Alternative semantics for McCarthy's amb
McCarthy's ambiguous operator amb is a locally angelic choice operator. A description of its semantics using power domains is much more complex than similar descriptions of globally angelic choice or erratic choice. Operational semantics based on rewrite rules are used here to compare the three kinds of choice. A natural problem shows erratic choice to be less expressive than the angelic forms of choice. Erratic choice and globally angelic choice share a pleasant distribution property that has been proposed as the basis for an algebra of nondeterministic programs. Locally angelic choice does not share this property. Locally angelic choice may play a role in otherwise globally angelic nondeterministic programs similar to the role played by the cut operator in Prolog.
KeywordsOperational Semantic Program Scheme Evaluation Rule Choice Operator Erratic Choice
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