Modular understanding of behaviors and flows of exceptions may help in their better use and handling. Such reasoning tasks about exceptions face unique challenges in event-based implicit invocation (II) languages that allow subjects to implicitly invoke observers, and run the observers in a chain. In this work, we illustrate these challenge in Ptolemy and propose Ptolemyχ that enables modular reasoning about behaviors and flows of exceptions for event announcement and handling. Ptolemyχ's exception-aware specification expressions and boundary exceptions limit the set of (un)checked exceptions of subjects and observers of an event. Exceptional postconditions specify the behaviors of these exceptions. Greybox specifications specify the flows of these exceptions among the observers in the chain. Ptolemyχ's type system and refinement rules enforce these specifications and thus enable its modular reasoning. We evaluate the utility of Ptolemyχ's exception flow reasoning by applying it to understand a set of aspect-oriented (AO) bug patterns. We also present Ptolemyχ's semantics including its sound static semantics.

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