{


    /*
    interface Observer<T> {
        next(t: T): void;
        error(e: Error): void
        complete(): void;
    }
    */
    class Observer {
       constructor(next, error, complete) {
           this.next = next;
           this.error = error;
           this.complete = complete;
       }
    }

    const incrementer = scanObservable(createIntervalObservable(100), function(s, item) {
        return s + 1;
    }, 0);

    const squaredIncrementer = mapObservable(incrementer, x => x * x);


    const firstTenSquares = take(squaredIncrementer, 10);

    // {{ square$ | async }}
    firstTenSquares({
        next(n) {
            console.log("heard", n);
        },
        complete() {
            console.log("finished");
        },
    })



    // type CancelSubscription = () => void;
    // type FunctionObservable<T> = (observer: Observer<T>) => CancelSubscription

    function createIntervalObservable(nMilliseconds) {
        return function(observer) {

            let intervalToken = setInterval(() => observer.next(), nMilliseconds);

            // cancellation
            return () => {
                if(intervalToken) {
                    clearInterval(intervalToken)
                    intervalToken = null;
                }
            }
        }
    }

    // mapObservable(o: FunctionObservable<T>, mapper: (t: T) => U): FunctionObservable<U>
    function mapObservable(observable, mapper) {
        return function(observer) {
            return observable({
                next(t) {
                    const u = mapper(t);
                    observer.next(u);
                },
                error: (e) => observer.error(e),
                complete: () => observer.complete(),
            })
        }
    }

    // scanObservable<S,T>(o: FunctionObservable<T>, reducer: (s: S, t: T) => S): FunctionObservable<S>
    function scanObservable(observable, reducer, state) {
        return function(observer) {
            return observable({
                next(t) {
                    state = reducer(state, t);
                    observer.next(state);
                },
                error: (e) => observer.error(e),
                complete: () => observer.complete(),
            })
        }
    }

    function take(observable, n) {
        return function(observer) {
            const unsubscribe = observable({
                next(t) {
                    if(n--) {
                        observer.next(t);
                    } else {
                        observer.complete();
                        unsubscribe();
                    }
                },
                error: (e) => observer.error(e),
                complete: () => observer.complete(),
            })

            return unsubscribe;
        }
    }





















}