yet been
expanded or matched.
:type rtext: list(str)
:param rtext: The portion of the text that is not yet
covered by ``tree``.
:type frontier: list of tuple of int
:param frontier: A list of the locations within ``tree`` of
all subtrees that have not yet been expanded, and all
leaves that have not yet been matched.
"""
tree_leaf = tree[frontier[0]]
if len(rtext) > 0 and tree_leaf == rtext[0]:
# If it's a terminal that matches rtext[0], then substitute
# in the token, and continue parsing.
newtree = tree.copy(deep=True)
newtree[frontier[0]] = rtext[0]
if self._trace:
self._trace_match(newtree, frontier[1:], rtext[0])
yield from self._parse(rtext[1:], newtree, frontier[1:])
else:
# If it's a non-matching terminal, fail.
if self._trace:
self._trace_backtrack(tree, frontier, rtext[:1])
def _expand(self, remaining_text, tree, frontier, production=None):
"""
:rtype: iter(Tree)
:return: An iterator of all parses that can be generated by
expanding the first element of ``frontier`` with
``production``. In particular, if the first element of
``frontier`` is a subtree whose node type is equal to
``production``'s left hand side, then add a child to that
subtree for each element of ``production``'s right hand
side; and return all parses that can be generated by
matching and expanding the remaining elements of
``frontier``. If the first element of ``frontier`` is not a
subtree whose node type is equal to ``production``'s left
hand side, then return an empty list. If ``production`` is
not specified, then return a list of all parses that can
be generated by expanding the first element of ``frontier``
with *any* CFG production.
:type tree: Tree
:param tree: A partial structure for the text that is
currently being parsed. The elements of ``tree``
that are specified by ``frontier`` have not yet been
expanded or matched.
:type remaining_text: list(str)
:param remaining_text: The portion of the text that is not yet
covered by ``tree``.
:type frontier: list(tuple(int))
:param frontier: A list of the locations within ``tree`` of
all subtrees that have not yet been expanded, and all
leaves that have not yet been matched.
"""
if production is None:
productions = self._grammar.productions()
else:
productions = [production]
for production in productions:
lhs = production.lhs().symbol()
if lhs == tree[frontier[0]].label():
subtree = self._production_to_tree(production)
if frontier[0] == ():
newtree = subtree
else:
newtree = tree.copy(deep=True)
newtree[frontier[0]] = subtree
new_frontier = [
frontier[0] + (i,) for i in range(len(production.rhs()))
]
if self._trace:
self._trace_expand(newtree, new_frontier, production)
yield from self._parse(
remaining_text, newtree, new_frontier + frontier[1:]
)
def _production_to_tree(self, production):
"""
:rtype: Tree
:return: The Tree that is licensed by ``production``.
In particular, given the production ``[lhs -> elt[1] ... elt[n]]``
return a tree that has a node ``lhs.symbol``, and
``n`` children. For each nonterminal element
``elt[i]`` in the production, the tree token has a
childless subtree with node value ``elt[i].symbol``; and
for each terminal element ``elt[j]``, the tree token has
a leaf token with type ``elt[j]``.
:param production: The CFG production that licenses the tree
token that should be returned.
:type production: Production
"""
children = []
for elt in production.rhs():
if isinstance(elt, Nonterminal):
children.append(Tree(elt.symbol(), []))
else:
# This will be matched.
children.append(elt)
return Tree(production.lhs().symbol(), children)
[docs] def trace(self, trace=2):
"""
Set the level of tracing output that should be generated when
parsing a text.
:type trace: int
:param trace: The trace level. A trace level of ``0`` will
generate no tracing output; and higher trace levels will
produce more verbose tracing output.
:rtype: None
"""
self._trace = trace
def _trace_fringe(self, tree, treeloc=None):
"""
Print trace output displaying the fringe of ``tree``. The
fringe of ``tree`` consists of all of its leaves and all of
its childless subtrees.
:rtype: None
"""
if treeloc == ():
print("*", end=" ")
if isinstance(tree, Tree):
if len(tree) == 0:
print(repr(Nonterminal(tree.label())), end=" ")
for i in range(len(tree)):
if treeloc is not None and i == treeloc[0]:
self._trace_fringe(tree[i], treeloc[1:])
else:
self._trace_fringe(tree[i])
else:
print(repr(tree), end=" ")
def _trace_tree(self, tree, frontier, operation):
"""
Print trace output displaying the parser's current state.
:param operation: A character identifying the operation that
generated the current state.
:rtype: None
"""
if self._trace == 2:
print(" %c [" % operation, end=" ")
else:
print(" [", end=" ")
if len(frontier) > 0:
self._trace_fringe(tree, frontier[0])
else:
self._trace_fringe(tree)
print("]")
def _trace_start(self, tree, frontier, text):
print("Parsing %r" % " ".join(text))
if self._trace > 2:
print("Start:")
if self._trace > 1:
self._trace_tree(tree, frontier, " ")
def _trace_expand(self, tree, frontier, production):
if self._trace > 2:
print("Expand: %s" % production)
if self._trace > 1:
self._trace_tree(tree, frontier, "E")
def _trace_match(self, tree, frontier, tok):
if self._trace > 2:
print("Match: %r" % tok)
if self._trace > 1:
self._trace_tree(tree, frontier, "M")
def _trace_succeed(self, tree, frontier):
if self._trace > 2:
print("GOOD PARSE:")
if self._trace == 1:
print("Found a parse:\n%s" % tree)
if self._trace > 1:
self._trace_tree(tree, frontier, "+")
def _trace_backtrack(self, tree, frontier, toks=None):
if self._trace > 2:
if toks:
print("Backtrack: %r match failed" % toks[0])
else:
print("Backtrack")
##//////////////////////////////////////////////////////
## Stepping Recursive Descent Parser
##//////////////////////////////////////////////////////
[docs]class SteppingRecursiveDescentParser(RecursiveDescentParser):
"""
A ``RecursiveDescentParser`` that allows you to step through the
parsing process, performing a single operation at a time.
The ``initialize`` method is used to start parsing a text.
``expand`` expands the first element on the frontier using a single
CFG production, and ``match`` matches the first element on the
frontier against the next text token. ``backtrack`` undoes the most
recent expand or match operation. ``step`` performs a single
expand, match, or backtrack operation. ``parses`` returns the set
of parses that have been found by the parser.
:ivar _history: A list of ``(rtext, tree, frontier)`` tripples,
containing the previous states of the parser. This history is
used to implement the ``backtrack`` operation.
:ivar _tried_e: A record of all productions that have been tried
for a given tree. This record is used by ``expand`` to perform
the next untried production.
:ivar _tried_m: A record of what tokens have been matched for a
given tree. This record is used by ``step`` to decide whether
or not to match a token.
:see: ``nltk.grammar``
"""
[docs] def __init__(self, grammar, trace=0):
super().__init__(grammar, trace)
self._rtext = None
self._tree = None
self._frontier = [()]
self._tried_e = {}
self._tried_m = {}
self._history = []
self._parses = []
# [XX] TEMPORARY HACK WARNING! This should be replaced with
# something nicer when we get the chance.
def _freeze(self, tree):
c = tree.copy()
# for pos in c.treepositions('leaves'):
# c[pos] = c[pos].freeze()
return ImmutableTree.convert(c)
[docs] def parse(self, tokens):
tokens = list(tokens)
self.initialize(tokens)
while self.step() is not None:
pass
return self.parses()
[docs] def initialize(self, tokens):
"""
Start parsing a given text. This sets the parser's tree to
the start symbol, its frontier to the root node, and its
remaining text to ``token['SUBTOKENS']``.
"""
self._rtext = tokens
start = self._grammar.start().symbol()
self._tree = Tree(start, [])
self._frontier = [()]
self._tried_e = {}
self._tried_m = {}
self._history = []
self._parses = []
if self._trace:
self._trace_start(self._tree, self._frontier, self._rtext)
[docs] def remaining_text(self):
"""
:return: The portion of the text that is not yet covered by the
tree.
:rtype: list(str)
"""
return self._rtext
[docs] def frontier(self):
"""
:return: A list of the tree locations of all subtrees that
have not yet been expanded, and all leaves that have not
yet been matched.
:rtype: list(tuple(int))
"""
return self._frontier
[docs] def tree(self):
"""
:return: A partial structure for the text that is
currently being parsed. The elements specified by the
frontier have not yet been expanded or matched.
:rtype: Tree
"""
return self._tree
[docs] def step(self):
"""
Perform a single parsing operation. If an untried match is
possible, then perform the match, and return the matched
token. If an untried expansion is possible, then perform the
expansion, and return the production that it is based on. If
backtracking is possible, then backtrack, and return True.
Otherwise, return None.
:return: None if no operation was performed; a token if a match
was performed; a production if an expansion was performed;
and True if a backtrack operation was performed.
:rtype: Production or String or bool
"""
# Try matching (if we haven't already)
if self.untried_match():
token = self.match()
if token is not None:
return token
# Try expanding.
production = self.expand()
if production is not None:
return production
# Try backtracking
if self.backtrack():
self._trace_backtrack(self._tree, self._frontier)
return True
# Nothing left to do.
return None
[docs] def expand(self, production=None):
"""
Expand the first element of the frontier. In particular, if
the first element of the frontier is a subtree whose node type
is equal to ``production``'s left hand side, then add a child
to that subtree for each element of ``production``'s right hand
side. If ``production`` is not specified, then use the first
untried expandable production. If all expandable productions
have been tried, do nothing.
:return: The production used to expand the frontier, if an
expansion was performed. If no expansion was performed,
return None.
:rtype: Production or None
"""
# Make sure we *can* expand.
if len(self._frontier) == 0:
return None
if not isinstance(self._tree[self._frontier[0]], Tree):
return None
# If they didn't specify a production, check all untried ones.
if production is None:
productions = self.untried_expandable_productions()
else:
productions = [production]
parses = []
for prod in productions:
# Record that we've tried this production now.
self._tried_e.setdefault(self._freeze(self._tree), []).append(prod)
# Try expanding.
for _result in self._expand(self._rtext, self._tree, self._frontier, prod):
return prod
# We didn't expand anything.
return None
[docs] def match(self):
"""
Match the first element of the frontier. In particular, if
the first element of the frontier has the same type as the
next text token, then substitute the text token into the tree.
:return: The token matched, if a match operation was
performed. If no match was performed, return None
:rtype: str or None
"""
# Record that we've tried matching this token.
tok = self._rtext[0]
self._tried_m.setdefault(self._freeze(self._tree), []).append(tok)
# Make sure we *can* match.
if len(self._frontier) == 0:
return None
if isinstance(self._tree[self._frontier[0]], Tree):
return None
for _result in self._match(self._rtext, self._tree, self._frontier):
# Return the token we just matched.
return self._history[-1][0][0]
return None
[docs] def backtrack(self):
"""
Return the parser to its state before the most recent
match or expand operation. Calling ``undo`` repeatedly return
the parser to successively earlier states. If no match or
expand operations have been performed, ``undo`` will make no
changes.
:return: true if an operation was successfully undone.
:rtype: bool
"""
if len(self._history) == 0:
return False
(self._rtext, self._tree, self._frontier) = self._history.pop()
return True
[docs] def expandable_productions(self):
"""
:return: A list of all the productions for which expansions
are available for the current parser state.
:rtype: list(Production)
"""
# Make sure we *can* expand.
if len(self._frontier) == 0:
return []
frontier_child = self._tree[self._frontier[0]]
if len(self._frontier) == 0 or not isinstance(frontier_child, Tree):
return []
return [
p
for p in self._grammar.productions()
if p.lhs().symbol() == frontier_child.label()
]
[docs] def untried_expandable_productions(self):
"""
:return: A list of all the untried productions for which
expansions are available for the current parser state.
:rtype: list(Production)
"""
tried_expansions = self._tried_e.get(self._freeze(self._tree), [])
return [p for p in self.expandable_productions() if p not in tried_expansions]
[docs] def untried_match(self):
"""
:return: Whether the first element of the frontier is a token
that has not yet been matched.
:rtype: bool
"""
if len(self._rtext) == 0:
return False
tried_matches = self._tried_m.get(self._freeze(self._tree), [])
return self._rtext[0] not in tried_matches
[docs] def currently_complete(self):
"""
:return: Whether the parser's current state represents a
complete parse.
:rtype: bool
"""
return len(self._frontier) == 0 and len(self._rtext) == 0
def _parse(self, remaining_text, tree, frontier):
"""
A stub version of ``_parse`` that sets the parsers current
state to the given arguments. In ``RecursiveDescentParser``,
the ``_parse`` method is used to recursively continue parsing a
text. ``SteppingRecursiveDescentParser`` overrides it to
capture these recursive calls. It records the parser's old
state in the history (to allow for backtracking), and updates
the parser's new state using the given arguments. Finally, it
returns ``[1]``, which is used by ``match`` and ``expand`` to
detect whether their operations were successful.
:return: ``[1]``
:rtype: list of int
"""
self._history.append((self._rtext, self._tree, self._frontier))
self._rtext = remaining_text
self._tree = tree
self._frontier = frontier
# Is it a good parse? If so, record it.
if len(frontier) == 0 and len(remaining_text) == 0:
self._parses.append(tree)
self._trace_succeed(self._tree, self._frontier)
return [1]
[docs] def parses(self):
"""
:return: An iterator of the parses that have been found by this
parser so far.
:rtype: list of Tree
"""
return iter(self._parses)
[docs] def set_grammar(self, grammar):
"""
Change the grammar used to parse texts.
:param grammar: The new grammar.
:type grammar: CFG
"""
self._grammar = grammar
##//////////////////////////////////////////////////////
## Demonstration Code
##//////////////////////////////////////////////////////
[docs]def demo():
"""
A demonstration of the recursive descent parser.
"""
from nltk import CFG, parse
grammar = CFG.fromstring(
"""
S -> NP VP
NP -> Det N | Det N PP
VP -> V NP | V NP PP
PP -> P NP
NP -> 'I'
N -> 'man' | 'park' | 'telescope' | 'dog'
Det -> 'the' | 'a'
P -> 'in' | 'with'
V -> 'saw'
"""
)
for prod in grammar.productions():
print(prod)
sent = "I saw a man in the park".split()
parser = parse.RecursiveDescentParser(grammar, trace=2)
for p in parser.parse(sent):
print(p)
if __name__ == "__main__":
demo()