Better Feedback for Incorrect Programs by Using Contracts

Abstract

Ask-Elle is a programming tutor that aims to help students learn the Haskell programming language. It does this by providing programming exercises and assisting the student during the process. The tutor can automatically provide hints to a solution when the student gets stuck or give counterexamples for incorrect submissions. It is these incorrect programs that we are interested in. The goal of this thesis is to improve the detail of feedback by automatically monitoring contracts on parts of the program. When running it with the counter example, the tutor should be able to explain more clearly what the mistake might be. We analyze a previous approach that mixes type inference with contracts and find that this is not always correct. We then formalize the problem statement and define a criterion for sound ways of monitoring an incorrect program. Finally, we have implemented the system in Haskell.