Option.ts

const _Some = Symbol("some");
const _None = Symbol("none");

type Some<T> = {
  type: typeof _Some;
  value: T;
};

type None = {
  type: typeof _None;
};

export type Option<T> = Some<T> | None;

export const isSome = <T>(option: Option<T>): option is Some<T> => option.type === _Some;
export const isNone = <T>(option: Option<T>): option is None => option.type === _None;

export const some = <T>(value: T): Some<T> => ({
  type: _Some,
  value,
});

export const none = (): None => ({ type: _None });

export const map = <T, A>(option: Option<T>, fn: (value: T) => A): Option<A> => {
  if (isSome(option)) {
    return some(fn(option.value));
  } else {
    return none();
  }
};

export const match = <T, A, B>(option: Option<T>, onSome: (value: T) => A, onNone: () => B) =>
  isSome(option) ? onSome(option.value) : onNone();