Functional Kotlin

Monad transformers

Either and Option are simple to use, but what happens if we combine both?

object UserService {     fun findAge(user: String): Either<String, Option<Int>> {       //Magic     }}

UserService.findAge returns Either<String, Option<Int>>; Left<String> for errors accessing the database or any other infrastructure, Right<None> for no value found on the database, and Right<Some<Int>> for a value found:

import arrow.core.*import arrow.syntax.function.pipefun main(args: Array<String>) { val anakinAge: Either<String, Option<Int>> = UserService.findAge("Anakin") anakinAge.fold(::identity, { op ->         op.fold({ "Not found" }, Int::toString)     }) pipe ::println }

To print an age, we need two nested folds, nothing too complicated. Problems arrive when ...