Concurrently iterates values yielded from an iterable, passing them to
    an asynchronous coroutine.

    This function is the flatten version of to ``paco.map()``.

    Mapped values will be returned as an ordered list.
    Items order is preserved based on origin iterable order.

    Concurrency level can be configurable via ``limit`` param.

    All coroutines will be executed in the same loop.

    This function is a coroutine.

    This function can be composed in a pipeline chain with ``|`` operator.

    Arguments:
        coro (coroutinefunction): coroutine iterator function that accepts
            iterable values.
        iterable (iterable|asynchronousiterable): an iterable collection
            yielding coroutines functions.
        limit (int): max iteration concurrency limit. Use ``0`` for no limit.
        collect (bool): return yielded values from coroutines. Default False.
        loop (asyncio.BaseEventLoop): optional event loop to use.
        return_exceptions (bool): enable/disable returning exceptions in case
            of error. `collect` param must be True.
        timeout (int|float): timeout can be used to control the maximum number
            of seconds to wait before returning. timeout can be an int or
            float. If timeout is not specified or None, there is no limit to
            the wait time.
        *args (mixed): optional variadic arguments to pass to the
            coroutine iterable function.

    Returns:
        results (list): ordered list of values yielded by coroutines

    Raises:
        TypeError: in case of invalid input arguments.

    Usage::

        async def mul_2(num):
            return num * 2

        await paco.flat_map(mul_2, [1, [2], [3, [4]], [(5,)]])
        # => [2, 4, 6, 8, 10]

        # Pipeline style
        await [1, [2], [3, [4]], [(5,)]] | paco.flat_map(mul_2)
        # => [2, 4, 6, 8, 10]

def flat_map(coro, iterable, limit=0, loop=None, timeout=None,
             return_exceptions=False, initializer=None, *args, **kw):
    """
    Concurrently iterates values yielded from an iterable, passing them to
    an asynchronous coroutine.

    This function is the flatten version of to ``paco.map()``.

    Mapped values will be returned as an ordered list.
    Items order is preserved based on origin iterable order.

    Concurrency level can be configurable via ``limit`` param.

    All coroutines will be executed in the same loop.

    This function is a coroutine.

    This function can be composed in a pipeline chain with ``|`` operator.

    Arguments:
        coro (coroutinefunction): coroutine iterator function that accepts
            iterable values.
        iterable (iterable|asynchronousiterable): an iterable collection
            yielding coroutines functions.
        limit (int): max iteration concurrency limit. Use ``0`` for no limit.
        collect (bool): return yielded values from coroutines. Default False.
        loop (asyncio.BaseEventLoop): optional event loop to use.
        return_exceptions (bool): enable/disable returning exceptions in case
            of error. `collect` param must be True.
        timeout (int|float): timeout can be used to control the maximum number
            of seconds to wait before returning. timeout can be an int or
            float. If timeout is not specified or None, there is no limit to
            the wait time.
        *args (mixed): optional variadic arguments to pass to the
            coroutine iterable function.

    Returns:
        results (list): ordered list of values yielded by coroutines

    Raises:
        TypeError: in case of invalid input arguments.

    Usage::

        async def mul_2(num):
            return num * 2

        await paco.flat_map(mul_2, [1, [2], [3, [4]], [(5,)]])
        # => [2, 4, 6, 8, 10]

        # Pipeline style
        await [1, [2], [3, [4]], [(5,)]] | paco.flat_map(mul_2)
        # => [2, 4, 6, 8, 10]

    """
    assert_corofunction(coro=coro)
    assert_iter(iterable=iterable)

    # By default do not collect yielded values from coroutines
    results = initializer if isiter(initializer) else []

    @asyncio.coroutine
    def reducer(buf, value):
        if isiter(value):
            yield from _reduce(value)
        else:
            buf.append((yield from coro(value)))
        return buf

    def _reduce(iterable):
        return reduce(reducer, iterable,
                      initializer=results, limit=limit, loop=loop)

    # Returns list of mapped reduced results
    return (yield from _reduce(iterable))