Is there a preferred way to approach companion function with a functional style ?

data class SomeData(val someValue: String) {
  companion object {
    fun someFunction() {}
  }
}

// OR

data class SomeData(val someValue: String) {
  companion object
}

fun SomeData.Companion.someFunction() {}

Not sure if this is related to FP 😅 Personally I would always put it in the

companion

itself when possible. I would say follow rule of thumb - simplest solution. It doesn't require an import that way (, and is better for IDEA).

Well I don’t really know where to put the limit of “try to have as much pure function as you can, avoid classes since it could hold internal variable that could turn your code in a non-pure way over time”. So this fall into this grey area for me where I’m not sure what to prefer

since it could hold internal variable that could turn your code in a non-pure way over time

I guess I'm more in the camp of "this should be caught at PR review time". But even then, this doesn't suffer from that problem since

companion object

is "static" so it doesn't have access to any "mutable state" that the class could potentially hold. I am also in favor of adding

fun toOther(): OtherDataClass

inside

SomeData

rather than an extension function for the same rationale of "not needing an import". Simple rule of thumb applies here:

data class

should only have `val`'s in the constructor, or pure functions inside the body or

companion object

.