Resource-Bounded Type Theory: Compositional Cost Analysis via Graded Modalities

Abstract: We present a compositional framework for certifying resource bounds in typed programs. Terms are typed with synthesized bounds drawn from an abstract resource lattice, enabling uniform treatment of time, memory, gas, and domain-specific costs. We introduce a graded feasibility modality with co-unit and monotonicity laws. Our main result is a syntactic cost soundness theorem for the recursion-free simply-typed fragment: if a closed term has synthesized bound b under a given budget, its operational cost is bounded by b. We provide a syntactic term model in the topos of presheaves over the lattice -- where resource bounds index a cost-stratified family of definable values -- with cost extraction as a natural transformation. We prove canonical forms via reification and establish initiality of the syntactic model: it embeds uniquely into all resource-bounded models. A case study demonstrates compositional reasoning for binary search using Lean's native recursion with separate bound proofs.