/// <summary>
/// Returns a <see cref="IEither{T,TError}" /> when awaited.
/// </summary>
/// <param name="source">
/// The <see cref="IAsyncEither{T,TError}" />.
/// </param>
/// <typeparam name="T">
/// of <see cref="IAsyncEither{T,TError}" />.
/// The <typeparamref name="T" />
/// </typeparam>
/// <typeparam name="TError">
/// The <typeparamref name="TError" /> of <see cref="IAsyncEither{T,TError}" />.
/// </typeparam>
/// <returns></returns>
public static TaskAwaiter <IEither <T, TError> > GetAwaiter <T, TError>(this IAsyncEither <T, TError> source) =>
source?.ToTaskEither().GetAwaiter() ?? throw new ArgumentNullException(nameof(source));
internal static async Task <IEither <T, TError> > ToTaskEither <T, TError>(this IAsyncEither <T, TError> asyncEither)
=> await asyncEither.HasValue.ConfigureAwait(false)
? Result.Value <T, TError>(asyncEither.Value).Either
: Result.Error <T, TError>(asyncEither.Error).Either;