Ohana trees and Taylor expansion for the $λ$I-calculus. No variable gets left behind or forgotten!

Although the $λ$I-calculus is a natural fragment of the $λ$-calculus, obtained by forbidding the erasure, its equational theories did not receive much attention. The reason is that all proper denotational models studied in the literature equate all non-normalizable $λ$I-terms, whence the associated theory is not very informative. The goal of this paper is to introduce a previously unknown theory of the $λ$I-calculus, induced by a notion of evaluation trees that we call "Ohana trees". The Ohana tree of a $λ$I-term is an annotated version of its Böhm tree, remembering all free variables that are hidden within its meaningless subtrees, or pushed into infinity along its infinite branches. We develop the associated theories of program approximation: the first approach -- more classic -- is based on finite trees and continuity, the second adapts Ehrhard and Regnier's Taylor expansion. We then prove a Commutation Theorem stating that the normal form of the Taylor expansion of a $λ$I-term coincides with the Taylor expansion of its Ohana tree. As a corollary, we obtain that the equality induced by Ohana trees is compatible with abstraction and application. We conclude by discussing the cases of Lévy-Longo and Berarducci trees, and generalizations to the full $λ$-calculus.