filter map function for Rx.Observable.

filter_map.ts

import {Option} from 'option-t';
import {Observable} from 'rx';

type FilterMapFn<T, U> = (v: T) => Option<U>;

function filterMapForObservable<T, U>(source: Rx.Observable<T>, op: FilterMapFn<T, U>): Rx.Observable<U> {
    const filtered = source.flatMap(function(v: T): Rx.Observable<U> {
        const mapped: Option<U> = op(v);
        const result: Rx.Observable<U> =
            mapped.mapOrElse(function def(): Rx.Observable<U> {
              return Rx.Observable.never<U>();
            }, function map(v: U): Rx.Observable<U> {
              return Rx.Observable.return<U>(v);
            });
        return result;
    });
    return filtered;
}