-type optional(T) :: T | undefined.

-type state_t() ::
          #state{socket :: partisan_peer_socket:t(),
                 peer_node :: node(),
                 channel :: partisan:channel(),
                 ref :: reference(),
                 ping_idle_timeout :: optional(non_neg_integer()),
                 ping_tref :: optional(reference()),
                 ping_retry :: optional(partisan_retry:t()),
                 ping_id :: optional(partisan:any_reference())}.