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fp-ts-training/README.md

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# Inato `fp-ts` training
This repo is a work in progress toward having a comprehensive training material
to onboard people on using `fp-ts` efficiently.
The exercices consist of unimplemented functions and their associated failing
tests.
But first, it is essential to understand why we are using `fp-ts`. I advice you to read this [article](https://medium.com/inato/our-journey-to-functional-programing-36854a370de1) and then start the exercices.
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To run the tests, simply run
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```sh
$ yarn test
```
You can also run them in watch mode:
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```sh
$ yarn test:watch
```
Finally, if you wish to only run the tests for a given exercice `exoN`, you can
run the following:
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```sh
$ yarn test[:watch] exoN
```
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The exercices are organized into `exoN` folders and most of what is required to
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complete each is detailed in the comments.
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## code style guide
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For readability purpose, we replace `ReaderTaskEither` by `rte`
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- Use `flow` instead of `pipe` when possible
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> Why? Using flow reduces the amount of variables to declare in a method, hence the visibility and readability of the code
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```typescript
// Bad
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const formatUserPhoneNumber = (user: User) =>
pipe(user, User.phoneNumber, User.formatPhoneNumber);
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// Good
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const formatUserPhoneNumber = flow(User.phoneNumber, User.formatPhoneNumber);
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```
- Avoid using `boolean` method `match` when unecessary
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> Why? boolean.match can lower the global understanding of a method and enforce nested pipes. Using classic `if/else` is often the best option
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```typescript
// Bad
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const triggerEmailCampaign = ({
user,
...emailSettings
}: {
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user: User} & EmailSettings) =>
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pipe(
user.nationality === 'FR',
boolean.match(
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() => triggerGlobalEmailCampaign({ to: user.email, emailSettings }),
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() => triggerFrenchEmailCampaign({ to: user.email, emailSettings }),
),
);
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// Good
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const triggerEmailCampaign = ({
user,
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...emailSettings
}: { user: User } & EmailSettings) => {
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if (user.nationality === 'FR') {
return triggerFrenchEmailCampaign({ to: user.email, emailSettings });
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}
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return triggerGlobalEmailCampaign({ to: user.email, emailSettings });
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```
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- 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
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```typescript
// Bad
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export const convertDollarAmountInCountryCurrency = ({
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countryName,
amountInDollar,
}: {
countryName: CountryName;
amountInDollar: number;
}) =>
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pipe(
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getCountryCode(countryName),
either.map(
countryCode =>
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pipe(
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getCountryCurrency(countryCode),
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option.map(
flow(
convertFromDollarAmount(amountInDollar),
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convertedAmount =>
console.log(
`converted amount for country ${countryCode} is ${convertedAmount}`,
),
),
),
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),
),
),
);
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// Good
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export const convertDollarAmountInCountryCurrency = (amountInDollar: number) =>
flow(
getCountryCode,
either.map(convertDollarAmountToCountryCodeCurrency(amountInDollar)),
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);
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const convertDollarAmountInCountryCodeCurrency =
(amountInDollar: number) => (countryCode: CountryCode) =>
pipe(
getCurrencyFromCountryCode(countryCode),
option.map(convertFromDollarAmount(amountInDollar)),
option.map(convertedAmount =>
console.log(
`converted amount for country ${countryCode} is ${convertedAmount}`,
),
),
);
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```