NOTE: C++ optional

Optional type std::optional<T> was introduced since C++17, we could include header <optional> to have it. I supply a few examples to explain this good abstraction.

A little step:

std::optional<int> i{1};
if (i) {
  std::cout << i.value() << std::endl;
}

This example shows how to check optional exist or not, and how to extract value from optional.

std::optional<int> i{std::nullopt};
std::cout << i.value_or(10) << std::endl;

This example shows if we know the fall back value, we can provide it and skip checking optional. There was still something missing that quite important, for example, we shouldn't use value_or, but more expressive helpers:

Prelude> fmap (+10) (Just 4) :: Maybe Int
Just 14
Prelude> (*) <$> (Just 4) <*> (Just 5)
Just 20

So that we use function than just a value, furthermore I also would like to see there had something like unwrap(), C++ provided Exception, I thought there had no reason to prevent this way.