Functor Option

functor-option.ts

import * as O from "fp-ts/lib/Option";

const MyOptionFunctor = {
  map: (option: O.Option<unknown>, f: (a: unknown) => unknown) =>
    O.isNone(option) ? O.none : O.some(f(option.value)), 
  // We check if 'option' is 'none'
  // if so return 'none'
  // else take out the value from 'option' and apply it to our function 'f'
  // then wrap it in an 'Option' again
};

// We define a pure function (a: A) => B
const double = (x: number) => x * 2;

MyOptionFunctor.map(O.some(4), double); // { _tag: 'Some', value: 8 }