Control discipline necessity: Making the language as general as the implementation

Abstract

The formal verification of control structure implementation techniques is a subject receiving increased attention among programming language researchers. Such correctness arguments relate source language control semantics to corresponding implementation effects. Most commonly, it is demonstrated that a particular implementation strategy α is “safe” is some source language control disciplineδ is observed. We term this asufficiency proof forδ with given α. In this paper, the companion issue of thenecessity ofδ is explored. That is, we consider the question of whether some more liberal disciplineδ might also be sufficient for α. The motivation is the observation that language designers often have particular control implementation strategies in minda priori, but at times over-restrict their final designs to ensure that the envisioned implementation remains applicable. The general notion of control discipline necessity proofs is illustrated by a proof that Wang and Dahl's CS condition on block-structured coroutines is necessary, given a simple scope-based deletion strategy, but that their CR condition is not necessary unless scope-based referencing constraints are taken into account.