Object Algebras in Kotlin

ObjectAlgebra.kt

// Basic setup

interface Exp
data class Lit(val x: Int): Exp
data class Add(val x: Exp, val y: Exp): Exp

interface IntAlg<A>
{
    fun lit(x: Int): A
    fun add(x: A, y: A): A
}

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

fun <A> exp(v: IntAlg<A>) = v.add(v.lit(42), v.lit(0x52))
val exp0: Exp = exp(IntFactory())

// Adding a print operation

interface Print { fun print(): String }

// Can I haz SAM-conversion plz?
fun Print(f: () -> String) = object: Print { override fun print() = f() }

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

val exp1: Print = exp(IntPrint())

// Adding new data variants

data class Bool(val x: Boolean): Exp
data class Iff(val x: Exp, val y: Exp, val z: Exp): Exp

interface IntBoolAlg<A>: IntAlg<A>
{
    fun bool(b: Boolean): A
    fun iff(x: A, y: A, z: A): A
}

open class IntBoolFactory: IntFactory(), IntBoolAlg<Exp>
{
    override fun bool(x: Boolean) = Bool(x)
    override fun iff(x: Exp, y: Exp, z: Exp) = Iff(x, y, z)
}

fun <A> exp2(v: IntBoolAlg<A>) = v.iff(v.bool(false), exp(v), v.lit(0))
val exp2: Exp = exp2(IntBoolFactory())

// Making the data variants printable

open class IntBoolPrint: IntPrint(), IntBoolAlg<Print>
{
    override fun bool(b: Boolean) = Print { b.toString() }
    override fun iff(x: Print, y: Print, z: Print)
        = Print { "if (${x.print()}) then ${y.print()} else ${z.print()}" }
}

val exp3: Print = exp2(IntBoolPrint())