OPINIONS: The Gemara relates that the Reish Galusa had an Avarneka in his orchard. An Avarneka is either a shady tree under which people sit on benches and relax, or a covered area built in the middle of an aristocrat's orchard. He wanted to be able to carry food to it from his home, through the orchard, on Shabbos. Rav Huna bar Chinena set up poles in a certain arrangement in the orchard in order to make it usable on Shabbos. Rava, however, removed them.
How did Rav Huna bar Chinena intend to use poles to permit carrying in the orchard, and why did Rava object?
(a) RASHI, in his first explanation, writes that an Avarneka is not an area of real residential use (Hukaf l'Dirah); people do not live there, they only relax there. Rav Huna bar Chinena placed poles in the ground in the form of an enclosed area to represent a small room in which a guard could live (i.e., a Dirah). His intent was to make the entire orchard Hukaf l'Dirah, for now its outer wall included a residential area as well as an orchard.
Rava maintained that the poles did not make the orchard Hukaf l'Dirah, for several reasons. First, that small area became Hukaf l'Dirah only after the orchard was enclosed by a fence. Second, as a temporary enclosure, it was not made as a permanent dwelling. Third, its primary function was to protect objects placed in it, and not to be used as a residence.
According to this explanation, Rav Huna bar Chinena erred in the Halachos of Hukaf l'Dirah when he erected poles in his attempt to permit carrying in the orchard.
(b) In his second approach, Rashi suggests that since people stay in the Avarneka for extended periods of time, it must be a room of some sort (and not just a shaded area, as in Rashi's first explanation). Rav Huna bar Chinena maintained that since the Avarneka was built after the orchard was enclosed, the Avarneka does not make the entire orchard considered Hukaf l'Dirah. Therefore, he placed poles in the arrangement of a path from the house to the Avarneka. This created a fenced-off area smaller than Beis Se'asayim, in which one is permitted to carry.
Rava maintained that the presence of the Avarneka made the entire area in front of it (i.e., the orchard) Hukaf l'Dirah. Therefore, the poles were unnecessary. The Gemara relates that the next day, Amora'im (including those who had originally sided with Rava) proved to Rava that even though the Avarneka was built for residential purposes, it was unable to make the entire orchard Hukaf l'Dirah because it was built after the orchard was completed.
(It is not clear why Rava himself did not reason this way. Perhaps he assumed that since the orchard was subordinate to the Avarneka, its status should be determined by the status of the Avarneka. The Avarneka itself was Hukaf l'Dirah, because its walls were built after the Avarneka was designated for Dirah. Even though the walls of the orchard were present before the Avarneka, Rava maintained that the entire orchard was subordinate to the Avarneka. Therefore, the orchard was also considered Hukaf l'Dirah, and it was not comparable to the usual case of "Hukaf uleva'Sof Yashav.")
The Gemara concludes that the outer Mechitzos of the orchard must be erected for the sake of residential purposes in order for the area to be considered Hukaf l'Dirah.
(c) RABEINU NESANEL (cited by Tosfos, the Ritva, and others) also suggests that Rava made a mistake and Rav Huna bar Chinena was correct. He explains that Rav Huna bar Chinena set a row of poles along the entire length of one wall of the orchard, in order to re-enclose the orchard after the Avarneka was built. This act made the entire orchard into an area that is Hukaf l'Dirah in which it is permitted to carry.
(Why did Rava mistakenly assume that they could rely on the Avarneka? Tosfos suggests that Rava maintained that the only time that "Hukaf uleva'Sof Yashav" does not make an area Hukaf l'Dirah is when a wall fell down and allowed passage between the house and the orchard. Since both the house and the orchard were there before passage to the orchard was facilitated, the orchard is not considered Hukaf l'Dirah as a result of the wall becoming breached. However, in the case of the Avarneka, an entirely new house (the Avarneka) was built after the wall of the orchard was erected. Rava thought that the area could be considered Hukaf l'Dirah once a new house is built there, even if it is built after the orchard's surrounding wall.)