Positive Inductive-Recursive Definitions

TL;DR

This paper introduces positive inductive-recursive definitions, a generalized theory of data types that extends previous work by lifting restrictions and providing new interpretations and applications.

Contribution

It extends inductive-recursive definitions to non-discrete categories, introduces codes and morphisms for these types, and applies the theory to nested data types and recursive functions.

Findings

Defined codes and morphisms for positive inductive-recursive types

Interpreted codes as functors on Fam(C) and morphisms as natural transformations

Applied the theory to nested data types and recursive functions

Abstract

A new theory of data types which allows for the definition of types as initial algebras of certain functors Fam(C) -> Fam(C) is presented. This theory, which we call positive inductive-recursive definitions, is a generalisation of Dybjer and Setzer's theory of inductive-recursive definitions within which C had to be discrete -- our work can therefore be seen as lifting this restriction. This is a substantial endeavour as we need to not only introduce a type of codes for such data types (as in Dybjer and Setzer's work), but also a type of morphisms between such codes (which was not needed in Dybjer and Setzer's development). We show how these codes are interpreted as functors on Fam(C) and how these morphisms of codes are interpreted as natural transformations between such functors. We then give an application of positive inductive-recursive definitions to the theory of nested data types and we give concrete examples of recursive functions defined on universes by using their elimination principle. Finally we justify the existence of positive inductive-recursive definitions by adapting Dybjer and Setzer's set-theoretic model to our setting.