Version 2 of Flattenin' Your Mappenin'

gistfile1.swift

//
// Version 2 of pagesFromData from Flattenin' Your Mappenin'
// http://robnapier.net/flatmap
//

import Foundation

infix operator >>== {}
func >>== <T,U>(x: T, f:T -> Result<U>) -> Result<U> {
  return x.flatMap(f)
}

func pagesFromData(data: NSData) -> Result<[Page]> {
  return asJSON(data)  >>== asJSONArray
    >>== secondElement >>== asStringList
    >>== asPages
}

func asJSON(data: NSData) -> Result<JSON> {
  var error: NSError?
  let json: AnyObject? = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions(0), error: &error)

  switch (json, error) {
  case (_, .Some(let error)): return .Failure(error)

  case (.Some(let json), _): return .Success(Box(json))

  default:
    fatalError("Received neither JSON nor an error")
    return .Failure(NSError())
  }
}

func asJSONArray(json: JSON) -> Result<JSONArray> {
  if let array = json as? JSONArray {
    return .Success(Box(array))
  } else {
    return .Failure(NSError(localizedDescription: "Expected array. Got: \(json)"))
  }
}

func secondElement(array: JSONArray) -> Result<JSON> {
  if array.count < 2 {
    return .Failure(NSError(localizedDescription:"Could not get second element. Array too short: \(array.count)"))
  }
  return .Success(Box(array[1]))
}

func asStringList(array: JSON) -> Result<[String]> {
  if let string = array as? [String] {
    return .Success(Box(string))
  } else {
    return .Failure(NSError(localizedDescription: "Unexpected string list: \(array)"))
  }
}

func asPages(titles: [String]) -> Result<[Page]> {
  return .Success(Box(titles.map { Page(title: $0) }))
}

enum Result<A> {
  case Success(Box<A>)
  case Failure(NSError)

  func flatMap<B>(f:A -> Result<B>) -> Result<B> {
    switch self {
    case Success(let value): return f(value.unbox)
    case Failure(let error): return .Failure(error)
    }
  }
}
final class Box<T> {
  let unbox: T
  init(_ value: T) { self.unbox = value }
}

extension Result: Printable {
  var description: String {
    switch self {
    case .Success(let box):
      return "Success: \(box.unbox)"
    case .Failure(let error):
      return "Failure: \(error.localizedDescription)"
      }
  }
}

struct Page {
  let title: String
}

extension Page: Printable {
  var description: String {
    return title
  }
}

typealias JSON = AnyObject
typealias JSONArray = [JSON]

extension NSError {
  convenience init(localizedDescription: String) {
    self.init(domain: "", code: 0, userInfo: [NSLocalizedDescriptionKey: localizedDescription])
  }
}

func asJSONData(string: NSString) -> NSData {
  return string.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
}

let goodPagesJson = asJSONData("[\"a\",[\"Animal\",\"Association football\",\"Arthropod\",\"Australia\",\"AllMusic\",\"African American (U.S. Census)\",\"Album\",\"Angiosperms\",\"Actor\",\"American football\",\"Austria\",\"Argentina\",\"American Civil War\",\"Administrative divisions of Iran\",\"Alternative rock\"]]")

pagesFromData(goodPagesJson).description

let corruptJson = asJSONData("a\",[\"Animal\",\"Association football\",\"Arthropod\",\"Australia\",\"AllMusic\",\"African American (U.S. Census)\",\"Album\",\"Angiosperms\",\"Actor\",\"American football\",\"Austria\",\"Argentina\",\"American Civil War\",\"Administrative divisions of Iran\",\"Alternative rock\"]]")

pagesFromData(corruptJson).description