Melbourne Compose Group is the monthly in-person meetup for functional programmers in Melbourne, which this month, is on the second Thursday of the month. Our November meeting will be 6:00-8:00 pm Thurs 12th of November at our regular venue, Activity Room 2 at Kathleen Syme Center in Carlton. Arrive from 5:30 for chat and socialising, talks start at 6:30pm. Please [RSVP via Luma](https://luma.com/i1j0ygwy). # Tristan Mc Leay - Mutation Testing With Sydtest Now that use of AI code generation tools has become prevalent, we need a way to keep our agents honest. Is our code well tested? Have we skipped tests? Are the tests sensible or meaningless? All of these could have been problems before AI, but they take on an increased significance when we can produce bigger changes than we might be able to genuinely concentrate on. Mutation testing is a way of checking our test quality as well as our test coverage: instead of just asking whether a line ran during testing, it asks whether changes to that line would have caused the tests to fail. In the case of a typed functional language, that might be swapping an operator, dropping a case, hard-coding a boolean instead of an if or a guard. If the tests continue to pass with a change in place, then the test suite is usually insufficiently sensitive to changes. Sydtest includes a mutation testing framework for GHC, implemented as a GHC plugin alongside driver code. It was designed precisely for this problem - as our coding switches to AI, we need AI to look at our mutation test results to know what to do next. In this presentation, I will take a tour of Sydtest's mutation testing and apply it to a code base. This code base has poor test coverage and was originally hand written but nowadays is essentially agent-coded. As always, newcomers welcome. Hope to see you there :) -Ben Hutchison & John Walker