Inato fp-ts training

Training material to onboard people on using fp-ts efficiently.

The exercises consist of unimplemented functions and their associated failing tests.

If needed, you can always refer to the solution branch where you will find the solutions for each exercices.

But first, it is essential to understand why we are using fp-ts. We suggest you to read this article and then start the exercises.

After cloning the repository, setup the project by running

$ yarn

To run the tests, simply run

$ yarn test

You can also run them in watch mode:

$ yarn test:watch

Finally, if you wish to only run the tests for a given exercise exoN, you can run the following:

$ yarn test[:watch] exoN

The exercises are organized into exoN folders and most of what is required to complete each is detailed in the comments.

code style guide

For readability purpose, we replace ReaderTaskEither by rte

• Use flow instead of pipe when possible

  Why? Using flow reduces the amount of variables to declare in a method, hence the visibility and readability of the code

// Bad
const formatUserPhoneNumber = (user: User) =>
  pipe(user, User.phoneNumber, User.formatPhoneNumber);

// Good
const formatUserPhoneNumber = flow(User.phoneNumber, User.formatPhoneNumber);

• Avoid using boolean method match when unecessary

  Why? boolean.match can lower the global understanding of a method and enforce nested pipes. Using classic if/else is often the best option

// Bad
const triggerEmailCampaign = ({
  user,
  ...emailSettings
}: {
  user: User} & EmailSettings) =>
  pipe(
    user.nationality === 'FR',
    boolean.match(
      () => triggerGlobalEmailCampaign({ to: user.email, emailSettings }),
      () => triggerFrenchEmailCampaign({ to: user.email, emailSettings }),
    ),
  );

// Good
const triggerEmailCampaign = ({
  user,
  ...emailSettings
}: { user: User } & EmailSettings) => {
  if (user.nationality === 'FR') {
    return triggerFrenchEmailCampaign({ to: user.email, emailSettings });
  }
  return triggerGlobalEmailCampaign({ to: user.email, emailSettings });

• Avoid nested pipes

  Why? They lower global understanding of the code. We allow ourselves 2 levels of piping maximum per function and tend to do atomic functions instead

// Bad
export const convertDollarAmountInCountryCurrency = ({
  countryName,
  amountInDollar,
}: {
  countryName: CountryName;
  amountInDollar: number;
}) =>
  pipe(
    getCountryCode(countryName),
    either.map(
      countryCode =>
        pipe(
          getCountryCurrency(countryCode),
          option.map(
            flow(
              convertFromDollarAmount(amountInDollar),
              convertedAmount =>
                console.log(
                  `converted amount for country ${countryCode} is ${convertedAmount}`,
                ),
              ),
            ),
          ),
        ),
    ),
  );

// Good
export const convertDollarAmountInCountryCurrency = (amountInDollar: number) =>
  flow(
    getCountryCode,
    either.map(convertDollarAmountToCountryCodeCurrency(amountInDollar)),
  );

const convertDollarAmountToCountryCodeCurrency =
  (amountInDollar: number) => (countryCode: CountryCode) =>
    pipe(
      getCountryCurrency(countryCode),
      option.map(convertFromDollarAmount(amountInDollar)),
      option.map(convertedAmount =>
        console.log(
          `converted amount for country ${countryCode} is ${convertedAmount}`,
        ),
      ),
    );