FLACK: Localizing Faults in Alloy Models

Fault localization can help developers identify buggy statements or expressions in programs. Existing fault localization techniques are often designed for imperative programs (e.g., C and Java) and rely on tests to compare correct and incorrect execution traces to identify suspicious statements. In this demo paper, we present FLACK, a tool to automatically locate faults for models written in Alloy, a declarative language where the models are not executed but instead converted into a logical formula and solved using a SAT solver. FLACK takes as input an Alloy model that violates some assertions and returns a ranked list of suspicious expressions contributing to the violation. The key idea is to analyze the differences between counterexamples, i.e., instances of the model that do not satisfy the assertion and instances that do satisfy the assertion to find suspicious expressions in the input model. An experiment with 157 Alloy models with various bugs shows the efficiency and accuracy of FLACK in localizing the causes of these bugs. FLACK and its evaluation benchmark and results can be downloaded from https://github.com/guolong-zheng/flack. The video demonstration is available at https://youtu.be/FKa2ohqIUms.