# Natural Language Toolkit: Shift-Reduce 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 Tree
##//////////////////////////////////////////////////////
## Shift/Reduce Parser
##//////////////////////////////////////////////////////
[docs]class ShiftReduceParser(ParserI):
"""
A simple bottom-up CFG parser that uses two operations, "shift"
and "reduce", to find a single parse for a text.
``ShiftReduceParser`` maintains a stack, which records the
structure of a portion of the text. This stack is a list of
strings and Trees that collectively cover a portion of
the text. For example, while parsing the sentence "the dog saw
the man" with a typical grammar, ``ShiftReduceParser`` will produce
the following stack, which covers "the dog saw"::
[(NP: (Det: 'the') (N: 'dog')), (V: 'saw')]
``ShiftReduceParser`` attempts to extend the stack to cover the
entire text, and to combine the stack elements into a single tree,
producing a complete parse for the sentence.
Initially, the stack is empty. It is extended to cover the text,
from left to right, by repeatedly applying two operations:
- "shift" moves a token from the beginning of the text to the
end of the stack.
- "reduce" uses a CFG production to combine the rightmost stack
elements into a single Tree.
Often, more than one operation can be performed on a given stack.
In this case, ``ShiftReduceParser`` uses the following heuristics
to decide which operation to perform:
- Only shift if no reductions are available.
- If multiple reductions are available, then apply the reduction
whose CFG production is listed earliest in the grammar.
Note that these heuristics are not guaranteed to choose an
operation that leads to a parse of the text. Also, if multiple
parses exists, ``ShiftReduceParser`` will return at most one of
them.
:see: ``nltk.grammar``
"""
[docs] def __init__(self, grammar, trace=0):
"""
Create a new ``ShiftReduceParser``, that uses ``grammar`` to
parse texts.
:type grammar: Grammar
: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
self._check_grammar()
[docs] def grammar(self):
return self._grammar
[docs] def parse(self, tokens):
tokens = list(tokens)
self._grammar.check_coverage(tokens)
# initialize the stack.
stack = []
remaining_text = tokens
# Trace output.
if self._trace:
print("Parsing %r" % " ".join(tokens))
self._trace_stack(stack, remaining_text)
# iterate through the text, pushing the token onto
# the stack, then reducing the stack.
while len(remaining_text) > 0:
self._shift(stack, remaining_text)
while self._reduce(stack, remaining_text):
pass
# Did we reduce everything?
if len(stack) == 1:
# Did we end up with the right category?
if stack[0].label() == self._grammar.start().symbol():
yield stack[0]
def _shift(self, stack, remaining_text):
"""
Move a token from the beginning of ``remaining_text`` to the
end of ``stack``.
:type stack: list(str and Tree)
:param stack: A list of strings and Trees, encoding
the structure of the text that has been parsed so far.
:type remaining_text: list(str)
:param remaining_text: The portion of the text that is not yet
covered by ``stack``.
:rtype: None
"""
stack.append(remaining_text[0])
remaining_text.remove(remaining_text[0])
if self._trace:
self._trace_shift(stack, remaining_text)
def _match_rhs(self, rhs, rightmost_stack):
"""
:rtype: bool
:return: true if the right hand side of a CFG production
matches the rightmost elements of the stack. ``rhs``
matches ``rightmost_stack`` if they are the same length,
and each element of ``rhs`` matches the corresponding
element of ``rightmost_stack``. A nonterminal element of
``rhs`` matches any Tree whose node value is equal
to the nonterminal's symbol. A terminal element of ``rhs``
matches any string whose type is equal to the terminal.
:type rhs: list(terminal and Nonterminal)
:param rhs: The right hand side of a CFG production.
:type rightmost_stack: list(string and Tree)
:param rightmost_stack: The rightmost elements of the parser's
stack.
"""
if len(rightmost_stack) != len(rhs):
return False
for i in range(len(rightmost_stack)):
if isinstance(rightmost_stack[i], Tree):
if not isinstance(rhs[i], Nonterminal):
return False
if rightmost_stack[i].label() != rhs[i].symbol():
return False
else:
if isinstance(rhs[i], Nonterminal):
return False
if rightmost_stack[i] != rhs[i]:
return False
return True
def _reduce(self, stack, remaining_text, production=None):
"""
Find a CFG production whose right hand side matches the
rightmost stack elements; and combine those stack elements
into a single Tree, with the node specified by the
production's left-hand side. If more than one CFG production
matches the stack, then use the production that is listed
earliest in the grammar. The new Tree replaces the
elements in the stack.
:rtype: Production or None
:return: If a reduction is performed, then return the CFG
production that the reduction is based on; otherwise,
return false.
:type stack: list(string and Tree)
:param stack: A list of strings and Trees, encoding
the structure of the text that has been parsed so far.
:type remaining_text: list(str)
:param remaining_text: The portion of the text that is not yet
covered by ``stack``.
"""
if production is None:
productions = self._grammar.productions()
else:
productions = [production]
# Try each production, in order.
for production in productions:
rhslen = len(production.rhs())
# check if the RHS of a production matches the top of the stack
if self._match_rhs(production.rhs(), stack[-rhslen:]):
# combine the tree to reflect the reduction
tree = Tree(production.lhs().symbol(), stack[-rhslen:])
stack[-rhslen:] = [tree]
# We reduced something
if self._trace:
self._trace_reduce(stack, production, remaining_text)
return production
# We didn't reduce anything
return None
[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
"""
# 1: just show shifts.
# 2: show shifts & reduces
# 3: display which tokens & productions are shifed/reduced
self._trace = trace
def _trace_stack(self, stack, remaining_text, marker=" "):
"""
Print trace output displaying the given stack and text.
:rtype: None
:param marker: A character that is printed to the left of the
stack. This is used with trace level 2 to print 'S'
before shifted stacks and 'R' before reduced stacks.
"""
s = " " + marker + " [ "
for elt in stack:
if isinstance(elt, Tree):
s += repr(Nonterminal(elt.label())) + " "
else:
s += repr(elt) + " "
s += "* " + " ".join(remaining_text) + "]"
print(s)
def _trace_shift(self, stack, remaining_text):
"""
Print trace output displaying that a token has been shifted.
:rtype: None
"""
if self._trace > 2:
print("Shift %r:" % stack[-1])
if self._trace == 2:
self._trace_stack(stack, remaining_text, "S")
elif self._trace > 0:
self._trace_stack(stack, remaining_text)
def _trace_reduce(self, stack, production, remaining_text):
"""
Print trace output displaying that ``production`` was used to
reduce ``stack``.
:rtype: None
"""
if self._trace > 2:
rhs = " ".join(production.rhs())
print(f"Reduce {production.lhs()!r} <- {rhs}")
if self._trace == 2:
self._trace_stack(stack, remaining_text, "R")
elif self._trace > 1:
self._trace_stack(stack, remaining_text)
def _check_grammar(self):
"""
Check to make sure that all of the CFG productions are
potentially useful. If any productions can never be used,
then print a warning.
:rtype: None
"""
productions = self._grammar.productions()
# Any production whose RHS is an extension of another production's RHS
# will never be used.
for i in range(len(productions)):
for j in range(i + 1, len(productions)):
rhs1 = productions[i].rhs()
rhs2 = productions[j].rhs()
if rhs1[: len(rhs2)] == rhs2:
print("Warning: %r will never be used" % productions[i])
##//////////////////////////////////////////////////////
## Stepping Shift/Reduce Parser
##//////////////////////////////////////////////////////
[docs]class SteppingShiftReduceParser(ShiftReduceParser):
"""
A ``ShiftReduceParser`` that allows you to setp through the parsing
process, performing a single operation at a time. It also allows
you to change the parser's grammar midway through parsing a text.
The ``initialize`` method is used to start parsing a text.
``shift`` performs a single shift operation, and ``reduce`` performs
a single reduce operation. ``step`` will perform a single reduce
operation if possible; otherwise, it will perform a single shift
operation. ``parses`` returns the set of parses that have been
found by the parser.
:ivar _history: A list of ``(stack, remaining_text)`` pairs,
containing all of the previous states of the parser. This
history is used to implement the ``undo`` operation.
:see: ``nltk.grammar``
"""
[docs] def __init__(self, grammar, trace=0):
super().__init__(grammar, trace)
self._stack = None
self._remaining_text = None
self._history = []
[docs] def parse(self, tokens):
tokens = list(tokens)
self.initialize(tokens)
while self.step():
pass
return self.parses()
[docs] def stack(self):
"""
:return: The parser's stack.
:rtype: list(str and Tree)
"""
return self._stack
[docs] def remaining_text(self):
"""
:return: The portion of the text that is not yet covered by the
stack.
:rtype: list(str)
"""
return self._remaining_text
[docs] def initialize(self, tokens):
"""
Start parsing a given text. This sets the parser's stack to
``[]`` and sets its remaining text to ``tokens``.
"""
self._stack = []
self._remaining_text = tokens
self._history = []
[docs] def step(self):
"""
Perform a single parsing operation. If a reduction is
possible, then perform that reduction, and return the
production that it is based on. Otherwise, if a shift is
possible, then perform it, and return True. Otherwise,
return False.
:return: False if no operation was performed; True if a shift was
performed; and the CFG production used to reduce if a
reduction was performed.
:rtype: Production or bool
"""
return self.reduce() or self.shift()
[docs] def shift(self):
"""
Move a token from the beginning of the remaining text to the
end of the stack. If there are no more tokens in the
remaining text, then do nothing.
:return: True if the shift operation was successful.
:rtype: bool
"""
if len(self._remaining_text) == 0:
return False
self._history.append((self._stack[:], self._remaining_text[:]))
self._shift(self._stack, self._remaining_text)
return True
[docs] def reduce(self, production=None):
"""
Use ``production`` to combine the rightmost stack elements into
a single Tree. If ``production`` does not match the
rightmost stack elements, then do nothing.
:return: The production used to reduce the stack, if a
reduction was performed. If no reduction was performed,
return None.
:rtype: Production or None
"""
self._history.append((self._stack[:], self._remaining_text[:]))
return_val = self._reduce(self._stack, self._remaining_text, production)
if not return_val:
self._history.pop()
return return_val
[docs] def undo(self):
"""
Return the parser to its state before the most recent
shift or reduce operation. Calling ``undo`` repeatedly return
the parser to successively earlier states. If no shift or
reduce 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._stack, self._remaining_text) = self._history.pop()
return True
[docs] def reducible_productions(self):
"""
:return: A list of the productions for which reductions are
available for the current parser state.
:rtype: list(Production)
"""
productions = []
for production in self._grammar.productions():
rhslen = len(production.rhs())
if self._match_rhs(production.rhs(), self._stack[-rhslen:]):
productions.append(production)
return productions
[docs] def parses(self):
"""
:return: An iterator of the parses that have been found by this
parser so far.
:rtype: iter(Tree)
"""
if (
len(self._remaining_text) == 0
and len(self._stack) == 1
and self._stack[0].label() == self._grammar.start().symbol()
):
yield self._stack[0]
# copied from nltk.parser
[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 shift-reduce 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'
"""
)
sent = "I saw a man in the park".split()
parser = parse.ShiftReduceParser(grammar, trace=2)
for p in parser.parse(sent):
print(p)
if __name__ == "__main__":
demo()