Using human-in-the-loop synthesis to author functional reactive programs

Programs that respond to asynchronous events are challenging to write; they are difficult to reason about and tricky to test and debug. Because these programs can have a huge space of possible input timings and interleaving, the programmer may easily miss corner cases. We propose applying synthesis to aid programmers in creating programs more easily and with a higher degree of confidence in their correctness. We have written an efficient encoding of functional reactive programming (FRP) semantics based on functional programming over lists lifted in Rosette. We demonstrate that this technique is state-of-the-art by first comparing its performance against two existing synthesis tools that produce list manipulation programs, and then by synthesizing a suite of benchmarks given complete specifications. We also propose an interactive tool in which a programmer provides some initial partial specification in the form of input/output examples or invariants; the tool finds ambiguity in the specification by synthesizing two candidate programs and gives the user an input that distinguishes them; the user updates the specification and continues iterating until the correct program is found. As evaluation, we demonstrate the use of the tool on a suite of benchmarks from the web programming and Internet of Things domains and walk through a sample interaction on a realistic web benchmark, showing that we can converge on the target program with a tractable number of interactions. As future work, we discuss encoding additional FRP languages to in order to explore metalinguistic features, strategies for decomposition that would allow the synthesis of larger programs, and improved programmer tools such as a GUI to more easily elicit specifications.