Do notation

[SandroMaglione]

This PR introduces the Do notation in fpdart. This is the result of the discussion in #26.

The Do notation aims to simplify (flatten) long chains to a more readable and linear code, while still keeping the functional programming advantages and guarantees.

Below and example of refactoring for the Option type:

/// Without the Do notation
String goShopping() => goToShoppingCenter()
    .alt(goToLocalMarket)
    .flatMap(
      (market) => market.buyBanana().flatMap(
            (banana) => market.buyApple().flatMap(
                  (apple) => market.buyPear().flatMap(
                        (pear) => Option.of('Shopping: $banana, $apple, $pear'),
                      ),
                ),
          ),
    )
    .getOrElse(
      () => 'I did not find 🍌 or 🍎 or 🍐, so I did not buy anything 🤷‍♂️',
    );

/// Using the Do notation
String goShoppingDo() => Option.Do(
      ($) {
        final market = $(goToShoppingCenter().alt(goToLocalMarket));
        final amount = $(market.buyAmount());

        final banana = $(market.buyBanana());
        final apple = $(market.buyApple());
        final pear = $(market.buyPear());

        return 'Shopping: $banana, $apple, $pear';
      },
    ).getOrElse(
      () => 'I did not find 🍌 or 🍎 or 🍐, so I did not buy anything 🤷‍♂️',
    );

Status

• Option
• Either
• TaskEither
• Task
• IO
• IOEither
• TaskOption

Note: The Do Notation will be released in a beta version for testing, before landing in an official fpdart release. We are looking for feedback 🤝

[tim-smart]

Changes look good 👍🏻

[SandroMaglione]

Released preview version v0.6.0-dev.1