Read the "Domain Modeling Made Functional" book, but did the code examples in Kotlin (and Arrow-kt where appropriate). I've published the repo at https://gitlab.com/mikezx6r/domain-functional. For the

Result

, I've used

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as

BIO

isn't quite available.

BIO

appears close, so I may use the 0.11.0-SNAPSHOT of Arrow, and add a version using that. Regardless, if you have any comments, feedback, or questions on the code, please let me know. Certainly increases my desire to use a strictly typed domain for writing code. You'd still want tests (of which I only have a few in this repo), but it simplifies testing. Mocking becomes providing a Lambda that conforms to the signature and returns the value you want. No complicated (and slow) mocking frameworks. Simple DI in the real code, explicit business case error vs exceptional situation separation. I see many benefits to this approach.

I never should have read that book. It made me quite jealous of certain F# features :(

I'm curious. Which features in particular? I've been able to implement the concepts in the book using Kotlin, and Arrow-kt quite cleanly, so I'm wondering what I missed.

Mostly really nice pattern matching. Also designing with types in Kotlin still needs a lot more boilerplate, even with data classes.

Agree on pattern matching, but as I've never really worked with a language that supported it, don't really notice. And it's coming to Java, so it should filter to Kotlin... Of course still a few years away. Hmm, yes, a bit more boilerplate, but I was surprised how close the Kotlin code is to the F# code.