@Effect() route$ = actions$
    .ofType(ACTION_A)
    .switchMap(() => this._store.let(getCapabilities)
        .filter(capabilities => !isOutOfDate(capabilities))
        .take(1)
    )
    .map( do some stuff );

// Selector function that causes excess observable emits
export const getCapabilities =
     compose(fromLocations.currentLocationLookup(fromLocations.getCapabilities), fromRoot.getLocations);

// Selector function works as expected
export const getCapabilitiesWorking = function (state$: Observable<AppState>): Observable<Capability> {
    return state$
        .let(fromRoot.getLocations)
        .map(l => l.capabilitiesByLocation[l.currentId]);
};

// fromLocations
// -------------

export function getCapabilities(state$: Observable<LocationsState>): Observable<{ [id: string]: Capability }> {
    return state$.map(s => s.capabilitiesByLocation);
}

// Find the value in 'collection' corresponding to the current location
export function currentLocationLookup<T>(
    collection: (state$: Observable<LocationsState>) => Observable<{ [id: string]: T }>
): (state$: Observable<LocationsState>) => Observable<T> {
    return (state$: Observable<LocationsState>): Observable<T> => {
        return Observable.combineLatest(
            state$.let(getCurrentLocation),
            state$.let(collection),
            (locId, map) => map[locId]
        );
    };
}