TypesafeEval.kt

sealed class Expression<A> {
    abstract val value: A
}
data class Num(override val value: Int) : Expression<Int>()
data class Bool(override val value: Boolean) : Expression<Boolean>()
data class Add(val a: Expression<Int>, val b: Expression<Int>) : Expression<Int>() {
    override val value: Int get() = a.value + b.value
}
data class Equals<A>(val first: Expression<A>, val second: Expression<A>) : Expression<Boolean>() {
    override val value: Boolean get() = first.value == second.value
}

fun main() {
    val exp = Equals(
        Equals(
            Num(3),
            Add(
                Num(1),
                Num(2)
            )
        ),
        Bool(true)
    )
    println(exp.value)
}