Defined in The verb object Call signatures Return value A monadic type of the same kind Observe the value for side effects passing the operand through unchanged The callback receives the value as const and must return void observation can neither mutate the operand nor replace it Where the carrier can be empty or hold an error the callback runs only when a value is present on choice and just it always runs over an uninhabited value side it is neither invoked nor instantiated Use through the fn inspect nielbloid inspect_t inspect 1 Observes the value in passing x | inspect f constexpr auto operator auto &&fn const > functor<inspect_t decltype fn > 1 Observe the value for side effects passing the operand through unchanged return A functor that will execute the function on the value fn auto && The function to observe the value invoked with no arguments where the value is void