Step-Indexed Relational Reasoning for Countable Nondeterminism

by Lars Birkedal and Aleš Bizjak and Jan Schwinghammer

Programming languages with countable nondeterministic choice are computationally interesting since countable nondeterminism arises when modeling fairness for concurrent systems. Because countable choice introduces non-continuous behaviour, it is well-known that developing semantic models for programming languages with countable nondeterminism is challenging. We present a step-indexed logical relations model of a higher-order functional programming language with countable nondeterminism and demonstrate how it can be used to reason about contextually defined may- and must-equivalence. In earlier step-indexed models, the indices have been drawn from ω. Here the step-indexed relations for must-equivalence are indexed over an ordinal greater than ω.

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