An interpretation of system F through bar recursion

TL;DR

This paper introduces a novel translation of system F into a language with bar recursion, enabling a new perspective on the computational interpretation of polymorphism and second-order arithmetic.

Contribution

It provides the first translation of system F into a simply-typed language with bar recursion, using realizability and compositional methods.

Findings

Bound on reduction steps for system F terms via realizability

Translation is compositional and relies on induction on typing derivations

Potential for a new approach to studying polymorphism through bar recursion

Abstract

There are two possible computational interpretations of second-order arithmetic: Girard's system F or Spector's bar recursion and its variants. While the logic is the same, the programs obtained from these two interpretations have a fundamentally different computational behavior and their relationship is not well understood. We make a step towards a comparison by defining the first translation of system F into a simply-typed total language with a variant of bar recursion. This translation relies on a realizability interpretation of second-order arithmetic. Due to G\"odel's incompleteness theorem there is no proof of termination of system F within second-order arithmetic. However, for each individual term of system F there is a proof in second-order arithmetic that it terminates, with its realizability interpretation providing a bound on the number of reduction steps to reach a normal form. Using this bound, we compute the normal form through primitive recursion. Moreover, since the normalization proof of system F proceeds by induction on typing derivations, the translation is compositional. The flexibility of our method opens the possibility of getting a more direct translation that will provide an alternative approach to the study of polymorphism, namely through bar recursion.