Ah… It sounds like you are asking about Haskell-style typeclasses? There’s no equivalent in Kotlin, unfortunately. There are type aliases… so you could do something like

typealias TransformerFn = (ByteArray) -> ByteArray

then

fun userTimer(t: TransformerFn): TransformerFn { ... }

instead of writing out

(ByteArray) -> ByteArray

repeatedly.