Kotlin Design Pattern: IntrusiveVisitor

IntrusiveVisitor.kt

abstract class Expression {
    abstract fun print(sb: StringBuilder)
}

class DoubleExpression(private val value: Double) : Expression() {
    override fun print(sb: StringBuilder) {
        sb.append(value)
    }
}

class AdditionExpression(
    private val left: Expression,
    private val right: Expression
) : Expression() {
    override fun print(sb: StringBuilder) {
        sb.append("(")
        left.print(sb)
        sb.append("+")
        right.print(sb)
        sb.append(")")
    }
}

fun main() {
    val expression = AdditionExpression(
        left = DoubleExpression(1.0),
        right = AdditionExpression(
            left = DoubleExpression(2.0),
            right = DoubleExpression(3.0)
        )
    )
    val sb = StringBuilder()
    expression.print(sb)
    println(sb) // (1.0+(2.0+3.0))
}