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We present a definition and implementation of TE that supports ML-style references and nested synchronizations, both of which were previously unnecessary due to Haskell's more restrictive type system. As in prior work, we have a high-level semantics that makes nondeterministic choices such that synchronizations succeed whenever possible and a low-level semantics that uses search to implement the high-level semantics soundly and completely. The key design trade-off in the semantics is to allow updates to mutable memory without requiring the implementation to consider all possible thread interleavings. Our solution uses first-class heaps and allows interleavings only when a message is sent or received. We have used Coq to prove our high- and low-level semantics equivalent.
We have implemented our approach by modifying the Objective Caml run-time system. By modifying the run-time system, rather than relying solely on a library, we can eliminate the potential for nonterminating computations within unsuccessful synchronizations to run forever.
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