fibonaci-lazy-generators.ts

/* Some monadic and helper functions */

function* pure<T>(value: T): IterableIterator<T> {
    yield value;
}

function* concat<T>(...iterators: Array<IterableIterator<T>>): IterableIterator<T> {
    for (const iterator of iterators) {
        yield* iterator;
    }
}

function* pairwise<T>(iterator: IterableIterator<T>): IterableIterator<[T, T]> {
    let value: T = iterator.next().value;

    while (true) {
        const res = iterator.next();
        if (res.done) {
            break;
        } else {
            yield [value, res.value];
            value = res.value;
        }
    }
}

/* Fibonacci! */

function* fibonacci(): IterableIterator<number> {
    yield* concat(pure(1), pure(2), map(([x, y]) => x + y, pairwise(fibonacci())));
}

/* Example: Let's get the first 10 values */

const fibs = fibonacci();
for (let i = 0; i < 10; i++ ){
    console.log(fibs.next().value);
}