Normalization By Evaluation Research Articles

Normalization by evaluation for modal dependent type theory

Abstract We present the Kripke-style modal type theory, Mint, which combines dependent types and the necessity modality. It extends the Kripke-style modal lambda-calculus by Pfenning and Davies to the full Martin-Löf type theory. As such it encompasses dependently typed variants of system K, T, K4, and S4. Further, Mint seamlessly supports a full universe hierarchy, usual inductive types, and large eliminations. In this paper, we give a modular sound and complete normalization-by-evaluation (NbE) proof for Mint based on an untyped domain model, which applies to all four aforementioned modal systems without modification. This NbE proof yields a normalization algorithm for Mint, which can be directly implemented. To further strengthen our results, our models and the NbE proof are fully mechanized in Agda and we extract a Haskell implementation of our NbE algorithm from it.

Normalization for fitch-style modal calculi

Fitch-style modal lambda calculi enable programming with necessity modalities in a typed lambda calculus by extending the typing context with a delimiting operator that is denoted by a lock. The addition of locks simplifies the formulation of typing rules for calculi that incorporate different modal axioms, but each variant demands different, tedious and seemingly ad hoc syntactic lemmas to prove normalization. In this work, we take a semantic approach to normalization, called normalization by evaluation (NbE), by leveraging the possible-world semantics of Fitch-style calculi to yield a more modular approach to normalization. We show that NbE models can be constructed for calculi that incorporate the K, T and 4 axioms of modal logic, as suitable instantiations of the possible-world semantics. In addition to existing results that handle 𝛽-equivalence, our normalization result also considers 𝜂-equivalence for these calculi. Our key results have been mechanized in the proof assistant Agda. Finally, we showcase several consequences of normalization for proving meta-theoretic properties of Fitch-style calculi as well as programming-language applications based on different interpretations of the necessity modality.

Normalization by Evaluation for the Lambek Calculus

The syntactic calculus of Lambek is a deductive system for the multiplicative fragment of intuitionistic non-commutative linear logic. As a fine-grained calculus of resources, it has many applications, mostly in formal computational investigations of natural languages. This paper introduces a calculus of beta-eta-long normal forms for derivations in the Lambek calculus with multiplicative unit and conjunction among its logical connectives. Reduction to normal form follows the normalization by evaluation (NbE) strategy: (i) evaluate a derivation in a Kripke model of Lambek calculus; (ii) extract normal forms from the obtained semantic values. The implementation of the NbE algorithm requires the presence of a strong monad in the Kripke interpretation of positive formulae, in analogy with the extension of intuitionistic propositional logic with falsity and disjunction. The NbE algorithm can also be instantiated with minor variations to calculi for related substructural logics, such as multiplicative and dual intuitionistic linear logic (MILL and DILL).