Hoare-style program logics and abstract interpretation are two of the principal frameworks for reasoning about programs, and their affinity has long been recognised informally: both derive over-approximate judgements about program behaviour by structural, syntax-directed reasoning. Turning this analogy into a formal correspondence runs into two challenges. First, the disjunction (or join) rule, sound and admissible in Hoare logic, becomes unsound over non-additive abstract semantics. Second, the extrapolation operators that make abstract interpretation effective on unbounded computations, i.e. widening and narrowing, have no analogue in the derivations of existing program logics. We introduce APPL (Abstract Program Property Logic), a Hoare-style program logic that provides a proof-theoretic foundation for abstract interpretation while addressing both challenges. The key technical idea is density: abstract semantics is additive over dense families of logical assertions. This leads to a density-guarded join rule that enables sound disjunctive reasoning over arbitrary abstract domains. We also internalise the application of widening and narrowing through inference rules for iterative commands. As a result, an entire analysis by abstract interpretation can be viewed as a logical derivation in APPL. The logic is sound over any abstract domain and, when the assertion language is expressive enough, it is also relatively complete. Finally, by interpreting nondeterminism through a monoidal operator, APPL provides a single logical framework that uniformly subsumes Hoare logic, incorrectness logic, and Hoare-style logics for hyperproperties, thereby enabling their abstraction. Remarkably, the density-guarded join rule introduced to handle non-additive abstract semantics is also fundamental in guaranteeing completeness for hyperproperties.

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