/// <summary></summary>
public static Either <L, T> NullToLeft <T, L>(
this Optional <T> self,
Func <L> nothing)
=> self.IfPresent(
o => Either <L, T> .Right(o),
() => Either <L, T> .Left(nothing()));
/// <summary></summary> public static Either <L, R> Return <L, R>(L left, TypeMarker <R> _) => Either <L, R> .Left(left);
/// <summary>LとRを交換</summary>
public static Either <R, L> Swap <R, L>(this Either <L, R> self)
=> self.IfRight(
r => Either <R, L> .Left(r),
l => Either <R, L> .Right(l));
/// <summary></summary> public static Either <L, R> Return <L, R>(L left) => Either <L, R> .Left(left);