Source code for nltk.inference.tableau

# Natural Language Toolkit: First-Order Tableau Theorem Prover
#
# Copyright (C) 2001-2018 NLTK Project
# Author: Dan Garrette <dhgarrette@gmail.com>
#
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT

"""
Module for a tableau-based First Order theorem prover.
"""
from __future__ import print_function, unicode_literals

from nltk.internals import Counter

from nltk.sem.logic import (VariableExpression, EqualityExpression,
                            ApplicationExpression, Expression,
                            AbstractVariableExpression, AllExpression,
                            NegatedExpression,
                            ExistsExpression, Variable, ImpExpression,
                            AndExpression, unique_variable,
                            LambdaExpression, IffExpression,
                            OrExpression, FunctionVariableExpression)

from nltk.inference.api import Prover, BaseProverCommand

_counter = Counter()

[docs]class ProverParseError(Exception): pass
[docs]class TableauProver(Prover): _assume_false=False def _prove(self, goal=None, assumptions=None, verbose=False): if not assumptions: assumptions = [] result = None try: agenda = Agenda() if goal: agenda.put(-goal) agenda.put_all(assumptions) debugger = Debug(verbose) result = self._attempt_proof(agenda, set(), set(), debugger) except RuntimeError as e: if self._assume_false and str(e).startswith('maximum recursion depth exceeded'): result = False else: if verbose: print(e) else: raise e return (result, '\n'.join(debugger.lines)) def _attempt_proof(self, agenda, accessible_vars, atoms, debug): (current, context), category = agenda.pop_first() #if there's nothing left in the agenda, and we haven't closed the path if not current: debug.line('AGENDA EMPTY') return False proof_method = { Categories.ATOM: self._attempt_proof_atom, Categories.PROP: self._attempt_proof_prop, Categories.N_ATOM: self._attempt_proof_n_atom, Categories.N_PROP: self._attempt_proof_n_prop, Categories.APP: self._attempt_proof_app, Categories.N_APP: self._attempt_proof_n_app, Categories.N_EQ: self._attempt_proof_n_eq, Categories.D_NEG: self._attempt_proof_d_neg, Categories.N_ALL: self._attempt_proof_n_all, Categories.N_EXISTS: self._attempt_proof_n_some, Categories.AND: self._attempt_proof_and, Categories.N_OR: self._attempt_proof_n_or, Categories.N_IMP: self._attempt_proof_n_imp, Categories.OR: self._attempt_proof_or, Categories.IMP: self._attempt_proof_imp, Categories.N_AND: self._attempt_proof_n_and, Categories.IFF: self._attempt_proof_iff, Categories.N_IFF: self._attempt_proof_n_iff, Categories.EQ: self._attempt_proof_eq, Categories.EXISTS: self._attempt_proof_some, Categories.ALL: self._attempt_proof_all, }[category] debug.line((current, context)) return proof_method(current, context, agenda, accessible_vars, atoms, debug) def _attempt_proof_atom(self, current, context, agenda, accessible_vars, atoms, debug): # Check if the branch is closed. Return 'True' if it is if (current, True) in atoms: debug.line('CLOSED', 1) return True if context: if isinstance(context.term, NegatedExpression): current = current.negate() agenda.put(context(current).simplify()) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) else: #mark all AllExpressions as 'not exhausted' into the agenda since we are (potentially) adding new accessible vars agenda.mark_alls_fresh(); return self._attempt_proof(agenda, accessible_vars|set(current.args), atoms|set([(current, False)]), debug+1) def _attempt_proof_n_atom(self, current, context, agenda, accessible_vars, atoms, debug): # Check if the branch is closed. Return 'True' if it is if (current.term, False) in atoms: debug.line('CLOSED', 1) return True if context: if isinstance(context.term, NegatedExpression): current = current.negate() agenda.put(context(current).simplify()) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) else: #mark all AllExpressions as 'not exhausted' into the agenda since we are (potentially) adding new accessible vars agenda.mark_alls_fresh(); return self._attempt_proof(agenda, accessible_vars|set(current.term.args), atoms|set([(current.term, True)]), debug+1) def _attempt_proof_prop(self, current, context, agenda, accessible_vars, atoms, debug): # Check if the branch is closed. Return 'True' if it is if (current, True) in atoms: debug.line('CLOSED', 1) return True #mark all AllExpressions as 'not exhausted' into the agenda since we are (potentially) adding new accessible vars agenda.mark_alls_fresh(); return self._attempt_proof(agenda, accessible_vars, atoms|set([(current, False)]), debug+1) def _attempt_proof_n_prop(self, current, context, agenda, accessible_vars, atoms, debug): # Check if the branch is closed. Return 'True' if it is if (current.term, False) in atoms: debug.line('CLOSED', 1) return True #mark all AllExpressions as 'not exhausted' into the agenda since we are (potentially) adding new accessible vars agenda.mark_alls_fresh(); return self._attempt_proof(agenda, accessible_vars, atoms|set([(current.term, True)]), debug+1) def _attempt_proof_app(self, current, context, agenda, accessible_vars, atoms, debug): f, args = current.uncurry() for i, arg in enumerate(args): if not TableauProver.is_atom(arg): ctx = f nv = Variable('X%s' % _counter.get()) for j,a in enumerate(args): ctx = (ctx(VariableExpression(nv)) if i == j else ctx(a)) if context: ctx = context(ctx).simplify() ctx = LambdaExpression(nv, ctx) agenda.put(arg, ctx) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) raise Exception('If this method is called, there must be a non-atomic argument') def _attempt_proof_n_app(self, current, context, agenda, accessible_vars, atoms, debug): f, args = current.term.uncurry() for i, arg in enumerate(args): if not TableauProver.is_atom(arg): ctx = f nv = Variable('X%s' % _counter.get()) for j,a in enumerate(args): ctx = (ctx(VariableExpression(nv)) if i == j else ctx(a)) if context: #combine new context with existing ctx = context(ctx).simplify() ctx = LambdaExpression(nv, -ctx) agenda.put(-arg, ctx) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) raise Exception('If this method is called, there must be a non-atomic argument') def _attempt_proof_n_eq(self, current, context, agenda, accessible_vars, atoms, debug): ########################################################################### # Since 'current' is of type '~(a=b)', the path is closed if 'a' == 'b' ########################################################################### if current.term.first == current.term.second: debug.line('CLOSED', 1) return True agenda[Categories.N_EQ].add((current,context)) current._exhausted = True return self._attempt_proof(agenda, accessible_vars|set([current.term.first, current.term.second]), atoms, debug+1) def _attempt_proof_d_neg(self, current, context, agenda, accessible_vars, atoms, debug): agenda.put(current.term.term, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_all(self, current, context, agenda, accessible_vars, atoms, debug): agenda[Categories.EXISTS].add((ExistsExpression(current.term.variable, -current.term.term), context)) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_some(self, current, context, agenda, accessible_vars, atoms, debug): agenda[Categories.ALL].add((AllExpression(current.term.variable, -current.term.term), context)) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_and(self, current, context, agenda, accessible_vars, atoms, debug): agenda.put(current.first, context) agenda.put(current.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_or(self, current, context, agenda, accessible_vars, atoms, debug): agenda.put(-current.term.first, context) agenda.put(-current.term.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_imp(self, current, context, agenda, accessible_vars, atoms, debug): agenda.put(current.term.first, context) agenda.put(-current.term.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) def _attempt_proof_or(self, current, context, agenda, accessible_vars, atoms, debug): new_agenda = agenda.clone() agenda.put(current.first, context) new_agenda.put(current.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) and \ self._attempt_proof(new_agenda, accessible_vars, atoms, debug+1) def _attempt_proof_imp(self, current, context, agenda, accessible_vars, atoms, debug): new_agenda = agenda.clone() agenda.put(-current.first, context) new_agenda.put(current.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) and \ self._attempt_proof(new_agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_and(self, current, context, agenda, accessible_vars, atoms, debug): new_agenda = agenda.clone() agenda.put(-current.term.first, context) new_agenda.put(-current.term.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) and \ self._attempt_proof(new_agenda, accessible_vars, atoms, debug+1) def _attempt_proof_iff(self, current, context, agenda, accessible_vars, atoms, debug): new_agenda = agenda.clone() agenda.put(current.first, context) agenda.put(current.second, context) new_agenda.put(-current.first, context) new_agenda.put(-current.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) and \ self._attempt_proof(new_agenda, accessible_vars, atoms, debug+1) def _attempt_proof_n_iff(self, current, context, agenda, accessible_vars, atoms, debug): new_agenda = agenda.clone() agenda.put(current.term.first, context) agenda.put(-current.term.second, context) new_agenda.put(-current.term.first, context) new_agenda.put(current.term.second, context) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) and \ self._attempt_proof(new_agenda, accessible_vars, atoms, debug+1) def _attempt_proof_eq(self, current, context, agenda, accessible_vars, atoms, debug): ######################################################################### # Since 'current' is of the form '(a = b)', replace ALL free instances # of 'a' with 'b' ######################################################################### agenda.put_atoms(atoms) agenda.replace_all(current.first, current.second) accessible_vars.discard(current.first) agenda.mark_neqs_fresh(); return self._attempt_proof(agenda, accessible_vars, set(), debug+1) def _attempt_proof_some(self, current, context, agenda, accessible_vars, atoms, debug): new_unique_variable = VariableExpression(unique_variable()) agenda.put(current.term.replace(current.variable, new_unique_variable), context) agenda.mark_alls_fresh() return self._attempt_proof(agenda, accessible_vars|set([new_unique_variable]), atoms, debug+1) def _attempt_proof_all(self, current, context, agenda, accessible_vars, atoms, debug): try: current._used_vars except AttributeError: current._used_vars = set() #if there are accessible_vars on the path if accessible_vars: # get the set of bound variables that have not be used by this AllExpression bv_available = accessible_vars - current._used_vars if bv_available: variable_to_use = list(bv_available)[0] debug.line('--> Using \'%s\'' % variable_to_use, 2) current._used_vars |= set([variable_to_use]) agenda.put(current.term.replace(current.variable, variable_to_use), context) agenda[Categories.ALL].add((current,context)) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) else: #no more available variables to substitute debug.line('--> Variables Exhausted', 2) current._exhausted = True agenda[Categories.ALL].add((current,context)) return self._attempt_proof(agenda, accessible_vars, atoms, debug+1) else: new_unique_variable = VariableExpression(unique_variable()) debug.line('--> Using \'%s\'' % new_unique_variable, 2) current._used_vars |= set([new_unique_variable]) agenda.put(current.term.replace(current.variable, new_unique_variable), context) agenda[Categories.ALL].add((current,context)) agenda.mark_alls_fresh() return self._attempt_proof(agenda, accessible_vars|set([new_unique_variable]), atoms, debug+1)
[docs] @staticmethod def is_atom(e): if isinstance(e, NegatedExpression): e = e.term if isinstance(e, ApplicationExpression): for arg in e.args: if not TableauProver.is_atom(arg): return False return True elif isinstance(e, AbstractVariableExpression) or \ isinstance(e, LambdaExpression): return True else: return False
[docs]class TableauProverCommand(BaseProverCommand): def __init__(self, goal=None, assumptions=None, prover=None): """ :param goal: Input expression to prove :type goal: sem.Expression :param assumptions: Input expressions to use as assumptions in the proof. :type assumptions: list(sem.Expression) """ if prover is not None: assert isinstance(prover, TableauProver) else: prover = TableauProver() BaseProverCommand.__init__(self, prover, goal, assumptions)
[docs]class Agenda(object): def __init__(self): self.sets = tuple(set() for i in range(21))
[docs] def clone(self): new_agenda = Agenda() set_list = [s.copy() for s in self.sets] new_allExs = set() for allEx,_ in set_list[Categories.ALL]: new_allEx = AllExpression(allEx.variable, allEx.term) try: new_allEx._used_vars = set(used for used in allEx._used_vars) except AttributeError: new_allEx._used_vars = set() new_allExs.add((new_allEx,None)) set_list[Categories.ALL] = new_allExs set_list[Categories.N_EQ] = set((NegatedExpression(n_eq.term),ctx) for (n_eq,ctx) in set_list[Categories.N_EQ]) new_agenda.sets = tuple(set_list) return new_agenda
def __getitem__(self, index): return self.sets[index]
[docs] def put(self, expression, context=None): if isinstance(expression, AllExpression): ex_to_add = AllExpression(expression.variable, expression.term) try: ex_to_add._used_vars = set(used for used in expression._used_vars) except AttributeError: ex_to_add._used_vars = set() else: ex_to_add = expression self.sets[self._categorize_expression(ex_to_add)].add((ex_to_add, context))
[docs] def put_all(self, expressions): for expression in expressions: self.put(expression)
[docs] def put_atoms(self, atoms): for atom, neg in atoms: if neg: self[Categories.N_ATOM].add((-atom,None)) else: self[Categories.ATOM].add((atom,None))
[docs] def pop_first(self): """ Pop the first expression that appears in the agenda """ for i,s in enumerate(self.sets): if s: if i in [Categories.N_EQ, Categories.ALL]: for ex in s: try: if not ex[0]._exhausted: s.remove(ex) return (ex, i) except AttributeError: s.remove(ex) return (ex, i) else: return (s.pop(), i) return ((None, None), None)
[docs] def replace_all(self, old, new): for s in self.sets: for ex,ctx in s: ex.replace(old.variable, new) if ctx is not None: ctx.replace(old.variable, new)
[docs] def mark_alls_fresh(self): for u,_ in self.sets[Categories.ALL]: u._exhausted = False
[docs] def mark_neqs_fresh(self): for neq,_ in self.sets[Categories.N_EQ]: neq._exhausted = False
def _categorize_expression(self, current): if isinstance(current, NegatedExpression): return self._categorize_NegatedExpression(current) elif isinstance(current, FunctionVariableExpression): return Categories.PROP elif TableauProver.is_atom(current): return Categories.ATOM elif isinstance(current, AllExpression): return Categories.ALL elif isinstance(current, AndExpression): return Categories.AND elif isinstance(current, OrExpression): return Categories.OR elif isinstance(current, ImpExpression): return Categories.IMP elif isinstance(current, IffExpression): return Categories.IFF elif isinstance(current, EqualityExpression): return Categories.EQ elif isinstance(current, ExistsExpression): return Categories.EXISTS elif isinstance(current, ApplicationExpression): return Categories.APP else: raise ProverParseError("cannot categorize %s" % \ current.__class__.__name__) def _categorize_NegatedExpression(self, current): negated = current.term if isinstance(negated, NegatedExpression): return Categories.D_NEG elif isinstance(negated, FunctionVariableExpression): return Categories.N_PROP elif TableauProver.is_atom(negated): return Categories.N_ATOM elif isinstance(negated, AllExpression): return Categories.N_ALL elif isinstance(negated, AndExpression): return Categories.N_AND elif isinstance(negated, OrExpression): return Categories.N_OR elif isinstance(negated, ImpExpression): return Categories.N_IMP elif isinstance(negated, IffExpression): return Categories.N_IFF elif isinstance(negated, EqualityExpression): return Categories.N_EQ elif isinstance(negated, ExistsExpression): return Categories.N_EXISTS elif isinstance(negated, ApplicationExpression): return Categories.N_APP else: raise ProverParseError("cannot categorize %s" % \ negated.__class__.__name__)
[docs]class Debug(object): def __init__(self, verbose, indent=0, lines=None): self.verbose = verbose self.indent = indent if not lines: lines = [] self.lines = lines def __add__(self, increment): return Debug(self.verbose, self.indent+1, self.lines)
[docs] def line(self, data, indent=0): if isinstance(data, tuple): ex, ctx = data if ctx: data = '%s, %s' % (ex, ctx) else: data = '%s' % ex if isinstance(ex, AllExpression): try: used_vars = "[%s]" % (",".join("%s" % ve.variable.name for ve in ex._used_vars)) data += ': %s' % used_vars except AttributeError: data += ': []' newline = '%s%s' % (' '*(self.indent+indent), data) self.lines.append(newline) if self.verbose: print(newline)
[docs]class Categories(object): ATOM = 0 PROP = 1 N_ATOM = 2 N_PROP = 3 APP = 4 N_APP = 5 N_EQ = 6 D_NEG = 7 N_ALL = 8 N_EXISTS = 9 AND = 10 N_OR = 11 N_IMP = 12 OR = 13 IMP = 14 N_AND = 15 IFF = 16 N_IFF = 17 EQ = 18 EXISTS = 19 ALL = 20
[docs]def testTableauProver(): tableau_test('P | -P') tableau_test('P & -P') tableau_test('Q', ['P', '(P -> Q)']) tableau_test('man(x)') tableau_test('(man(x) -> man(x))') tableau_test('(man(x) -> --man(x))') tableau_test('-(man(x) and -man(x))') tableau_test('(man(x) or -man(x))') tableau_test('(man(x) -> man(x))') tableau_test('-(man(x) and -man(x))') tableau_test('(man(x) or -man(x))') tableau_test('(man(x) -> man(x))') tableau_test('(man(x) iff man(x))') tableau_test('-(man(x) iff -man(x))') tableau_test('all x.man(x)') tableau_test('all x.all y.((x = y) -> (y = x))') tableau_test('all x.all y.all z.(((x = y) & (y = z)) -> (x = z))') # tableau_test('-all x.some y.F(x,y) & some x.all y.(-F(x,y))') # tableau_test('some x.all y.sees(x,y)') p1 = 'all x.(man(x) -> mortal(x))' p2 = 'man(Socrates)' c = 'mortal(Socrates)' tableau_test(c, [p1, p2]) p1 = 'all x.(man(x) -> walks(x))' p2 = 'man(John)' c = 'some y.walks(y)' tableau_test(c, [p1, p2]) p = '((x = y) & walks(y))' c = 'walks(x)' tableau_test(c, [p]) p = '((x = y) & ((y = z) & (z = w)))' c = '(x = w)' tableau_test(c, [p]) p = 'some e1.some e2.(believe(e1,john,e2) & walk(e2,mary))' c = 'some e0.walk(e0,mary)' tableau_test(c, [p]) c = '(exists x.exists z3.((x = Mary) & ((z3 = John) & sees(z3,x))) <-> exists x.exists z4.((x = John) & ((z4 = Mary) & sees(x,z4))))' tableau_test(c)
# p = 'some e1.some e2.((believe e1 john e2) and (walk e2 mary))' # c = 'some x.some e3.some e4.((believe e3 x e4) and (walk e4 mary))' # tableau_test(c, [p])
[docs]def testHigherOrderTableauProver(): tableau_test('believe(j, -lie(b))', ['believe(j, -lie(b) & -cheat(b))']) tableau_test('believe(j, lie(b) & cheat(b))', ['believe(j, lie(b))']) tableau_test('believe(j, lie(b))', ['lie(b)']) #how do we capture that John believes all things that are true tableau_test('believe(j, know(b, cheat(b)))', ['believe(j, know(b, lie(b)) & know(b, steals(b) & cheat(b)))']) tableau_test('P(Q(y), R(y) & R(z))', ['P(Q(x) & Q(y), R(y) & R(z))']) tableau_test('believe(j, cheat(b) & lie(b))', ['believe(j, lie(b) & cheat(b))']) tableau_test('believe(j, -cheat(b) & -lie(b))', ['believe(j, -lie(b) & -cheat(b))'])
[docs]def tableau_test(c, ps=None, verbose=False): pc = Expression.fromstring(c) pps = ([Expression.fromstring(p) for p in ps] if ps else []) if not ps: ps = [] print('%s |- %s: %s' % (', '.join(ps), pc, TableauProver().prove(pc, pps, verbose=verbose)))
[docs]def demo(): testTableauProver() testHigherOrderTableauProver()
if __name__ == '__main__': demo()