Defunctionalized interpreters for programming languages

Abstract

This document illustrates how functional implementations of formal semantics (structural operational semantics, reduction semantics, small-step and big-step abstract machines, natural semantics, and denotational semantics) can be transformed into each other. These transformations were foreshadowed by Reynolds in "Definitional Interpreters for Higher-Order Programming Languages" for functional implementations of denotational semantics, natural semantics, and big-step abstract machines using closure conversion, CPS transformation, and defunctionalization. Over the last few years, the author and his students have further observed that functional implementations of small-step and of big-step abstract machines are related using fusion by fixed-point promotion and that functional implementations of reduction semantics and of small-step abstract machines are related using refocusing and transition compression. It furthermore appears that functional implementations of structural operational semantics and of reduction semantics are related as well, also using CPS transformation and defunctionalization. This further relation provides an element of answer to Felleisen's conjecture that any structural operational semantics can be expressed as a reduction semantics: for deterministic languages, a reduction semantics is a structural operational semantics in continuation style, where the reduction context is a defunctionalized continuation. As the defunctionalized counterpart of the continuation of a one-step reduction function, a reduction context represents the rest of the reduction, just as an evaluation context represents the rest of the evaluation since it is the defunctionalized counterpart of the continuation of an evaluation function.