path: root/abdl/predicates.py

# This file is part of A Boneless Datastructure Language
# Copyright (C) 2020  Soni L.
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

"""Classes for use with ABDL's Predicate system.

See ``abdl.predicates.Predicate`` and the language reference for details.
"""

# pylint: disable=too-few-public-methods

class Predicate:
    """A predicate checks if an object is accepted in an ABDL expression.
    """

    def accept(self, obj):
        """Checks if ``obj`` is accepted by this predicate.

        Args:
            obj: The object to be accepted.

        Returns:
            bool: True if the object is accepted, False otherwise.
        """
        raise NotImplementedError

class IsInstance(Predicate):
    """A ``Predicate`` that accepts objects according to ``isinstance``.

    Used to implement ``:$foo`` when ``defs['foo']`` is neither a ``Predicate`` nor a
    ``tuple``.

    One generally does not need to explicitly create instances of this class, as abdl
    accepts types wherever instances of this class are accepted.

    Args:
        ty (type): The expected type.
    """

    def __init__(self, type_):
        self.type_ = type_
        """The expected type."""

    def accept(self, obj):
        return isinstance(obj, self.type_)

class Union(Predicate):
    """A ``Predicate`` built from multiple predicates.

    Used to implement ``:$foo`` when ``defs['foo']`` is not a ``Predicate`` but is a
    ``tuple``. Can be constructed from any iterable. If the iterable yields different
    results on different runs (e.g. is a generator), the behaviour is undefined.

    Has semantics equivalent to ``any(p.accept(obj) for p in predicates)``, except p
    can also be a type or a tuple.

    One generally does not need to explicitly create instances of this class, as abdl
    accepts tuples wherever instances of this class are accepted.

    Args:
        predicates: An iterable of predicates, types and tuples that yields the same
            sequence on different iterations.
    """

    def __init__(self, predicates):
        self.predicates = predicates
        """The iterable of predicates, types and tuples."""

    def accept(self, obj):
        for predicate in self.predicates:
            if _to_predicate(predicate).accept(obj):
                return True
        return False

def _to_predicate(obj):
    if isinstance(obj, Predicate):
        return obj
    if isinstance(obj, tuple):
        return Union(obj)
    # I don't know if anyone relies on the old behaviour of passing the thing directly to isinstance
    # but this lets the exceptions be raised almost exactly like before
    return IsInstance(obj)