Monoid Type Class Option

Monoid-option.ts

// When we can find a Monoid instance for A we can create one for Option<A>
  
// When we take a look at the fp-ts implementation it will become clearer
export function getApplySemigroup<A>(S: Semigroup<A>): Semigroup<Option<A>> {
  return {
    concat: (x, y) => (isSome(x) && isSome(y) ? some(S.concat(x.value, y.value)) : none)
  }
}

export function getApplyMonoid<A>(M: Monoid<A>): Monoid<Option<A>> {
  return {
    ...getApplySemigroup(M),
    empty: some(M.empty)
  }
}
// We see that getApplyMonoid uses getApplySemigroup and just extends it with empty
// which just returns the empty value of the Monoid A wrapped in an Option, so we get Option<A>
// So if A has a empty value and can thus become an instance of the Monoid type class, we can also derive a Monoid instance for Option<A>