Plugin Interface Proposal Example for Kotlin

PluginInterface.kt

// Basic setup

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

val exp0 = Add(Lit(42), Lit(0x52))

// Adding a print operation

plugin interface Print { fun print(): String }

abstract instance Exp: Print

instance Lit: Print {
    override fun print() = x.toString()
}
instance Add: Print {
    override fun print() = "${x.print()} + ${y.print()}"
}

// Adding new data variants

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

// Making the data variants printable

instance Bool: Print {
    override fun print() = x.toString()
}
instance Iff: Print {
    override fun print() = "if (${x.print()}) then ${y.print()} else ${z.print()}"
}