10 research outputs found
Normalisation by evaluation for dependent types
We develop normalisation by evaluation (NBE) for dependent types based on presheaf categories. Our construction is formulated using internal type theory using quotient inductive types. We use a typed presentation hence there are no preterms or realizers in our construction. NBE for simple types is using a logical relation between the syntax and the presheaf interpretation. In our construction, we merge the presheaf interpretation and the logical relation into a proof-relevant logical predicate. We have formalized parts of the construction in Agda
Combinatory Logic and Lambda Calculus Are Equal, Algebraically
It is well-known that extensional lambda calculus is equivalent to extensional combinatory logic. In this paper we describe a formalisation of this fact in Cubical Agda. The distinguishing features of our formalisation are the following: (i) Both languages are defined as generalised algebraic theories, the syntaxes are intrinsically typed and quotiented by conversion; we never mention preterms or break the quotients in our construction. (ii) Typing is a parameter, thus the un(i)typed and simply typed variants are special cases of the same proof. (iii) We define syntaxes as quotient inductive-inductive types (QIITs) in Cubical Agda; we prove the equivalence and (via univalence) the equality of these QIITs; we do not rely on any axioms, the conversion functions all compute and can be experimented with
Normalisation by evaluation for type theory, in type theory
We develop normalisation by evaluation (NBE) for dependent types based on presheaf categories. Our construction is formulated in the metalanguage of type theory using quotient inductive types. We use a typed presentation hence there are no preterms or realizers in our construction, and every construction respects the conversion relation. NBE for simple types uses a logical relation between the syntax and the presheaf interpretation. In our construction, we merge the presheaf interpretation and the logical relation into a proof-relevant logical predicate. We prove normalisation, completeness, stability and decidability of definitional equality. Most of the constructions were formalized in Agda
Cubical Syntax for Reflection-Free Extensional Equality
We contribute XTT, a cubical reconstruction of Observational Type Theory
which extends Martin-L\"of's intensional type theory with a dependent equality
type that enjoys function extensionality and a judgmental version of the
unicity of identity types principle (UIP): any two elements of the same
equality type are judgmentally equal. Moreover, we conjecture that the typing
relation can be decided in a practical way. In this paper, we establish an
algebraic canonicity theorem using a novel cubical extension (independently
proposed by Awodey) of the logical families or categorical gluing argument
inspired by Coquand and Shulman: every closed element of boolean type is
derivably equal to either 'true' or 'false'.Comment: Extended version; International Conference on Formal Structures for
Computation and Deduction (FSCD), 201
Multimodal Dependent Type Theory
We introduce MTT, a dependent type theory which supports multiple modalities.
MTT is parametrized by a mode theory which specifies a collection of modes,
modalities, and transformations between them. We show that different choices of
mode theory allow us to use the same type theory to compute and reason in many
modal situations, including guarded recursion, axiomatic cohesion, and
parametric quantification. We reproduce examples from prior work in guarded
recursion and axiomatic cohesion, thereby demonstrating that MTT constitutes a
simple and usable syntax whose instantiations intuitively correspond to
previous handcrafted modal type theories. In some cases, instantiating MTT to a
particular situation unearths a previously unknown type theory that improves
upon prior systems. Finally, we investigate the metatheory of MTT. We prove the
consistency of MTT and establish canonicity through an extension of recent
type-theoretic gluing techniques. These results hold irrespective of the choice
of mode theory, and thus apply to a wide variety of modal situations
Logical Predicates in Higher-Order Mathematical Operational Semantics
We present a systematic approach to logical predicates based on universal
coalgebra and higher-order abstract GSOS, thus making a first step towards a
unifying theory of logical relations. We first observe that logical predicates
are special cases of coalgebraic invariants on mixed-variance functors. We then
introduce the notion of a locally maximal logical refinement of a given
predicate, with a view to enabling inductive reasoning, and identify sufficient
conditions on the overall setup in which locally maximal logical refinements
canonically exist. Finally, we develop induction-up-to techniques that simplify
inductive proofs via logical predicates on systems encoded as (certain classes
of) higher-order GSOS laws by identifying and abstracting away from their
boiler-plate part.Comment: Extended versio
Multimodal Dependent Type Theory
We introduce MTT, a dependent type theory which supports multiple modalities.
MTT is parametrized by a mode theory which specifies a collection of modes,
modalities, and transformations between them. We show that different choices of
mode theory allow us to use the same type theory to compute and reason in many
modal situations, including guarded recursion, axiomatic cohesion, and
parametric quantification. We reproduce examples from prior work in guarded
recursion and axiomatic cohesion, thereby demonstrating that MTT constitutes a
simple and usable syntax whose instantiations intuitively correspond to
previous handcrafted modal type theories. In some cases, instantiating MTT to a
particular situation unearths a previously unknown type theory that improves
upon prior systems. Finally, we investigate the metatheory of MTT. We prove the
consistency of MTT and establish canonicity through an extension of recent
type-theoretic gluing techniques. These results hold irrespective of the choice
of mode theory, and thus apply to a wide variety of modal situations
A Cubical Language for Bishop Sets
We present XTT, a version of Cartesian cubical type theory specialized for
Bishop sets \`a la Coquand, in which every type enjoys a definitional version
of the uniqueness of identity proofs. Using cubical notions, XTT reconstructs
many of the ideas underlying Observational Type Theory, a version of
intensional type theory that supports function extensionality. We prove the
canonicity property of XTT (that every closed boolean is definitionally equal
to a constant) using Artin gluing