Exceptionally Safe Futures

A future is a well-known programming construct used to introduce concurrency to sequential programs. Computations annotated as futures are executed asynchronously and run concurrently with their continuations. Typically, futures are not transparent annotations: a program with futures need not produce the same result as the sequential program from which it was derived. Safe futures guarantee a future-annotated program produce the same result as its sequential counterpart. Ensuring safety is especially challenging in the presence of constructs such as exceptions that permit the expression of non-local control-flow. For example, a future may raise an exception whose handler is in its continuation. To ensure safety, we must guarantee the continuation does not discard this handler regardless of the continuation’s own internal control-flow (e.g. exceptions it raises or futures it spawns). In this paper, we present a formulation of safe futures for a higher-order functional language with first-class exceptions. Safety can be guaranteed dynamically by stalling the execution of a continuation that has an exception handler potentially required by its future until the future completes. To enable greater concurrency, we develop a static analysis and instrumentation and formalize the runtime behavior for instrumented programs that allows execution to discard handlers precisely when it is safe to do so.