Freeze After Writing Quasi-Deterministic Parallel Programming with LVars and Handlers

Deterministic-by-construction parallel programming models offer programmers the promise of freedom from subtle, hard-toreproduce nondeterministic bugs in parallel code. A principled approach to deterministic-by-construction parallel programming with shared state is offered by LVars: shared memory locations whose semantics are defined in terms of a user-specified lattice. Writes to an LVar take the least upper bound of the old and new values with respect to the lattice, while reads from an LVar can observe only that its contents have crossed a specified threshold in the lattice. Although it guarantees determinism, this interface is quite limited. We extend LVars in two ways. First, we add the ability to “freeze” and then read the contents of an LVar directly. Second, we add the ability to attach callback functions to an LVar, allowing events to be triggered by writes to it. Together, callbacks and freezing enable an expressive and useful style of parallel programming. We prove that in a language where communication takes place through freezable LVars, programs are at worst quasi-deterministic: on every run, they either produce the same answer or raise an error. We demonstrate the viability of our approach by implementing a library for Haskell supporting a variety of LVar-based data structures, together with two case studies that illustrate the programming model and yield promising parallel speedup.