AI vs Human - Kotlin Refactoring Showdown
A key change that we can make to improve our code is to limit the scope of mutability - to program with calculations and values rather than actions. In contrast, the Kotlin code that the AI wrote for us to generate a Mermaid chart of test runtimes worked by recursively adding to a shared StringBuilder, making it hard to reason with and change.
Just for fun then, today we refactor the AI code to work with immutable lists, giving a simpler algorithm that I think better reflects the structure of the problem.
It was only having recorded this that I thought - I wonder if the AI could have fixed its own code? So I asked it, and it did.
I for one welcome our new AI overlords.
In this episode
00:00:45 Review the AI generated code
00:02:45 Convert to extension function
00:03:12 Extract a constant to the top level
00:04:16 Simplify output formatting
00:04:40 Un-nest the recursive function by passing it the state it mutates
00:07:49 Remove shared mutablity by giving functions their own mutable state
00:09:22 Now reduce mutablitity within the function
00:11:06 Revisit names when we change an action to a calculation
00:11:38 Indenting seems to be applied at the wrong level
00:12:30 Prefer List<String> to multi-line strings
00:15:12 Reduce the number of actions by calculating and batching
00:16:07 A change to the data model allows a better algorithm
00:17:24 Sealed interfaces make ad-hoc polymorphism work
00:18:08 How to handle optional things immutably
00:24:21 Now let the AI fix it's mess
00:29:04 Wrap up
There is a playlist of TDD Gilded Rose episodes - • Test Driven Gilded Rose in Kotlin
and one for testing • Testing
The codebase is available on GitHub https://github.com/dmcg/gilded-rose-tdd
If you are going to be at KotlinConf 2025, or even just in Copenhagen in May, then you should sign up for the workshop that Nat Pryce and I are running. It’s called Refactoring to Functional Kotlin, and will give you hands-on experience of taking legacy code and safely migrating it to a functional style. Places are limited, so buy now at https://kotlinconf.com/workhops
If you like this video, you’ll probably like my book Java to Kotlin, A Refactoring Guidebook (http://java-to-kotlin.dev). It's about far more than just the syntax differences between the languages - it shows how to upgrade your thinking to a more functional style.
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