Turning JSON into URLQueryParams

params.swift

func f(_ jsonValue: Any, prefix: String? = nil) throws -> [URLQueryItem] {
  var items: [URLQueryItem] = []
  
  switch jsonValue {
  case let d as Dictionary<String, Any>:
    try d.forEach {
      let newPrefix = prefix == nil ? $0.key : prefix! + "[\($0.key)]"
      items.append(contentsOf: try f($0.value, prefix: newPrefix))
    }
    
  case let a as Array<Any>:
    guard let somePrefix = prefix else {
      throw(ParamsError.invalidTopLevelType)
    }
    try a.forEach { it in
      items.append(contentsOf: try f(it, prefix: somePrefix + "[]"))
    }
    
  default:
    guard let somePrefix = prefix else {
      throw(ParamsError.invalidTopLevelType)
    }
    items.append(URLQueryItem(name: somePrefix, value: String(describing: jsonValue)))
  }
  
  return items
}