360 lines
10 KiB
Python
360 lines
10 KiB
Python
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# Natural Language Toolkit: CCG Categories
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#
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# Copyright (C) 2001-2018 NLTK Project
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# Author: Graeme Gange <ggange@csse.unimelb.edu.au>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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from __future__ import unicode_literals
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from functools import total_ordering
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from abc import ABCMeta, abstractmethod
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from six import add_metaclass
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from nltk.internals import raise_unorderable_types
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from nltk.compat import (python_2_unicode_compatible, unicode_repr)
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@add_metaclass(ABCMeta)
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@total_ordering
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class AbstractCCGCategory(object):
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'''
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Interface for categories in combinatory grammars.
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'''
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@abstractmethod
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def is_primitive(self):
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"""
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Returns true if the category is primitive.
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"""
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@abstractmethod
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def is_function(self):
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"""
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Returns true if the category is a function application.
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"""
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@abstractmethod
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def is_var(self):
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"""
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Returns true if the category is a variable.
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"""
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@abstractmethod
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def substitute(self, substitutions):
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"""
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Takes a set of (var, category) substitutions, and replaces every
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occurrence of the variable with the corresponding category.
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"""
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@abstractmethod
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def can_unify(self, other):
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"""
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Determines whether two categories can be unified.
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- Returns None if they cannot be unified
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- Returns a list of necessary substitutions if they can.
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"""
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# Utility functions: comparison, strings and hashing.
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@abstractmethod
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def __str__(self):
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pass
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def __eq__(self, other):
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return (self.__class__ is other.__class__ and
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self._comparison_key == other._comparison_key)
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def __ne__(self, other):
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return not self == other
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def __lt__(self, other):
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if not isinstance(other, AbstractCCGCategory):
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raise_unorderable_types("<", self, other)
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if self.__class__ is other.__class__:
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return self._comparison_key < other._comparison_key
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else:
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return self.__class__.__name__ < other.__class__.__name__
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def __hash__(self):
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try:
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return self._hash
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except AttributeError:
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self._hash = hash(self._comparison_key)
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return self._hash
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@python_2_unicode_compatible
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class CCGVar(AbstractCCGCategory):
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'''
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Class representing a variable CCG category.
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Used for conjunctions (and possibly type-raising, if implemented as a
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unary rule).
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'''
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_maxID = 0
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def __init__(self, prim_only=False):
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"""Initialize a variable (selects a new identifier)
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:param prim_only: a boolean that determines whether the variable is
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restricted to primitives
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:type prim_only: bool
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"""
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self._id = self.new_id()
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self._prim_only = prim_only
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self._comparison_key = self._id
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@classmethod
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def new_id(cls):
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"""
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A class method allowing generation of unique variable identifiers.
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"""
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cls._maxID = cls._maxID + 1
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return cls._maxID - 1
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@classmethod
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def reset_id(cls):
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cls._maxID = 0
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def is_primitive(self):
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return False
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def is_function(self):
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return False
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def is_var(self):
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return True
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def substitute(self, substitutions):
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"""If there is a substitution corresponding to this variable,
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return the substituted category.
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"""
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for (var, cat) in substitutions:
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if var == self:
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return cat
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return self
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def can_unify(self, other):
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""" If the variable can be replaced with other
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a substitution is returned.
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"""
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if other.is_primitive() or not self._prim_only:
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return [(self, other)]
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return None
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def id(self):
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return self._id
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def __str__(self):
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return "_var" + str(self._id)
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@total_ordering
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@python_2_unicode_compatible
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class Direction(object):
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'''
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Class representing the direction of a function application.
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Also contains maintains information as to which combinators
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may be used with the category.
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'''
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def __init__(self, dir, restrictions):
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self._dir = dir
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self._restrs = restrictions
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self._comparison_key = (dir, tuple(restrictions))
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# Testing the application direction
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def is_forward(self):
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return self._dir == '/'
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def is_backward(self):
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return self._dir == '\\'
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def dir(self):
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return self._dir
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def restrs(self):
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"""A list of restrictions on the combinators.
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'.' denotes that permuting operations are disallowed
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',' denotes that function composition is disallowed
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'_' denotes that the direction has variable restrictions.
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(This is redundant in the current implementation of type-raising)
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"""
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return self._restrs
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def is_variable(self):
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return self._restrs == '_'
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# Unification and substitution of variable directions.
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# Used only if type-raising is implemented as a unary rule, as it
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# must inherit restrictions from the argument category.
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def can_unify(self, other):
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if other.is_variable():
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return [('_', self.restrs())]
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elif self.is_variable():
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return [('_', other.restrs())]
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else:
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if self.restrs() == other.restrs():
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return []
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return None
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def substitute(self, subs):
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if not self.is_variable():
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return self
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for (var, restrs) in subs:
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if var == '_':
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return Direction(self._dir, restrs)
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return self
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# Testing permitted combinators
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def can_compose(self):
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return (',' not in self._restrs)
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def can_cross(self):
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return ('.' not in self._restrs)
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def __eq__(self, other):
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return (self.__class__ is other.__class__ and
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self._comparison_key == other._comparison_key)
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def __ne__(self, other):
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return not self == other
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def __lt__(self, other):
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if not isinstance(other, Direction):
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raise_unorderable_types("<", self, other)
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if self.__class__ is other.__class__:
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return self._comparison_key < other._comparison_key
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else:
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return self.__class__.__name__ < other.__class__.__name__
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def __hash__(self):
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try:
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return self._hash
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except AttributeError:
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self._hash = hash(self._comparison_key)
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return self._hash
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def __str__(self):
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r_str = ""
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for r in self._restrs:
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r_str = r_str + "%s" % r
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return "%s%s" % (self._dir, r_str)
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# The negation operator reverses the direction of the application
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def __neg__(self):
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if self._dir == '/':
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return Direction('\\', self._restrs)
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else:
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return Direction('/', self._restrs)
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@python_2_unicode_compatible
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class PrimitiveCategory(AbstractCCGCategory):
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'''
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Class representing primitive categories.
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Takes a string representation of the category, and a
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list of strings specifying the morphological subcategories.
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'''
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def __init__(self, categ, restrictions=[]):
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self._categ = categ
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self._restrs = restrictions
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self._comparison_key = (categ, tuple(restrictions))
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def is_primitive(self):
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return True
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def is_function(self):
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return False
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def is_var(self):
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return False
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def restrs(self):
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return self._restrs
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def categ(self):
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return self._categ
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# Substitution does nothing to a primitive category
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def substitute(self, subs):
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return self
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# A primitive can be unified with a class of the same
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# base category, given that the other category shares all
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# of its subclasses, or with a variable.
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def can_unify(self, other):
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if not other.is_primitive():
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return None
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if other.is_var():
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return [(other, self)]
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if other.categ() == self.categ():
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for restr in self._restrs:
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if restr not in other.restrs():
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return None
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return []
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return None
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def __str__(self):
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if self._restrs == []:
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return "%s" % self._categ
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restrictions = "[%s]" % ",".join(unicode_repr(r) for r in self._restrs)
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return "%s%s" % (self._categ, restrictions)
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@python_2_unicode_compatible
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class FunctionalCategory(AbstractCCGCategory):
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'''
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Class that represents a function application category.
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Consists of argument and result categories, together with
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an application direction.
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'''
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def __init__(self, res, arg, dir):
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self._res = res
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self._arg = arg
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self._dir = dir
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self._comparison_key = (arg, dir, res)
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def is_primitive(self):
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return False
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def is_function(self):
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return True
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def is_var(self):
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return False
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# Substitution returns the category consisting of the
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# substitution applied to each of its constituents.
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def substitute(self, subs):
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sub_res = self._res.substitute(subs)
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sub_dir = self._dir.substitute(subs)
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sub_arg = self._arg.substitute(subs)
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return FunctionalCategory(sub_res, sub_arg, self._dir)
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# A function can unify with another function, so long as its
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# constituents can unify, or with an unrestricted variable.
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def can_unify(self, other):
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if other.is_var():
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return [(other, self)]
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if other.is_function():
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sa = self._res.can_unify(other.res())
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sd = self._dir.can_unify(other.dir())
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if sa is not None and sd is not None:
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sb = self._arg.substitute(sa).can_unify(
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other.arg().substitute(sa))
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if sb is not None:
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return sa + sb
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return None
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# Constituent accessors
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def arg(self):
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return self._arg
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def res(self):
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return self._res
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def dir(self):
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return self._dir
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def __str__(self):
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return "(%s%s%s)" % (self._res, self._dir, self._arg)
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