TypeScript를 활용한 함수형 프로그래밍 온보딩

1. CH04_05. Option 소개

export const isSome = <A>(oa: Option<A>): oa is Some<A> => oa._tag === "Some";
export const isNone = <A>(oa: Option<A>): oa is None => oa._tag === "None";