feat: add flat and flatMap methods to Maybe functor

[ortense]

description

Add flat and flatMap methods to Maybe functor

.flat

A method to removing one level of nesting, if applicable.

const data = maybe(maybe('data')) //? Maybe<Maybe<string>>

const flatted = data.flat() //? Maybe<string>

const nonNestedFlatted = flatted.flat() //? Maybe<string> 

.flatMap

Useful to combine functions that returns Maybe

const divide = (denominator: number) =>
  (numerator: number) => denominator === 0
    ? maybe(null) : maybe(numerator/denominator)

const getFromLocalStorage = (key: string) =>
  maybe(localStorage.getItem(key))

When a function returns a Maybe

getFromLocalStorage('count') //? Maybe<string>
  .map(Number) //? Maybe<number>
  .map(divide(2)) //? Maybe<Maybe<number>>

getFromLocalStorage('count') //? Maybe<string>
  .map(Number) //? Maybe<number>
  .flatMap(divide(2)) //? Maybe<number>

When needs to map a nested Maybe

getFromLocalStorage('count') //? Maybe<string>
  .map(Number) //? Maybe<number>
  .map(divide(2)) //? Maybe<Maybe<number>>
  .flatMap(value => value + 1) // Maybe<number>