Representing Nonterminating Rewriting with $\mathbf{F}_2^μ$

Abstract: We specify a second-order type system $\mathbf{F}_2^\mu$ that is tailored for representing nonterminations. The nonterminating trace of a term $t$ in a rewrite system $\mathcal{R}$ corresponds to a productive inhabitant $e$ such that $\Gamma_{\mathcal{R}} \vdash e : t$ in $\mathbf{F}_2^\mu$, where $\Gamma_{\mathcal{R}}$ is the environment representing the rewrite system. We prove that the productivity checking in $\mathbf{F}_2^\mu$ is decidable via a mapping to the $\lambda$-Y calculus. We develop a type checking algorithm for $\mathbf{F}_2^\mu$ based on second-order matching. We implement the type checking algorithm in a proof-of-concept type checker.