Identification of an adequate invariant is valuablefor reasoning about the correctness of code involving a loop, informally or formally. Almost every modern system for automated verification demands that programmers annotate their code with assertions, such as invariants to facilitate automation. But many learners struggle to grasp how to arrive at an assertion that remains an invariant and is sufficiently strong to prove subsequent assertions reliant on the outcome of the loop. The objective of this research is to present a method to help understand the difficulties students face in developing suitable loop invariants, and assist them in the process. We describe results from an experimentation in a software engineering classroom where students were charged with developing verified component-based code using a web-based front end for a verifying compiler. We collected data in the background as students attempted to produce verified code with loop invariants in in-class activities and take-home projects.Initial results show what kinds of information we can expect to see and what kinds of feedback might be useful.

This paper summarizes our experience in communicating the elements of reasoning about correctness, and the central role of formal specifications in reasoning about modular, component-based software using a language and an integrated Web IDE designed for the purpose. Our experience in using such an IDE, supported by a 'push-button' verifying compiler in a classroom setting, reveals the highly iterative process learners use to arrive at suitably specified, automatically provable code. We explain how the IDE facilitates reasoning at each step of this process by providing human readable verification conditions (VCs) and feedback from an integrated prover that clearly indicates unprovable VCs to help identify obstacles to completing proofs. The paper discusses the IDE's usage in verified software development using several examples drawn from actual classroom lectures and student assignments to illustrate principles of design-by-contract and the iterative process of creating and subsequently refining assertions, such as loop invariants in object-based code.Comment: In Proceedings F-IDE 2015, arXiv:1508.0338

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