Map a functions to all Strings in circe

mapStrings.scala

import io.circe._
import io.circe.syntax._  

def mapStrings(json: Json)(f: String => String): Json = json.fold(
  json,  // null
  _ => json,  // boolean
  _ => json, // number
  str => f(str).asJson,
  arr => arr.map(mapStrings(_)(f)).asJson,
  obj => obj.mapValues(mapStrings(_)(f)).asJson
)