Defined in The verb object Call signatures Return value A monadic type of the same kind On the operation is rejected an has no error value to map Use to act on the empty state instead Map the error of the expected keeping the carrier s shape The operation that narrows a graded error set over a graded copack error side the matching is exhaustive so the branches may map diverse alternatives into one common type the grade collapses to its singular copack or into a narrower copack Rejected on optional which has no error value to map on the identity expected it is vacuous well formed with the callback neither invoked nor instantiated Use through the fn transform_error nielbloid transform_error_t transform_error 1 Maps the error staying in the carrier x | transform_error f constexpr auto operator auto &&fn const > functor<transform_error_t decltype fn > 1 Map the error of the expected keeping the carrier s shape return A functor that will execute the function on the error fn auto && The function to execute on the error optional optional or_else