If you want to treat your error result as a first class value, and transport it around, then your sum type can't use the same implementation as exceptions, whic

If you want to treat your error result as a first class value, and transport it around, then your sum type can't use the same implementation as exceptions, which can use data-driven stack unwinding to have zero cost in the success case, the data being generated by the compiler and consumed by a stack unwinder after it has been invoked by an exception raise.