Object algebra approach to the Expression problem in Kotlin

ObjectAlgebra.kt

/**
 * Object algebra approach to the Expression problem in Kotlin.
 *
 * See also <http://lambda-the-ultimate.org/node/4572>.
 */

// Cannot put this fun inside main() in Kotlin M2
fun exp2plus3<A>(f: IntAlg<A>): A = f.add(f.lit(2), f.lit(3))

fun main(args : Array<String>) {
    println("${exp2plus3(IntPrint()).print()} = ${exp2plus3(IntFactory()).eval()}")
}

trait Exp {
    fun eval(): Int
}

class Lit(val x: Int) : Exp {
    override fun eval() = x
}

class Add(val e1: Exp, val e2: Exp) : Exp {
    override fun eval() = e1.eval() + e2.eval()
}

trait IntAlg<A> {
    fun lit(x: Int): A
    fun add(e1: A, e2: A): A
}

class IntFactory : IntAlg<Exp> {
    override fun lit(x: Int) = Lit(x)
    override fun add(e1: Exp, e2: Exp) = Add(e1, e2)
}

trait Print {
    fun print(): String
}

class IntPrint : IntAlg<Print> {
    override fun add(e1: Print, e2: Print) = object : Print {
        override fun print() = "${e1.print()} + ${e2.print()}"
    }
    override fun lit(x: Int) = object : Print {
        override fun print() = x.toString()
    }
}