context(raise: Raise<E>)
inline fun <E, A> catchingWhen(toError: (Throwable) -> E?, block: () -> A): A =
  catch(block) { e -> raise.raise(toError(e) ?: throw e) }

// use
val id = catchingWhen({ e ->
  when (e) {
    is SQLIntegrityConstraintViolationException -> DuplicateUser(email)
    is SQLTimeoutException -> StoreUnavailable
    else -> null            // unexpected: stays an exception
  }
}) { insertUser(email) }