from collections import defaultdict
from typing import Any, Callable, Dict, List, Set, Tuple, Type, Union
import inspect
import logging
import sys
import traceback
import types

from nltk import Tree

from allennlp.common.util import START_SYMBOL

from allennlp_semparse.common import util, ParsingError, ExecutionError

logger = logging.getLogger(__name__)

# We rely heavily on the typing module and its type annotations for our grammar induction code.
# Unfortunately, the behavior of the typing module changed somewhat substantially between python
# 3.6 and 3.7, so we need to do some gymnastics to get some of our checks to work with both.
# That's what these three methods are about.

def is_callable(type_: Type) -> bool:
    if sys.version_info < (3, 7):
        from typing import CallableMeta  # type: ignore

        return isinstance(type_, CallableMeta)  # type: ignore
        return getattr(type_, "_name", None) == "Callable"

def is_generic(type_: Type) -> bool:
    if sys.version_info < (3, 7):
        from typing import GenericMeta  # type: ignore

        return isinstance(type_, GenericMeta)  # type: ignore
        from typing import _GenericAlias  # type: ignore

        return isinstance(type_, _GenericAlias)  # type: ignore

def get_generic_name(type_: Type) -> str:
    if sys.version_info < (3, 7):
        origin = type_.__origin__.__name__
        # In python 3.7, type_.__origin__ switched to the built-in class, instead of the typing
        # class.
        origin = type_._name
    args = type_.__args__
    return f'{origin}[{",".join(arg.__name__ for arg in args)}]'

class PredicateType:
    A base class for `types` in a domain language.  This serves much the same purpose as
    ``typing.Type``, but we add a few conveniences to these types, so we construct separate classes
    for them and group them together under ``PredicateType`` to have a good type annotation for
    these types.

    def get_type(type_: Type) -> "PredicateType":
        Converts a python ``Type`` (as you might get from a type annotation) into a
        ``PredicateType``.  If the ``Type`` is callable, this will return a ``FunctionType``;
        otherwise, it will return a ``BasicType``.

        ``BasicTypes`` have a single ``name`` parameter - we typically get this from
        ``type_.__name__``.  This doesn't work for generic types (like ``List[str]``), so we handle
        those specially, so that the ``name`` for the ``BasicType`` remains ``List[str]``, as you
        would expect.
        if is_callable(type_):
            callable_args = type_.__args__
            argument_types = [PredicateType.get_type(t) for t in callable_args[:-1]]
            return_type = PredicateType.get_type(callable_args[-1])
            return FunctionType(argument_types, return_type)
        elif is_generic(type_):
            # This is something like List[int].  type_.__name__ doesn't do the right thing (and
            # crashes in python 3.7), so we need to do some magic here.
            name = get_generic_name(type_)
            name = type_.__name__
        return BasicType(name)

    def get_function_type(
        arg_types: List["PredicateType"], return_type: "PredicateType"
    ) -> "PredicateType":
        Constructs an NLTK ``ComplexType`` representing a function with the given argument and
        return types.
        # TODO(mattg): We might need to generalize this to just `get_type`, so we can handle
        # functions as arguments correctly in the logic below.
        if not arg_types:
            # Functions with no arguments are basically constants whose type match their return
            # type.
            return return_type
        return FunctionType(arg_types, return_type)

class BasicType(PredicateType):
    A ``PredicateType`` representing a zero-argument predicate (which could technically be a
    function with no arguments or a constant; both are treated the same here).

    def __init__(self, name: str) -> None: = name

    def __repr__(self):

    def __hash__(self):
        return hash(

    def __eq__(self, other):
        if isinstance(self, other.__class__):
            return ==
        return NotImplemented

class FunctionType(PredicateType):
    A ``PredicateType`` representing a function with arguments.  When seeing this as a string, it
    will be in angle brackets, with argument types separated by commas, and the return type
    separated from argument types with a colon.  For example, ``def f(a: str) -> int:`` would look
    like ``<str:int>``, and ``def g(a: int, b: int) -> int`` would look like ``<int,int:int>``.

    def __init__(self, argument_types: List[PredicateType], return_type: PredicateType) -> None:
        self.argument_types = argument_types
        self.return_type = return_type = f'<{",".join(str(arg) for arg in argument_types)}:{return_type}>'

    def __repr__(self):

    def __hash__(self):
        return hash(

    def __eq__(self, other):
        if isinstance(self, other.__class__):
            return ==
        return NotImplemented

def predicate(function: Callable) -> Callable:
    This is intended to be used as a decorator when you are implementing your ``DomainLanguage``.
    This marks a function on a ``DomainLanguage`` subclass as a predicate that can be used in the
    language.  See the :class:`DomainLanguage` docstring for an example usage, and for what using
    this does.
    setattr(function, "_is_predicate", True)
    return function

def predicate_with_side_args(side_arguments: List[str]) -> Callable:
    Like :func:`predicate`, but used when some of the arguments to the function are meant to be
    provided by the decoder or other state, instead of from the language.  For example, you might
    want to have a function use the decoder's attention over some input text when a terminal was
    predicted.  That attention won't show up in the language productions.  Use this decorator, and
    pass in the required state to :func:`DomainLanguage.execute_action_sequence`, if you need to
    ignore some arguments when doing grammar induction.

    In order for this to work out, the side arguments `must` be after any non-side arguments.  This
    is because we use ``*args`` to pass the non-side arguments, and ``**kwargs`` to pass the side
    arguments, and python requires that ``*args`` be before ``**kwargs``.

    def decorator(function: Callable) -> Callable:
        setattr(function, "_side_arguments", side_arguments)
        return predicate(function)

    return decorator

def nltk_tree_to_logical_form(tree: Tree) -> str:
    Given an ``nltk.Tree`` representing the syntax tree that generates a logical form, this method
    produces the actual (lisp-like) logical form, with all of the non-terminal symbols converted
    into the correct number of parentheses.

    This is used in the logic that converts action sequences back into logical forms.  It's very
    unlikely that you will need this anywhere else.
    # nltk.Tree actually inherits from `list`, so you use `len()` to get the number of children.
    # We're going to be explicit about checking length, instead of using `if tree:`, just to avoid
    # any funny business nltk might have done (e.g., it's really odd if `if tree:` evaluates to
    # `False` if there's a single leaf node with no children).
    if len(tree) == 0:
        return tree.label()
    if len(tree) == 1:
        return tree[0].label()
    return "(" + " ".join(nltk_tree_to_logical_form(child) for child in tree) + ")"

class DomainLanguage:
    A ``DomainLanguage`` specifies the functions available to use for a semantic parsing task.  You
    write execution code for these functions, and we will automatically induce a grammar from those
    functions and give you a lisp interpreter that can use those functions.  For example:

    .. code-block:: python

        class Arithmetic(DomainLanguage):
            def add(self, num1: int, num2: int) -> int:
                return num1 + num2

            def halve(self, num: int) -> int:
                return num / 2


    Instantiating this class now gives you a language object that can parse and execute logical
    forms, can convert logical forms to action sequences (linearized abstract syntax trees) and
    back again, and can list all valid production rules in a grammar induced from the specified

    .. code-block:: python

        >>> l = Arithmetic()
        >>> l.execute("(add 2 3)")
        >>> l.execute("(halve (add 12 4))")
        >>> l.logical_form_to_action_sequence("(add 2 3)")
        # See the docstring for this function for an description of what these strings mean.
        ['@start@ -> int', 'int -> [<int,int:int>, int, int]', '<int,int:int> -> add',
         'int -> 2', 'int -> 3']
        >>> l.action_sequence_to_logical_form(l.logical_form_to_action_sequence('(add 2 3)'))
        '(add 2 3)'
        >>> l.get_nonterminal_productions()
        {'<int,int:int>': ['add', 'divide', 'multiply', 'subtract'], '<int:int>': ['halve'], ...}

    This is done with some reflection magic, with the help of the ``@predicate`` decorator and type
    annotations.  For a method you define on a ``DomainLanguage`` subclass to be included in the
    language, it *must* be decorated with ``@predicate``, and it *must* have type annotations on
    all arguments and on its return type.  You can also add predicates and constants to the
    language using the :func:`add_predicate` and :func:`add_constant` functions, if you choose
    (minor point: constants with generic types (like ``Set[int]``) must currently be specified as
    predicates, as the ``allowed_constants`` dictionary doesn't pass along the generic type

    The language we construct is purely functional - no defining variables or using lambda
    functions, or anything like that.  If you would like to extend this code to handle more complex
    languages, open an issue on github.

    We have rudimentary support for class hierarchies in the types that you provide.  This is done
    through adding constants multiple times with different types.  For example, say you have a
    ``Column`` class with ``NumberColumn`` and ``StringColumn`` subclasses.  You can have functions
    that take the base class ``Column`` as an argument, and other functions that take the
    subclasses.  These will get types like ``<List[Row],Column:List[str]>`` (for a "select"
    function that returns whatever cell text is in that column for the given rows), and
    ``<List[Row],NumberColumn,Number:List[Row]>`` (for a "greater_than" function that returns rows
    with a value in the column greater than the given number).  These will generate argument types
    of ``Column`` and ``NumberColumn``, respectively.  ``NumberColumn`` is a subclass of
    ``Column``, so we want the ``Column`` production to include all ``NumberColumns`` as options.
    This is done by calling ``add_constant()`` with each ``NumberColumn`` twice: once without a
    ``type_`` argument (which infers the type as ``NumberColumn``), and once with ``type_=Column``.
    You can see a concrete example of how this works in the

    def __init__(
        self, allowed_constants: Dict[str, Any] = None, start_types: Set[Type] = None
    ) -> None:
        self._functions: Dict[str, Callable] = {}
        self._function_types: Dict[str, List[PredicateType]] = defaultdict(list)
        self._start_types: Set[PredicateType] = {
            PredicateType.get_type(type_) for type_ in start_types
        for name in dir(self):
            if isinstance(getattr(self, name), types.MethodType):
                function = getattr(self, name)
                if getattr(function, "_is_predicate", False):
                    side_arguments = getattr(function, "_side_arguments", None)
                    self.add_predicate(name, function, side_arguments)
        if allowed_constants:
            for name, value in allowed_constants.items():
                self.add_constant(name, value)
        # Caching this to avoid recomputing it every time `get_nonterminal_productions` is called.
        self._nonterminal_productions: Dict[str, List[str]] = None

    def execute(self, logical_form: str):
        """Executes a logical form, using whatever predicates you have defined."""
        if not hasattr(self, "_functions"):
            raise RuntimeError("You must call super().__init__() in your Language constructor")
        logical_form = logical_form.replace(",", " ")
        expression = util.lisp_to_nested_expression(logical_form)
        return self._execute_expression(expression)

    def execute_action_sequence(
        self, action_sequence: List[str], side_arguments: List[Dict] = None
        Executes the program defined by an action sequence directly, without needing the overhead
        of translating to a logical form first.  For any given program, :func:`execute` and this
        function are equivalent, they just take different representations of the program, so you
        can use whichever is more efficient.

        Also, if you have state or side arguments associated with particular production rules
        (e.g., the decoder's attention on an input utterance when a predicate was predicted), you
        `must` use this function to execute the logical form, instead of :func:`execute`, so that
        we can match the side arguments with the right functions.
        # We'll strip off the first action, because it doesn't matter for execution.
        first_action = action_sequence[0]
        left_side = first_action.split(" -> ")[0]
        if left_side != "@start@":
            raise ExecutionError("invalid action sequence")
        remaining_actions = action_sequence[1:]
        remaining_side_args = side_arguments[1:] if side_arguments else None
        return self._execute_sequence(remaining_actions, remaining_side_args)[0]

    def get_nonterminal_productions(self) -> Dict[str, List[str]]:
        Induces a grammar from the defined collection of predicates in this language and returns
        all productions in that grammar, keyed by the non-terminal they are expanding.

        This includes terminal productions implied by each predicate as well as productions for the
        `return type` of each defined predicate.  For example, defining a "multiply" predicate adds
        a "<int,int:int> -> multiply" terminal production to the grammar, and `also` a "int ->
        [<int,int:int>, int, int]" non-terminal production, because I can use the "multiply"
        predicate to produce an int.
        if not self._nonterminal_productions:
            actions: Dict[str, Set[str]] = defaultdict(set)
            # If you didn't give us a set of valid start types, we'll assume all types we know
            # about (including functional types) are valid start types.
            if self._start_types:
                start_types = self._start_types
                start_types = set()
                for type_list in self._function_types.values():
            for start_type in start_types:
                actions[START_SYMBOL].add(f"{START_SYMBOL} -> {start_type}")
            for name, function_type_list in self._function_types.items():
                for function_type in function_type_list:
                    actions[str(function_type)].add(f"{function_type} -> {name}")
                    if isinstance(function_type, FunctionType):
                        return_type = function_type.return_type
                        arg_types = function_type.argument_types
                        right_side = f"[{function_type}, {', '.join(str(arg_type) for arg_type in arg_types)}]"
                        actions[str(return_type)].add(f"{return_type} -> {right_side}")
            self._nonterminal_productions = {key: sorted(value) for key, value in actions.items()}
        return self._nonterminal_productions

    def all_possible_productions(self) -> List[str]:
        Returns a sorted list of all production rules in the grammar induced by
        all_actions = set()
        for action_set in self.get_nonterminal_productions().values():
        return sorted(all_actions)

    def logical_form_to_action_sequence(self, logical_form: str) -> List[str]:
        Converts a logical form into a linearization of the production rules from its abstract
        syntax tree.  The linearization is top-down, depth-first.

        Each production rule is formatted as "LHS -> RHS", where "LHS" is a single non-terminal
        type, and RHS is either a terminal or a list of non-terminals (other possible values for
        RHS in a more general context-free grammar are not produced by our grammar induction

        Non-terminals are `types` in the grammar, either basic types (like ``int``, ``str``, or
        some class that you define), or functional types, represented with angle brackets with a
        colon separating arguments from the return type.  Multi-argument functions have commas
        separating their argument types.  For example, ``<int:int>`` is a function that takes an
        integer and returns an integer, and ``<int,int:int>`` is a function that takes two integer
        arguments and returns an integer.

        As an example translation from logical form to complete action sequence, the logical form
        ``(add 2 3)`` would be translated to ``['@start@ -> int', 'int -> [<int,int:int>, int, int]',
        '<int,int:int> -> add', 'int -> 2', 'int -> 3']``.
        expression = util.lisp_to_nested_expression(logical_form)
            transitions, start_type = self._get_transitions(expression, expected_type=None)
            if self._start_types and start_type not in self._start_types:
                raise ParsingError(
                    f"Expression had unallowed start type of {start_type}: {expression}"
        except ParsingError as error:
            logger.error(f"Error parsing logical form: {logical_form}: {error}")
        transitions.insert(0, f"@start@ -> {start_type}")
        return transitions

    def action_sequence_to_logical_form(self, action_sequence: List[str]) -> str:
        Takes an action sequence as produced by :func:`logical_form_to_action_sequence`, which is a
        linearization of an abstract syntax tree, and reconstructs the logical form defined by that
        abstract syntax tree.
        # Basic outline: we assume that the bracketing that we get in the RHS of each action is the
        # correct bracketing for reconstructing the logical form.  This is true when there is no
        # currying in the action sequence.  Given this assumption, we just need to construct a tree
        # from the action sequence, then output all of the leaves in the tree, with brackets around
        # the children of all non-terminal nodes.

        remaining_actions = [action.split(" -> ") for action in action_sequence]
        tree = Tree(remaining_actions[0][1], [])

            remaining_actions = self._construct_node_from_actions(tree, remaining_actions[1:])
        except ParsingError:
            logger.error("Error parsing action sequence: %s", action_sequence)

        if remaining_actions:
            logger.error("Error parsing action sequence: %s", action_sequence)
            logger.error("Remaining actions were: %s", remaining_actions)
            raise ParsingError("Extra actions in action sequence")
        return nltk_tree_to_logical_form(tree)

    def add_predicate(self, name: str, function: Callable, side_arguments: List[str] = None):
        Adds a predicate to this domain language.  Typically you do this with the ``@predicate``
        decorator on the methods in your class.  But, if you need to for whatever reason, you can
        also call this function yourself with a (type-annotated) function to add it to your

        name : ``str``
            The name that we will use in the induced language for this function.
        function : ``Callable``
            The function that gets called when executing a predicate with the given name.
        side_arguments : ``List[str]``, optional
            If given, we will ignore these arguments for the purposes of grammar induction.  This
            is to allow passing extra arguments from the decoder state that are not explicitly part
            of the language the decoder produces, such as the decoder's attention over the question
            when a terminal was predicted.  If you use this functionality, you also `must` use
            ``language.execute_action_sequence()`` instead of ``language.execute()``, and you must
            pass the additional side arguments needed to that function.  See
            :func:`execute_action_sequence` for more information.
        side_arguments = side_arguments or []
        signature = inspect.signature(function)
        argument_types = [
            for name, param in signature.parameters.items()
            if name not in side_arguments
        return_type = signature.return_annotation
        argument_nltk_types: List[PredicateType] = [
            PredicateType.get_type(arg_type) for arg_type in argument_types
        return_nltk_type = PredicateType.get_type(return_type)
        function_nltk_type = PredicateType.get_function_type(argument_nltk_types, return_nltk_type)
        self._functions[name] = function

    def add_constant(self, name: str, value: Any, type_: Type = None):
        Adds a constant to this domain language.  You would typically just pass in a list of
        constants to the ``super().__init__()`` call in your constructor, but you can also call
        this method to add constants if it is more convenient.

        Because we construct a grammar over this language for you, in order for the grammar to be
        finite we cannot allow arbitrary constants.  Having a finite grammar is important when
        you're doing semantic parsing - we need to be able to search over this space, and compute
        normalized probability distributions.
        value_type = type_ if type_ else type(value)
        constant_type = PredicateType.get_type(value_type)
        self._functions[name] = lambda: value

    def is_nonterminal(self, symbol: str) -> bool:
        Determines whether an input symbol is a valid non-terminal in the grammar.
        nonterminal_productions = self.get_nonterminal_productions()
        return symbol in nonterminal_productions

    def _execute_expression(self, expression: Any):
        This does the bulk of the work of executing a logical form, recursively executing a single
        expression.  Basically, if the expression is a function we know about, we evaluate its
        arguments then call the function.  If it's a list, we evaluate all elements of the list.
        If it's a constant (or a zero-argument function), we evaluate the constant.
        if isinstance(expression, list):
            if isinstance(expression[0], list):
                function = self._execute_expression(expression[0])
            elif expression[0] in self._functions:
                function = self._functions[expression[0]]
                if isinstance(expression[0], str):
                    raise ExecutionError(f"Unrecognized function: {expression[0]}")
                    raise ExecutionError(f"Unsupported expression type: {expression}")
            arguments = [self._execute_expression(arg) for arg in expression[1:]]
                return function(*arguments)
            except (TypeError, ValueError):
                raise ExecutionError(
                    f"Error executing expression {expression} (see stderr for stack trace)"
        elif isinstance(expression, str):
            if expression not in self._functions:
                raise ExecutionError(f"Unrecognized constant: {expression}")
            # This is a bit of a quirk in how we represent constants and zero-argument functions.
            # For consistency, constants are wrapped in a zero-argument lambda.  So both constants
            # and zero-argument functions are callable in `self._functions`, and are `BasicTypes`
            # in `self._function_types`.  For these, we want to return
            # `self._functions[expression]()` _calling_ the zero-argument function.  If we get a
            # `FunctionType` in here, that means we're referring to the function as a first-class
            # object, instead of calling it (maybe as an argument to a higher-order function).  In
            # that case, we return the function _without_ calling it.
            # Also, we just check the first function type here, because we assume you haven't
            # registered the same function with both a constant type and a `FunctionType`.
            if isinstance(self._function_types[expression][0], FunctionType):
                return self._functions[expression]
                return self._functions[expression]()
            return self._functions[expression]
            raise ExecutionError(
                "Not sure how you got here. Please open a github issue with details."

    def _execute_sequence(
        self, action_sequence: List[str], side_arguments: List[Dict]
    ) -> Tuple[Any, List[str], List[Dict]]:
        This does the bulk of the work of :func:`execute_action_sequence`, recursively executing
        the functions it finds and trimming actions off of the action sequence.  The return value
        is a tuple of (execution, remaining_actions), where the second value is necessary to handle
        the recursion.
        if not action_sequence:
            raise ExecutionError("invalid action sequence")
        first_action = action_sequence[0]
        remaining_actions = action_sequence[1:]
        remaining_side_args = side_arguments[1:] if side_arguments else None
        right_side = first_action.split(" -> ")[1]
        if right_side in self._functions:
            function = self._functions[right_side]
            # mypy doesn't like this check, saying that Callable isn't a reasonable thing to pass
            # here.  But it works just fine; I'm not sure why mypy complains about it.
            if isinstance(function, Callable):  # type: ignore
                function_arguments = inspect.signature(function).parameters
                if not function_arguments:
                    # This was a zero-argument function / constant that was registered as a lambda
                    # function, for consistency of execution in `execute()`.
                    execution_value = function()
                elif side_arguments:
                    kwargs = {}
                    non_kwargs = []
                    for argument_name in function_arguments:
                        if argument_name in side_arguments[0]:
                            kwargs[argument_name] = side_arguments[0][argument_name]
                    if kwargs and non_kwargs:
                        # This is a function that has both side arguments and logical form
                        # arguments - we curry the function so only the logical form arguments are
                        # left.
                        def curried_function(*args):
                            return function(*args, **kwargs)

                        execution_value = curried_function
                    elif kwargs:
                        # This is a function that _only_ has side arguments - we just call the
                        # function and return a value.
                        execution_value = function(**kwargs)
                        # This is a function that has logical form arguments, but no side arguments
                        # that match what we were given - just return the function itself.
                        execution_value = function
                    execution_value = function
            return execution_value, remaining_actions, remaining_side_args
            # This is a non-terminal expansion, like 'int -> [<int:int>, int, int]'.  We need to
            # get the function and its arguments, then call the function with its arguments.
            # Because we linearize the abstract syntax tree depth first, left-to-right, we can just
            # recursively call `_execute_sequence` for the function and all of its arguments, and
            # things will just work.
            right_side_parts = right_side.split(", ")

            # We don't really need to know what the types are, just how many of them there are, so
            # we recurse the right number of times.
            function, remaining_actions, remaining_side_args = self._execute_sequence(
                remaining_actions, remaining_side_args
            arguments = []
            for _ in right_side_parts[1:]:
                argument, remaining_actions, remaining_side_args = self._execute_sequence(
                    remaining_actions, remaining_side_args
            return function(*arguments), remaining_actions, remaining_side_args

    def _get_transitions(
        self, expression: Any, expected_type: PredicateType
    ) -> Tuple[List[str], PredicateType]:
        This is used when converting a logical form into an action sequence.  This piece
        recursively translates a lisp expression into an action sequence, making sure we match the
        expected type (or using the expected type to get the right type for constant expressions).
        if isinstance(expression, (list, tuple)):
            function_transitions, return_type, argument_types = self._get_function_transitions(
                expression[0], expected_type
            if len(argument_types) != len(expression[1:]):
                raise ParsingError(f"Wrong number of arguments for function in {expression}")
            argument_transitions = []
            for argument_type, subexpression in zip(argument_types, expression[1:]):
                argument_transitions.extend(self._get_transitions(subexpression, argument_type)[0])
            return function_transitions + argument_transitions, return_type
        elif isinstance(expression, str):
            if expression not in self._functions:
                raise ParsingError(f"Unrecognized constant: {expression}")
            constant_types = self._function_types[expression]
            if len(constant_types) == 1:
                constant_type = constant_types[0]
                # This constant had only one type; that's the easy case.
                if expected_type and expected_type != constant_type:
                    raise ParsingError(
                        f"{expression} did not have expected type {expected_type} "
                        f"(found {constant_type})"
                return [f"{constant_type} -> {expression}"], constant_type
                if not expected_type:
                    raise ParsingError(
                        "With no expected type and multiple types to pick from "
                        f"I don't know what type to use (constant was {expression})"
                if expected_type not in constant_types:
                    raise ParsingError(
                        f"{expression} did not have expected type {expected_type} "
                        f"(found these options: {constant_types}; none matched)"
                return [f"{expected_type} -> {expression}"], expected_type

            raise ParsingError(
                "Not sure how you got here. Please open an issue on github with details."

    def _get_function_transitions(
        self, expression: Union[str, List], expected_type: PredicateType
    ) -> Tuple[List[str], PredicateType, List[PredicateType]]:
        A helper method for ``_get_transitions``.  This gets the transitions for the predicate
        itself in a function call.  If we only had simple functions (e.g., "(add 2 3)"), this would
        be pretty straightforward and we wouldn't need a separate method to handle it.  We split it
        out into its own method because handling higher-order functions is complicated (e.g.,
        something like "((negate add) 2 3)").
        # This first block handles getting the transitions and function type (and some error
        # checking) _just for the function itself_.  If this is a simple function, this is easy; if
        # it's a higher-order function, it involves some recursion.
        if isinstance(expression, list):
            # This is a higher-order function.  TODO(mattg): we'll just ignore type checking on
            # higher-order functions, for now.
            transitions, function_type = self._get_transitions(expression, None)
        elif expression in self._functions:
            name = expression
            function_types = self._function_types[expression]
            if len(function_types) != 1:
                raise ParsingError(
                    f"{expression} had multiple types; this is not yet supported for functions"
            function_type = function_types[0]
            transitions = [f"{function_type} -> {name}"]
            if isinstance(expression, str):
                raise ParsingError(f"Unrecognized function: {expression[0]}")
                raise ParsingError(f"Unsupported expression type: {expression}")
        if not isinstance(function_type, FunctionType):
            raise ParsingError(f"Zero-arg function or constant called with arguments: {name}")

        # Now that we have the transitions for the function itself, and the function's type, we can
        # get argument types and do the rest of the transitions.
        argument_types = function_type.argument_types
        return_type = function_type.return_type
        right_side = f'[{function_type}, {", ".join(str(arg) for arg in argument_types)}]'
        first_transition = f"{return_type} -> {right_side}"
        transitions.insert(0, first_transition)
        if expected_type and expected_type != return_type:
            raise ParsingError(
                f"{expression} did not have expected type {expected_type} " f"(found {return_type})"
        return transitions, return_type, argument_types

    def _construct_node_from_actions(
        self, current_node: Tree, remaining_actions: List[List[str]]
    ) -> List[List[str]]:
        Given a current node in the logical form tree, and a list of actions in an action sequence,
        this method fills in the children of the current node from the action sequence, then
        returns whatever actions are left.

        For example, we could get a node with type ``c``, and an action sequence that begins with
        ``c -> [<r,c>, r]``.  This method will add two children to the input node, consuming
        actions from the action sequence for nodes of type ``<r,c>`` (and all of its children,
        recursively) and ``r`` (and all of its children, recursively).  This method assumes that
        action sequences are produced `depth-first`, so all actions for the subtree under ``<r,c>``
        appear before actions for the subtree under ``r``.  If there are any actions in the action
        sequence after the ``<r,c>`` and ``r`` subtrees have terminated in leaf nodes, they will be
        if not remaining_actions:
            logger.error("No actions left to construct current node: %s", current_node)
            raise ParsingError("Incomplete action sequence")
        left_side, right_side = remaining_actions.pop(0)
        if left_side != current_node.label():
            logger.error("Current node: %s", current_node)
            logger.error("Next action: %s -> %s", left_side, right_side)
            logger.error("Remaining actions were: %s", remaining_actions)
            raise ParsingError("Current node does not match next action")
        if right_side[0] == "[":
            # This is a non-terminal expansion, with more than one child node.
            for child_type in right_side[1:-1].split(", "):
                child_node = Tree(child_type, [])
                current_node.append(child_node)  # you add a child to an nltk.Tree with `append`
                # For now, we assume that all children in a list like this are non-terminals, so we
                # recurse on them.  I'm pretty sure that will always be true for the way our
                # grammar induction works.  We can revisit this later if we need to.
                remaining_actions = self._construct_node_from_actions(child_node, remaining_actions)
            # The current node is a pre-terminal; we'll add a single terminal child.  By
            # construction, the right-hand side of our production rules are only ever terminal
            # productions or lists of non-terminals.
                Tree(right_side, [])
            )  # you add a child to an nltk.Tree with `append`
        return remaining_actions

    def __len__(self):
        # This method exists just to make it easier to use this in a MetadataField.  Kind of
        # annoying, but oh well.
        return 0