kotlin typeclasses

kotlin-typeclasses.kt

object Fintech5 {
    data class ValidationError(val message: String)

    data class CreatePortfolio(val userId: String, val amount: Double)
    data class ChangePortfolio(val userId: String, val stock: String, val quantity: Int)

    interface Validator<T> {
        fun T.check(): EitherNel<ValidationError, T>
    }

    val createPortfolioValidator = object : Validator<CreatePortfolio> {
        override fun CreatePortfolio.check(): EitherNel<ValidationError, CreatePortfolio>  =
            EitherNel.zipOrAccumulate(
                if (!Storage.users.contains(userId)) Either.Left(ValidationError("Error: user $userId does not exist"))
                else Either.Right(this),

                if (amount < 0) Either.Left(ValidationError("Error: amount is negative"))
                else Either.Right(this)
            ) { a, _ -> a }
    }

    val changePortfolioValidator = object : Validator<ChangePortfolio> {
        override fun ChangePortfolio.check(): EitherNel<ValidationError, ChangePortfolio> =
            EitherNel.zipOrAccumulate(
                if (!Storage.users.contains(userId)) Either.Left(ValidationError("Error: user $userId does not exist"))
                else Either.Right(this),
                if (!Storage.stocks.contains(stock)) Either.Left(ValidationError("Error: user $stock is invalid"))
                else Either.Right(this)
            ) { a, _ -> a }
    }

    context(Validator<T>)
    fun <T> process(event: T) = either {
        val validated: T = event.check().bind()
        println("Successfully processed the event: $validated")
    }.onLeft { errors -> println("Found validation errors: ${errors.map { it.message }}") }

    @JvmStatic
    fun main(args: Array<String>) {
        with(createPortfolioValidator) {
            process(CreatePortfolio("asdf", -123.0))
            process(CreatePortfolio("user-123", 123.0))
        }

        with(changePortfolioValidator) {
            process(ChangePortfolio("asdf", "qwer", -12))
            process(ChangePortfolio("user-444","TSLA", 12))
        }
    }
}