lib/partial.bzl

# Copyright 2018 The Bazel Authors. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Starlark module for working with partial function objects.

Partial function objects allow some parameters are bound before the call.

Similar to https://docs.python.org/3/library/functools.html#functools.partial.
"""

# create instance singletons to avoid unnecessary allocations
_a_dict_type = type({})
_a_tuple_type = type(())
_a_struct_type = type(struct())

def _call(partial, *args, **kwargs):
    """Calls a partial created using `make`.

    Args:
      partial: The partial to be called.
      *args: Additional positional arguments to be appended to the ones given to
             make.
      **kwargs: Additional keyword arguments to augment and override the ones
                given to make.

    Returns:
      Whatever the function in the partial returns.
    """
    function_args = partial.args + args
    function_kwargs = dict(partial.kwargs)
    function_kwargs.update(kwargs)
    return partial.function(*function_args, **function_kwargs)

def _make(func, *args, **kwargs):
    """Creates a partial that can be called using `call`.

    A partial can have args assigned to it at the make site, and can have args
    passed to it at the call sites.

    A partial 'function' can be defined with positional args and kwargs:

      # function with no args
      ```
      def function1():
        ...
      ```

      # function with 2 args
      ```
      def function2(arg1, arg2):
        ...
      ```

      # function with 2 args and keyword args
      ```
      def function3(arg1, arg2, x, y):
        ...
      ```

    The positional args passed to the function are the args passed into make
    followed by any additional positional args given to call. The below example
    illustrates a function with two positional arguments where one is supplied by
    make and the other by call:

      # function demonstrating 1 arg at make site, and 1 arg at call site
      ```
      def _foo(make_arg1, func_arg1):
        print(make_arg1 + " " + func_arg1 + "!")
      ```

    For example:

      ```
      hi_func = partial.make(_foo, "Hello")
      bye_func = partial.make(_foo, "Goodbye")
      partial.call(hi_func, "Jennifer")
      partial.call(hi_func, "Dave")
      partial.call(bye_func, "Jennifer")
      partial.call(bye_func, "Dave")
      ```

    prints:

      ```
      "Hello, Jennifer!"
      "Hello, Dave!"
      "Goodbye, Jennifer!"
      "Goodbye, Dave!"
      ```

    The keyword args given to the function are the kwargs passed into make
    unioned with the keyword args given to call. In case of a conflict, the
    keyword args given to call take precedence. This allows you to set a default
    value for keyword arguments and override it at the call site.

    Example with a make site arg, a call site arg, a make site kwarg and a
    call site kwarg:

      ```
      def _foo(make_arg1, call_arg1, make_location, call_location):
        print(make_arg1 + " is from " + make_location + " and " +
              call_arg1 + " is from " + call_location + "!")

      func = partial.make(_foo, "Ben", make_location="Hollywood")
      partial.call(func, "Jennifer", call_location="Denver")
      ```

    Prints "Ben is from Hollywood and Jennifer is from Denver!".

      ```
      partial.call(func, "Jennifer", make_location="LA", call_location="Denver")
      ```

    Prints "Ben is from LA and Jennifer is from Denver!".

    Note that keyword args may not overlap with positional args, regardless of
    whether they are given during the make or call step. For instance, you can't
    do:

    ```
    def foo(x):
      pass

    func = partial.make(foo, 1)
    partial.call(func, x=2)
    ```

    Args:
      func: The function to be called.
      *args: Positional arguments to be passed to function.
      **kwargs: Keyword arguments to be passed to function. Note that these can
                be overridden at the call sites.

    Returns:
      A new `partial` that can be called using `call`
    """
    return struct(function = func, args = args, kwargs = kwargs)

def _is_instance(v):
    """Returns True if v is a partial created using `make`.

    Args:
      v: The value to check.

    Returns:
      True if v was created by `make`, False otherwise.
    """

    # Note that in bazel 3.7.0 and earlier, type(v.function) is the same
    # as the type of a function even if v.function is a rule. But we
    # cannot rely on this in later bazels due to breaking change
    # https://github.com/bazelbuild/bazel/commit/e379ece1908aafc852f9227175dd3283312b4b82
    #
    # Since this check is heuristic anyway, we simply check for the
    # presence of a "function" attribute without checking its type.
    return type(v) == _a_struct_type and \
           hasattr(v, "function") and \
           hasattr(v, "args") and type(v.args) == _a_tuple_type and \
           hasattr(v, "kwargs") and type(v.kwargs) == _a_dict_type

partial = struct(
    make = _make,
    call = _call,
    is_instance = _is_instance,
)