GhettoParser.kt

fun <A> A.toJson(): String {

    fun <A> Iterable<A>.toJsonArray(): String =
        joinToString(", ", "[", "]") { it.toJson() }

    fun <A, B> Map<A, B>.toJsonObject() =
        entries.joinToString(", ", "{", "}") { (key, value) -> "\"$key\" : ${value.toJson()}" }

    return when(this) {
        null -> "null"
        is String -> "\"$this\""
        is Array<*> -> this.toList().toJsonArray()
        is Iterable<*> -> this.toJsonArray()
        is Map<*, *> -> this.toJsonObject()
        else -> toString()
    }

}

Usage.kt

val json = mapOf("users" to names.toList().map {
    mapOf(
        "name" to "No name",
        "id" to it,
        "avatar_url" to "https://gravatar.com/img/2124"
    )
}).toJson()