/**
 * True if not empty
 */
isDefined(): this is Some<A>;
/**
 * Return value, throw exception if empty
 */
get(): A;
/**
 * True if empty
 */
isEmpty(): this is None<A>;
/**
 * True if not empty
 */
nonEmpty(): this is Some<A>;
/**
 * Evaluate and return alternate optional value if empty
 */
orElse(alternative: option<A>): option<A>;
/**
 * Evaluate and return alternate value if empty
 */
getOrElse(alternative: A): A;
/**
 * Apply function on optional value, return default if empty
 */
fold<B>(fallback: B, folder: (value: A) => B): B;
/**
 * Apply a function on the optional value
 */
map<B>(map: (value: A) => B): option<B>;
/**
 * Same as map but function must return an optional value
 */
flatMap<B>(map: (value: A) => option<B>): option<B>;
/**
 * Apply a procedure on option value
 */
forEach(consumer: (value: A) => void): void;
/**
 * Apply partial pattern match on optional value
 */
collect<B>(collector: (value: A) => B | undefined): option<B>;
/**
 * An optional value satisfies predicate
 */
filter(filter: (value: A) => boolean): option<A>;
/**
 * An optional value doesn't satisfy predicate
 */
filterNot(filter: (value: A) => boolean): option<A>;
/**
 * Apply predicate on optional value, or false if empty
 */
exists(predicate: (value: A) => boolean): boolean;
/**
 * Apply predicate on optional value, or true if empty
 */
forAll(predicate: (value: A) => boolean): boolean;
/**
 * Checks if value equals optional value, or false if empty
 */
contains<B extends A>(element: B): boolean;
/**
 * Combine two optional values to make a paired optional value
 */
zip<B>(that: option<B>): option<[A, B]>;
/**
 * Unary list of optional value, otherwise the empty list
 */
toList(): A[];

// Assume a function
function maybe(): option<number>

let n = maybe()

// This acts as a guard, so now n is of type Some<number>
if (n.isDefined()) {
  console.log(`n = ${n.value}`)
}

// Or at runtime
console.log(`n = ${n.get()}`)
// This throws if the value wasn't present.

for (const value of maybe()) {
  console.log(`value = ${value}`)
}