This paper provides a new mathematical foundation for the locally nameless representation of syntax with binders, one informed by nominal techniques. It gives an equational axiomatization of two key locally nameless operations, "variable opening" and "variable closing" and shows that a lot of the locally nameless infrastructure can be defined from that in a syntax-independent way, including crucially a "shift" functor for name binding. That functor operates on a category whose objects we call locally nameless sets . Functors combining shift with sums and products have initial algebras that recover the usual locally nameless representation of syntax with binders in the finitary case. We demonstrate this by uniformly constructing such an initial locally nameless set for each instance of Plotkin's notion of binding signature. We also show by example that the shift functor is useful for locally nameless sets of a semantic rather than a syntactic character. The category of locally nameless sets is proved to be isomorphic to a known topos of finitely supported M-sets, where M is the full transformation monoid on a countably infinite set. A corollary of the proof is that several categories that have been used in the literature to model variable renaming operations on syntax with binders are all equivalent to each other and to the category of locally nameless sets.

This paper introduces an expressive class of indexed quotient-inductive types, called QWI types, within the framework of constructive type theory. They are initial algebras for indexed families of equational theories with possibly infinitary operators and equations. We prove that QWI types can be derived from quotient types and inductive types in the type theory of toposes with natural number object and universes, provided those universes satisfy the Weakly Initial Set of Covers (WISC) axiom. We do so by constructing QWI types as colimits of a family of approximations to them defined by well-founded recursion over a suitable notion of size, whose definition involves the WISC axiom. We developed the proof and checked it using the Agda theorem prover.

Abstract This paper introduces an expressive class of quotient-inductive types, called QW-types. We show that in dependent type theory with uniqueness of identity proofs, even the infinitary case of QW-types can be encoded using the combination of inductive-inductive definitions involving strictly positive occurrences of Hofmann-style quotient types, and Abel’s size types. The latter, which provide a convenient constructive abstraction of what classically would be accomplished with transfinite ordinals, are used to prove termination of the recursive definitions of the elimination and computation properties of our encoding of QW-types. The development is formalized using the Agda theorem prover.

Constructs that involve taking a quotient are commonplace in mathematics. Here I will consider how they are treated within dependent type theory. The notion of quotient type has its origins in the Nuprl theorem-proving system [R. L. Constable et al., 1986] for extensional type theory. Later Hofmann formulated a version for intensional type theory in his thesis [M. Hofmann, 1995]. This depends on having a pre-existing notion of intensional identity type. Hofmann used Martin-Löf’s notion, the indexed family inductively generated from proofs of reflexivity [B. Nordström et al., 1990]. The recent homotopical view of identity in terms of path types [The Univalent Foundations Program, 2013] gives a more liberal perspective and has brought with it the notion of higher inductive type (HIT) [P. L. Lumsdaine and M. Shulman, 2019], subsuming both inductive and quotient types. Inductively defined indexed families of types (in all their various forms) are perhaps the most useful concept that dependent type theory has contributed to the practice of computer assistance for formalizing mathematical proofs. However, it is often the case that a particular application of such types needs not only to inductively generate a collection of objects, but also to make identifications between the objects. In classical mathematics one can first generate and then identify, using the Axiom of Choice to lift infinitary constructions to the quotient. HITs can allow one to avoid such non-constructive uses of choice by inter-twining generation and identification. Perhaps more important than the constructive/non-constructive issue is that simultaneously declaring how to generate and how to identify can be a very natural way of defining some construct from the user’s point of view. This is why HITs promise to be so useful, once we have robust and convenient mechanisms in theorem-proving systems for defining HITs and defining functions out of HITs. Although some HITs have been axiomatized in various systems, at the moment the only system I know of with built in support for defining quite general forms of HIT and using them is the implementation of cubical type theory [C. Cohen et al., 2018] within recent versions of the Agda system [A. Vezzosi et al., 2019]. The higher dimensional aspect of identity in cubical type theory is fascinating; nevertheless, the simpler one-dimensional version of identity, in which one has uniqueness of identity proofs (UIP), is adequate for many applications. Although some regard UIP as a bug of early versions of Agda with it’s original form of dependent pattern matching [T. Coquand, 1992], it is by choice a feature of the Lean prover [J. Avigad et al., 2020]. Altenkirch and Kaposi [T. Altenkirch and A. Kaposi, 2016] have termed the one-dimensional version of HITs quotient inductive types (QITs) and they promise to be very useful even in the setting of type theory with UIP. In this talk I survey some of these developments, including a recent reduction of QITs to quotients [M. P. Fiore et al., 2020], and the prospects for better support in theorem-provers for quotient constructions.

Abstract In recent years, we have seen several new models of dependent type theory extended with some form of modal necessity operator, including nominal type theory, guarded and clocked type theory and spatial and cohesive type theory. In this paper, we study modal dependent type theory: dependent type theory with an operator satisfying (a dependent version of) the K axiom of modal logic. We investigate both semantics and syntax. For the semantics, we introduce categories with families with a dependent right adjoint (CwDRA) and show that the examples above can be presented as such. Indeed, we show that any category with finite limits and an adjunction of endofunctors give rise to a CwDRA via the local universe construction. For the syntax, we introduce a dependently typed extension of Fitch-style modal λ-calculus, show that it can be interpreted in any CwDRA, and build a term model. We extend the syntax and semantics with universes.

The usual homogeneous form of equality type in Martin-Löf Type Theory contains identifications between elements of the same type. By contrast, the heterogeneous form of equality contains identifications between elements of possibly different types. This short note introduces a simple set of axioms for such types. The axioms are shown to be equivalent to the combination of systematic elimination rules for both forms of equality, albeit with typal (also known as “propositional”) computation properties, together with Streicher’s Axiom K, or equivalently, the principle of uniqueness of identity proofs.

The homotopical approach to intensional type theory views proofs of equality as paths. We explore what is required of an object $I$ in a topos to give such a path-based model of type theory in which paths are just functions with domain $I$. Cohen, Coquand, Huber and M\"ortberg give such a model using a particular category of presheaves. We investigate the extent to which their model construction can be expressed in the internal type theory of any topos and identify a collection of quite weak axioms for this purpose. This clarifies the definition and properties of the notion of uniform Kan filling that lies at the heart of their constructive interpretation of Voevodsky's univalence axiom. (This paper is a revised and expanded version of a paper of the same name that appeared in the proceedings of the 25th EACSL Annual Conference on Computer Science Logic, CSL 2016.)

We introduce Polinode, an online tool for performing network analysis. Polinode is aimed at commercial and non-commercial users alike and supports both research-related use cases as well as teaching network analysis to students. One of its primary advantages is that it is web-based. It therefore doesn't require any software downloads and opens up new avenues for collaboration and the incorporation of online content into network analysis. With Polinode you can upload arbitrary network data for online visualisation and analysis and you can also use the built-in relationship-based survey functionality to collect network data from respondents.

The homotopical approach to intensional type theory views proofs of equality as paths. We explore what is required of an interval-like object I in a topos to give a model of type theory in which elements of identity types are functions with domain I. Cohen, Coquand, Huber and Mörtberg give such a model using a particular category of presheaves. We investigate the extent to which their model construction can be expressed in the internal type theory of any topos and identify a collection of quite weak axioms for this purpose. This clarifies the definition and properties of the notion of uniform Kan filling that lies at the heart of their constructive interpretation of Voevodsky's univalence axiom. Furthermore, since our axioms can be satisfied in a number of different ways, we show that there is a range of topos-theoretic models of homotopy type theory in this style.

Nominal sets provide a mathematical theory for some of the key concepts that arise when representing and computing with data involving atomic (or 'pure') names: freshness, abstraction and scoping of names, and finiteness modulo symmetry. This tutorial introduces the notion of nominal set and explains selected applications of it to logic in computer science, to automata, languages and programming.

When defining computations over syntax as data, one often runs into tedious issues concerning α -equivalence and semantically correct manipulations of binding constructs. Here we study a semantic framework in which these issues can be dealt with automatically by the programming language. We take the user-friendly “nominal” approach in which bound objects are named. In particular, we develop a version of Scott domains within nominal sets and define two programming languages whose denotational semantics are based on those domains. The first language, λν -PCF, is an extension of Plotkin’s PCF with names that can be swapped, tested for equality and locally scoped; although simple, it already exposes most of the semantic subtleties of our approach. The second language, PNA, extends the first with name abstraction and concretion so that it can be used for metaprogramming over syntax with binders. For both languages, we prove a full abstraction result for nominal Scott domains analogous to Plotkin’s classic result about PCF and conventional Scott domains: two program phrases have the same observable operational behaviour in all contexts if and only if they denote equal elements of the nominal Scott domain model. This is the first full abstraction result we know of for languages combining higher-order functions with some form of locally scoped names which uses a domain theory based on ordinary extensional functions, rather than using the more intensional approach of game semantics. To obtain full abstraction, we need to add two functionals, one for existential quantification over names and one for “definite description” over names. Only adding one of them is not enough, as we give counter-examples to full abstraction in both cases.

We develop a domain theory within nominal sets and present programming language constructs and results that can be gained from this approach. The development is based on the concept of orbit-finite subset, that is, a subset of a nominal sets that is both finitely supported and contained in finitely many orbits. This concept appears prominently in the recent research programme of Bojanczyk et al. on automata over infinite languages, and our results establish a connection between their work and a characterisation of topological compactness discovered, in a quite different setting, by Winskel and Turner as part of a nominal domain theory for concurrency. We use this connection to derive a notion of Scott domain within nominal sets. The functionals for existential quantification over names and `definite description' over names turn out to be compact in the sense appropriate for nominal Scott domains. Adding them, together with parallel-or, to a programming language for recursively defined higher-order functions with name abstraction and locally scoped names, we prove a full abstraction result for nominal Scott domains analogous to Plotkin's classic result about PCF and conventional Scott domains: two program phrases have the same observable operational behaviour in all contexts if and only if they denote equal elements of the nominal Scott domain model. This is the first full abstraction result we know of for higher-order functions with local names that uses a domain theory based on ordinary extensional functions, rather than using the more intensional approach of game semantics.

Abstract Correct handling of names and binders is an important issue in meta-programming. This paper presents an embedding of constraint logic programming into the αML functional programming language, which provides a provably correct means of implementing proof search computations over inductive definitions involving names and binders modulo α-equivalence. We show that the execution of proof search in the αML operational semantics is sound and complete with regard to the model-theoretic semantics of formulae, and develop a theory of contextual equivalence for the subclass of αML expressions which correspond to inductive definitions and formulae. In particular, we prove that αML expressions, which denote inductive definitions, are contextually equivalent precisely when those inductive definitions have the same model-theoretic semantics. This paper is a revised and extended version of the conference paper (Lakin, M. R. & Pitts, A. M. (2009) Resolving inductive definitions with binders in higher-order typed functional programming. In Proceedings of the 18th European Symposium on Programming (ESOP 2009) , Castagna, G. (ed), Lecture Notes in Computer Science, vol. 5502. Berlin, Germany: Springer-Verlag, pp. 47–61) and draws on material from the first author's PhD thesis (Lakin, M. R. (2010) An Executable Meta-Language for Inductive Definitions with Binders . University of Cambridge, UK).

Abstract This paper introduces a new recursion principle for inductively defined data modulo α-equivalence of bound names that makes use of Odersky-style local names when recursing over bound names. It is formulated in simply typed λ-calculus extended with names that can be restricted to a lexical scope, tested for equality, explicitly swapped and abstracted. The new recursion principle is motivated by the nominal sets notion of ‘α-structural recursion’, whose use of names and associated freshness side-conditions in recursive definitions formalizes common practice with binders. The new calculus has a simple interpretation in nominal sets equipped with name-restriction operations. It is shown to adequately represent α-structural recursion while avoiding the need to verify freshness side-conditions in definitions and computations. The paper is a revised and expanded version of Pitts (Nominal System T. In Proceedings of the 37th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages, POPL 2010 (Madrid, Spain) . ACM Press, pp. 159–170, 2010).

This paper introduces a new recursion principle for inductive data modulo α-equivalence of bound names. It makes use of Odersky-style local names when recursing over bound names. It is formulated in an extension of Gödel's System T with names that can be tested for equality, explicitly swapped in expressions and restricted to a lexical scope. The new recursion principle is motivated by the nominal sets notion of "α-structural recursion", whose use of names and associated freshness side-conditions in recursive definitions formalizes common practice with binders. The new Nominal System T presented here provides a calculus of total functions that is shown to adequately represent α-structural recursion while avoiding the need to verify freshness side-conditions in definitions and computations. Adequacy is proved via a normalization-by-evaluation argument that makes use of a new semantics of local names in Gabbay-Pitts nominal sets.

This paper is concerned with the form of typed name binding used by the FreshML family of languages. Its characteristic feature is that a name binding is represented by an abstract (name,value)-pair that may only be deconstructed via the generation of fresh bound names. The paper proves a new result about what operations on names can co-exist with this construct. In FreshML the only observation one can make of names is to test whether or not they are equal. This restricted amount of observation was thought necessary to ensure that there is no observable difference between alpha-equivalent name binders. Yet from an algorithmic point of view it would be desirable to allow other operations and relations on names, such as a total ordering. This paper shows that, contrary to expectations, one may add not just ordering, but almost any relation or numerical function on names without disturbing the fundamental correctness result about this form of typed name binding (that object-level alpha-equivalence precisely corresponds to contextual equivalence at the programming meta-level), so long as one takes the state of dynamically created names into account.

The nominal approach to abstract syntax deals with the issues of bound names and α-equivalence by considering constructions and properties that are invariant with respect to permuting names. The use of permutations gives rise to an attractively simple formalization of common, but often technically incorrect uses of structural recursion and induction for abstract syntax modulo α-equivalence. At the heart of this approach is the notion of finitely supported mathematical objects. This article explains the idea in as concrete a way as possible and gives a new derivation within higher-order classical logic of principles of α- structural recursion and induction for α-equivalence classes from the ordinary versions of these principles for abstract syntax trees.

FreshML extends ML with elegant and practical constructs for declaring and manipulating syntactical data involving statically scoped binding operations. User-declared FreshML datatypes involving binders are concrete, in the sense that values of these types can be deconstructed by matching against patterns naming bound variables explicitly. This may have the computational effect of swapping bound names with freshly generated ones; previous work on FreshML used a complicated static type system inferring information about the 'freshness' of names for expressions in order to tame this effect. The main contribution of this paper is to show (perhaps surprisingly) that a standard type system without freshness inference, coupled with a conventional treatment of fresh name generation, suffices for FreshML's crucial correctness property that values of datatypes involving binders are operationally equivalent if and only if they represent a-equivalent pieces of object-level syntax. This is established via a novel denotational semantics. FreshML without static freshness inference is no more impure than ML and experience with it shows that it supports a programming style pleasingly close to informal practice when it comes to dealing with object-level syntax modulo a-equivalence.

Abstract. The permutation model of set theory with atoms (FM-sets), devised by Fraenkel and Mostowski in the 1930s, supports notions of ‘name-abstraction’ and ‘fresh name’ that provide a new way to represent, compute with, and reason about the syntax of formal systems involving variable-binding operations. Inductively defined FM-sets involving the name-abstraction set former (together with Cartesian product and disjoint union) can correctly encode syntax modulo renaming of bound variables. In this way, the standard theory of algebraic data types can be extended to encompass signatures involving binding operators. In particular, there is an associated notion of structural recursion for defining syntax-manipulating functions (such as capture avoiding substitution, set of free variables, etc.) and a notion of proof by structural induction, both of which remain pleasingly close to informal practice in computer science.

The notion of tripos (Hyland et al . 1980; Pitts 1981) was motivated by the desire to explain in what sense Higg's description of sheaf toposes as H -valued sets and Hyland's realizability toposes are instances of the same construction. The construction itself can be seen as the universal solution to the problem of realizing the predicates of a first order hyperdoctrine as subobjects in a logos with effective equivalence relations. In this note it is shown that the resulting logos is actually a topos if and only if the original hyperdoctrine satisfies a certain comprehension property. Triposes satisfy this property, but there are examples of non-triposes satisfying this form of comprehension.

Tell category theorists about the concept of abstract syntax for a language and they may say "that's just the initial algebra for a sum-of-products functor on the category of sets". Despite what you might think, they are trying to be helpful since the initiality property is the common denominator of both definitions by structural recursion and proofs by structural induction [5, Sect. 4.4]. In recent years we have learned how to extend this initial algebra view of abstract syntax to encompass languages with statically scoped binders. In the presence of such binders one wants to abstract away from the specific names of bound variables, either by quotienting parse trees by a suitable notion of alpha-equivalence, or by replacing conventional trees with ones containing de Bruijn indices [1]. By changing from the category of sets to other well-known, but still 'set-like' categories of sheaves or presheaves, one can regain an initial algebra view of this even more than normally abstract syntax---the pay-off being new and automatically generated forms of structural recursion and induction that respect alpha-equivalence [2, 3]. One good test of these new ideas is to see if they give rise to new forms of functional programming. In fact they do. The paper [6] sketches a functional programming language for representing and manipulating syntactical structure involving binders, based on the mathematical model of variable-binding in [3, 4]. In this ML-like language there are new forms of type for names and name-binding that come along with facilities for declaring fresh names, for binding names in abstractions and for pulling apart such name-abstractions via pattern-matching. The key idea is that properly abstract uses of names, i.e. ones that do not descend below the level of alpha-conversion, can be imposed on the user by a static type system that deduces information about the freshness of names. Even though we appear to be giving users a 'gensym' facility, the type system restricts the way it can be used to the extent that we keep within effect-free functional programming, in the sense that the usual laws of pure functional programming remain valid (augmented with new laws for names and name-abstractions). In this talk I will introduce this new approach to representing languages static binders in functional programming and discuss some of the difficulties we have had verifying its semantic properties.

The Fraenkel-Mostowski permutation model of set theory with atoms (FM-sets) can serve as the semantic basis of meta-logics for specifying and reasoning about formal systems involving name binding, /spl alpha/-conversion, capture avoiding substitution, and so on. We show that in FM-set theory one can express statements quantifying over 'fresh' names and we use this to give a novel set-theoretic interpretation of name abstraction. Inductively defined FM-sets involving this name abstraction set former (together with cartesian product and disjoint union) can correctly encode object-level syntax module e-conversion. In this way, the standard theory of algebraic data types can be extended to encompass signatures involving binding operators. In particular, there is an associated notion of structural recursion for defining syntax-manipulating functions (such as capture avoiding substitution, set of free variables, etc.) and a notion of proof by structural induction, both of which remain pleasingly close to informal practice.

A parametric logical relation between the phrases of an Algol-like language is presented. Its definition involves the structural operational semantics of the language, but was inspired by recent denotationally-based work of O'Hearn and Reynolds on translating Algol into a predicatively polymorphic linear lambda calculus. The logical relation yields an applicative characterisation of contextual equivalence for the language and provides a useful (and complete) method for proving equivalences. Its utility is illustrated by giving simple and direct proofs of some contextual equivalences, including an interesting equivalence due to O'Hearn which hinges upon the undefinability of 'snapback' operations (and which goes beyond the standard suite of 'Meyer-Sieber' examples). Whilst some of the mathematical intricacies of denotational semantics are avoided, the hard work in this operational approach lies in establishing the 'fundamental property' for the logical relation-the proof of which makes use of a compactness property of fixpoint recursion with respect to evaluation of phrases. But once this property has been established, the logical relation provides a verification method with an attractively low mathematical overhead.

A mixed induction/coinduction property of relations on recursively defined domains is described, working within a general framework for relations on domains and for actions of type constructors on relations introduced by P.W. O'Hearn and R.D. Tennent (1993), and drawing upon P.J. Freyd's analysis (1991) of recursive types in terms of a simultaneous initiality/finality property. The utility of the mixed induction/coinducton property is demonstrated by deriving a number of families of proof principles from it. One instance of the relational framework yields a family of induction principles for admissible subsets of general recursively defined domains which extends the principle of structural induction for inductively defined sets. Another instance of the framework yields the coinduction principle studied elsewhere by the author, by which equalities between elements of recursively defined domains may be proved via bisimulations.>

Abstract We prove the following surprising property of Heyting's intuitionistic propositional calculus, IpC. Consider the collection of formulas, ϕ, built up from propositional variables (p, q, r, …) and falsity (⊥) using conjunction (∧), disjunction (∨) and implication (→). Write ⊢ϕ to indicate that such a formula is intuitionistically valid. We show that for each variablepand formula ϕ there exists a formulaApϕ (effectively computable from ϕ), containing only variables not equal topwhich occur in ϕ, and such that for all formulas ψ not involvingp, ⊢ψ →Apϕ if and only if ⊢ψ → ϕ. Consequently quantification over propositional variables can be modelled in IpC, and there is an interpretation of the second order propositional calculus, IpC2, in IpC which restricts to the identity on first order propositions. An immediate corollary is the strengthening of the usual interpolation theorem for IpC to the statement that there are least and greatest interpolant formulas for any given pair of formulas. The result also has a number of interesting consequences for the algebraic counterpart of IpC, the theory of Heyting algebras. In particular we show that a model of IpC2can be constructed whose algebra of truth-values is equal to any given Heyting algebra.

A novel higher-order typed constructive predicate logic for fixpoint computations which exploits the categorical semantics of computations introduced by E. Moggi (1989) and contains a strong version of P. Martin-Lof's (1983) iteration type is introduced. The type system enforces a separation of computations from values. The logic contains a novel form of fixpoint induction and can express partial and total correctness statements about evaluation of computations to values. The constructive nature of the logic is witnessed by strong metalogical properties which are proved using a category-theoretic version of the logical relations method.>

A new, limitative relation between the polymorphic lambda calculus and the kind of higher-order type theory embodied in the logic of topoi is established. It is shown that any embedding in a topos of the Cartesian closed category of (closed) types of a model of the polymorphic lambda calculus must place the polymorphic types well away from the power types sigma to Omega of the topos, in the sense that sigma to Omega is a subtype of a polymorphic type only in the case that sigma is empty (and hence sigma to Omega is terminal). As corollaries, strengthening of the Reynolds result on the nonexistence of set-theoretic models of polymorphism are obtained.>