Gradualizing the Calculus of Inductive Constructions (CIC) involves dealing with subtle tensions between normalization, graduality, and conservativity with respect to CIC. Recently, GCIC has been proposed as a parametrized gradual type theory that admits three variants, each sacrificing one of these properties. For devising a gradual proof assistant based on CIC, normalization and conservativity with respect to CIC are key, but the tension with graduality needs to be addressed. Additionally, several challenges remain: (1) The presence of two wildcard terms at any type-the error and unknown terms-enables trivial proofs of any theorem, jeopardizing the use of a gradual type theory in a proof assistant; (2) Supporting general indexed inductive families, most prominently equality, is an open problem; (3) Theoretical accounts of gradual typing and graduality so far do not support handling type mismatches detected during reduction; (4) Precision and graduality are external notions not amenable to reasoning within a gradual type theory. All these issues manifest primally in CastCIC, the cast calculus used to define GCIC. In this work, we present an alternative to CastCIC called GRIP. GRIP is a reasonably gradual type theory that addresses the issues above, featuring internal precision and general exception handling. GRIP features an impure (gradual) sort of types inhabited by errors and unknown terms, and a pure (non-gradual) sort of strict propositions for consistent reasoning about gradual terms. By adopting a novel interpretation of the unknown term that carefully accounts for universe levels, GRIP satisfies graduality for a large and well-defined class of terms, in addition to being normalizing and a conservative extension of CIC. Internal precision supports reasoning about graduality within GRIP itself, for instance to characterize gradual exception-handling terms, and supports gradual subset types. We develop the metatheory of GRIP using a model formalized in Coq, and provide a prototype implementation of GRIP in Agda.
Mon 12 SepDisplayed time zone: Belgrade, Bratislava, Budapest, Ljubljana, Prague change
15:50 - 16:50 | |||
15:50 20mTalk | Propositional Equality for Gradual Dependently Typed Programming ICFP Papers and Events Joseph Eremondi University of British Columbia, Ronald Garcia University of British Columbia, Éric Tanter University of Chile DOI | ||
16:10 20mTalk | A Reasonably Gradual Type Theory ICFP Papers and Events Kenji Maillard Inria Nantes & University of Chile, Meven Lennon-Bertrand Inria – LS2N, Université de Nantes, Nicolas Tabareau Inria, Éric Tanter University of Chile DOI | ||
16:30 20mTalk | A completely unique account of enumeration ICFP Papers and Events DOI |