Hallel Of The Megillah

The Gemarah Megillah (14a) has three reasons why there is no hallel said on Purim.  The reason the Rambam codifies (3:6) is the reason of Rav Nachman that the reading of the Megillah is the fulfillment of hallel.  The simple read of the Gemarah is that there technically hallel should be said but is is fulfilled via reading the Megillah.  This is fact is the opinion of the Meiri that holds if one is not hearing the Megillah of Purim for some reason, then they must recite hallel.  This leads to questions discussed in the Achronim such as if there should be hallel said on cities that read on the 15^th when that falls out on Shabbos and the Megillah is not read on Shabbos like this year or why we don't have to stand throughout the Megillah like when reading hallel (see Chasam Sofer siman 51 and article by Rav Zvi Ryzmanץ) 

Both the Rebbe (Likutay Sichos volume 36) and Rav Solevetchik (Mesorah journal volume 18) have a different take on this Gemarah.  They explain that the Gemarah means not that there is a din to say hallel on Purim and it is fulfilled through the Megillah but the only correct form of hallel on Purim is reading the megillah.  The explanation is that we only say hallel on an overt, open miracles.  The Purim story did not contain any jaw dropping miraculous events and it is only through the Megillah that connects the dots for us that we can glimpse the mircale that took place.  קריאתה זו הילולא means that the only only hallel experience that is applicable on Purim is that of the Megillah which allows us to acknowledge the covert miracles that took place.  

The opinion of Tosfos Megillah (4a) is that even though the beracha of shechiyanu is said on the night time reading of the Megillah, it is repeated again before the day time reading for the main obligation is the day time reading.  The Rambam (1:3, see Magid Mishna) disagrees and holds there is no new shechiyanu in the day.  Rav Zolti (Mishnas Yaavetz siman 77) says the Rambam agrees to the assertion of Tosfos that the main reading is in the day time but nonetheless disagrees with the conclusion of Tosfos that this should require a new shechiyanu blessing.  His explanation of this idea is that the advantage of the daytime reading over the night time reading is that it is also a fulfilment of the law of hallel.  However, since the essence of the obligation of the reading of the Megillah is the same, the shechiyanu on the night time reading covers the day time reading as the added element of hallel does not obligate shechiyanu for we never say shechiyanu on hallel.  [In light of his explanation, Tosfos would have to hold the fact that there is an added element to the reading of the daytime defines the daytime reading as a different type of reading and that enables a shechiyanu or Tosfos holds there are other advantages to the daytime reading.]  Rav Solevetchik however, uses his approach to explain the opinion of Tosfos.  Since the reading of the Megillah is not in place of hallel but rather a unique form of hallel expressed only through the Megillah, that form of krias megillah deserves its own shechiyanu.