Efficient Well-Definedness Checking

Formal specifications often contain partial functions that may lead to ill-defined terms. A common technique to eliminate ill-defined terms is to require well-definedness conditions to be proven. The main advantage of this technique is that it allows us to reason in a two-valued logic even if the underlying specification language has a three-valued semantics. Current approaches generate well-definedness conditions that grow exponentially with respect to the input formula. As a result, many tools prove shorter, but stronger approximations of these well-definedness conditions instead. We present a procedure which generates well-definedness conditions that grow linearly with respect to the input formula. The procedure has been implemented in the Spec# verification tool. We also present empirical results that demonstrate the improvements made.