  We notice that the type of catch :: c a -> (e -> c a) -> c a operator is a
special case of monadic bind operator (>>=) :: m a -> (a -> m b) -> m b, the
semantics (surprisingly) matches, and this observation has many interesting
consequences.
  For instance, the reader is probably aware that the monadic essence of the
(>>=) operator of the error monad $\lambda A.E \lor A$ is to behave like
identity monad for "normal" values and to stop on "errors". The unappreciated
fact is that handling of said "errors" with a catch operator of the "flipped"
"conjoined" error monad $\lambda E.E \lor A$ is, too, a monadic computation
that treats still unhandled "errors" as "normal" values and stops when an
"error" is finally handled.
  We show that for an appropriately indexed type of computations such a
"conjoined" structure naturally follows from the conventional operational
semantics of throw and catch operators. Consequently, we show that this
structure uniformly generalizes all conventional monadic error handling
mechanisms we are aware of. We also demonstrate several more interesting
instances of this structure of which at least bi-indexed monadic parser
combinators and conventional exceptions implemented via continuations have
immediate practical applications. Finally, we notice that these observations
provide surprising perspectives on error handling in general and point to a
largely unexplored trail in programming language design space.
