Building higher-order functions with callables

We can define higher-order functions as callable classes. This builds on the idea of writing generator functions; we'll write callables because we need statement features of Python. In addition to using statements, we can also apply a static configuration when creating the higher-order functions.

What's important about a callable class definition is that the class object, created by the class statement, defines a function that emits a function. Commonly, we'll use a callable object to create a composite function that combines two other functions into something relatively complex.

To emphasize this, consider the following class:

from typing import Callable, Optional, Any

class NullAware:
    def __init__(
            self, some_func: Callable[[Any], Any]) -> None:
        self.some_func = some_func
    def __call__(self, arg: Optional[Any]) -> Optional[Any]:
        return None if arg is None else self.some_func(arg)

This class is used to...