Print ANTLR4 tree (Kotlin)

print_antlr4_tree.kt

import org.antlr.v4.runtime.ParserRuleContext
import org.antlr.v4.runtime.tree.ParseTree
import org.antlr.v4.runtime.tree.TerminalNode

fun printSyntaxTree(root: ParserRuleContext): String {
    fun recursive(aRoot: ParseTree, buf: StringBuilder, offset: Int) {
        buf.append("  ".repeat(offset))
        if (aRoot is TerminalNode) {
            buf.appendln("'${aRoot.text}'")
        } else {
            buf.appendln(aRoot.javaClass.simpleName)
        }
        if (aRoot is ParserRuleContext) {
            if (aRoot.children != null) {
                for (child in aRoot.children) {
                    recursive(child, buf, offset + 1)
                }
            }
        }
    }
    val buf = StringBuilder()
    recursive(root, buf, 0)
    return buf.toString()
}

// Usage:
// ParseTree tree = parser.compilationUnit();
// System.out.println(printSyntaxTree(tree));