Defined in The verb object Call signatures Return value A monadic type of the same kind Observe the error for side effects passing the operand through unchanged The callback receives the error as const and must return void on optional it is invoked with no arguments the empty state carries no error value Rejected on choice and just which have no error side on the identity expected it is vacuous the operand passes through and the callback is never instantiated Use through the fn inspect_error nielbloid inspect_error_t inspect_error 1 Observes the error in passing x | inspect_error f constexpr auto operator auto &&fn const > functor<inspect_error_t decltype fn > 1 Observe the error for side effects passing the operand through unchanged return A functor that will execute the function on the error fn auto && The function to observe the error invoked with no arguments on optional