public static async Task <Maybe <T> > WhereAsync <T>(
this Maybe <T> @this,
Func <T, Task <bool> > predicate)
{
if (predicate is null)
{
throw new Anexn(nameof(predicate));
}
return(await @this.BindAsync(__).ConfigureAwait(false));
async Task <Maybe <T> > __(T x)
=> await predicate(x).ConfigureAwait(false) ? @this : Maybe <T> .None;
}
