/// <summary>
/// Asynchronously creates a <see cref="T:System.Collections.Generic.List`1" />
/// from an <see cref="T:System.Linq.IQueryable`1" /> by enumerating it asynchronously.
/// </summary>
/// <typeparam name="TSource">The type of the value.</typeparam>
/// <param name="observable">The parent observable.</param>
/// <returns>An instance of <see cref="IProviderObservable{TSource}"/>.</returns>
public static IProviderObservable <List <TSource> > ToListAsync <TSource>(
this IProviderObservable <IQueryable <TSource> > observable) =>
observable.MapAsync(async s => await s.ToListAsync());
/// <summary>
/// Emits the first element of a sequence, or a default
/// value if the sequence contains no elements.
/// </summary>
/// <typeparam name="TSource">The type of the value.</typeparam>
/// <param name="observable">The parent observable.</param>
/// <returns>An instance of <see cref="IProviderObservable{TSource}"/>.</returns>
public static IProviderObservable <TSource> FirstOrDefaultAsync <TSource>(
this IProviderObservable <IQueryable <TSource> > observable) =>
observable.MapAsync(async s => await s.FirstOrDefaultAsync());
/// <summary>
/// Emits the first element of a sequence that satisfies a specified condition.
/// </summary>
/// <typeparam name="TSource">The type of the value.</typeparam>
/// <param name="observable">The parent observable.</param>
/// <param name="predicate">A function to test each element for a condition.</param>
/// <returns>An instance of <see cref="IProviderObservable{TSource}"/>.</returns>
public static IProviderObservable <TSource> FirstAsync <TSource>(
this IProviderObservable <IQueryable <TSource> > observable,
Expression <Func <TSource, bool> > predicate) =>
observable.MapAsync(async s => await s.FirstAsync(predicate));
/// <summary>
/// Emits the only element of a sequence, and throws an exception
/// if there is not exactly one element in the sequence.
/// </summary>
/// <typeparam name="TSource">The type of the value.</typeparam>
/// <param name="observable">The parent observable.</param>
/// <returns>An instance of <see cref="IProviderObservable{TSource}"/>.</returns>
public static IProviderObservable <TSource> SingleAsync <TSource>(
this IProviderObservable <IQueryable <TSource> > observable) =>
observable.MapAsync(async s => await s.SingleAsync());
