Kotlin 如何使用 Either monad 并避免嵌套 flatMap

我正在try 设置一些数据，然后调用服务.每一步都可能失败，所以我try 使用Arrow's来管理这一点.

但我最终得到了很多嵌套平面图.

下面的代码片段说明了我要做的事情:

import arrow.core.Either
import arrow.core.flatMap

typealias ErrorResponse = String
typealias SuccessResponse = String

data class Foo(val userId: Int, val orderId: Int, val otherField: String)
data class User(val userId: Int, val username: String)
data class Order(val orderId: Int, val otherField: String)

interface MyService {
    fun doSomething(foo: Foo, user: User, order: Order): Either<ErrorResponse, SuccessResponse> {
        return Either.Right("ok")
    }
}

fun parseJson(raw: String): Either<ErrorResponse, Foo> = TODO()
fun lookupUser(userId: Int): Either<ErrorResponse, User> = TODO()
fun lookupOrder(orderId: Int): Either<ErrorResponse, Order> = TODO()

fun start(rawData: String, myService: MyService): Either<ErrorResponse, SuccessResponse> {
    val foo = parseJson(rawData)
    val user = foo.flatMap {
        lookupUser(it.userId)
    }
    //I want to lookupOrder only when foo and lookupUser are successful
    val order = user.flatMap {
        foo.flatMap { lookupOrder(it.orderId) }
    }
    //Only when all 3 are successful, call the service
    return foo.flatMap { f ->
        user.flatMap { u ->
            order.flatMap { o ->
                myService.doSomething(f, u, o)
            }
        }
    }
}

我相信有更好的方法.有人能帮我一个惯用的方法吗？