Introducing the Reader Monad - Functional Kotlin

Duncan refactors Kotlin code to use the Reader monad as an alternative functional approach for providing transaction context.

In our last episode (youtu.be/uZdKRSkiWjc) we looked at using Kotlin context receivers to provide transactions to functions that need them. Uberto Barbini's upcoming book, From Objects to Functions (pragprog.com/titles/uboop/from-objects-to-functions/) details a more traditional functional approach to providing context - the Reader Monad. We would like to understand more about this approach, so in this episode we backtrack a little, refactoring code that takes a transaction as a parameter to instead return a RequiresTransaction type, and then refactor that to a Reader monad. This is part 64 of an exploration of where a Test Driven Development implementation of the Gilded Rose stock control system might take us in Kotlin. You can see the whole series as a playlist youtube.com/playlist?list=PL1ssMPpyqociJNwykAOB9_KEZVW7BW7m2 and the code on GitHub - this particular experiment is on a branch github.com/dmcg/gilded-rose-tdd/tree/plain-reader-monad There is another version (youtu.be/mjVL1xQ7oPc) of this topic that derives Reader from context receivers code. It probably makes less sense, especially if you haven't internalised context receivers; but if you've come here from a future where context receivers are commonplace, who knows, it might be a better bet. If you like this, you’ll probably like my book Java to Kotlin, A Refactoring Guidebook (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. I have some free time between producing videos and working for team Gilded Rose. If you like these videos I'd like to work with you - please get in touch - duncan@oneeyedmen.com