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: ReaderWriterStateT[F, R, W, S, S] = ReaderWriterStateT((_, s) => F.point((W.zero, s, s))) def get = init def put(s: S): ReaderWriterStateT[F, R, W, S, Unit] = ReaderWriterStateT((r, _) => F.point((W.zero, (), s))) override def modify(f: S => S): ReaderWriterStateT[F, R, W, S, Unit] = ReaderWriterStateT((r, s) => F.point((W.zero, (), f(s)))) override def gets[A](f: S => A): ReaderWriterStateT[F, R, W, S, A] = ReaderWriterStateT((_, s) => F.point((W.zero, f(s), s))) def writer[A](w: W, v: A): ReaderWriterStateT[F, R, W, S, A] = ReaderWriterStateT((_, s) => F.point((w, v, s))) override def tell(w: W): ReaderWriterStateT[F, R, W, S, Unit] = ReaderWriterStateT((_, s) => F.point((w, (), s))) def listen[A](ma: ReaderWriterStateT[F, R, W, S, A]): ReaderWriterStateT[F, R, W, S, (A, W)] = ReaderWriterStateT((r, s) => F.map(ma.run(r, s)) { case (w, a, s1) => (w, (a, w), s1)}) }
我们示范用这个ReaderWriterState来写一段程序：模拟一段通讯端口使用程序并把使用情况记录下来。先传入一个端口号，在程序中可以重设使用的端口号：
1 val program: ReaderWriterState[Config, List[String], Int, Int] = for { 2 _ <- log("Start - r: %s, s: %s") 3 res <- invokeService 4 _ <- log("Between - r: %s, s: %s") 5 _ <- setService(8,"Com8") 6 _ <- invokeService 7 _ <- log("Done - r: %s, s: %s") 8 } yield res //> program : scalaz.RWS[Exercises.rws.Config,List[String],Int,Int] = scalaz.I 9 //| ndexedReaderWriterStateT$$anon$5@223191a6
这倒像是一段高级语言写的程序。细节都在几个功能函数里。它们都必须返回ReaderWriterState类型：
1 case class Config(var port: Int, var portName: String) 2 def log[R, S](msg: String): RWS[R, List[String], S, Unit] =
3 ReaderWriterState { 4 case (r, s) => (msg.format(r, s) :: Nil, (), s) //.point[Identity]
5 } //> log: [R, S](msg: String)scalaz.RWS[R,List[String],S,Unit]
6 def invokeService: ReaderWriterState[Config, List[String], Int, Int] =
7 ReaderWriterState { 8 case (cfg, invocationCount) => ( 9 List("Invoking service with port: " + cfg.portName), 10 scala.util.Random.nextInt(100), 11 invocationCount + 1
12 ) //.point[Identity]
13 } //> invokeService: => scalaz.ReaderWriterState[Exercises.rws.Config,List[String 14 //| ],Int,Int]
15 def setService(p: Int, n: String): ReaderWriterState[Config, List[String], Int, Int] =
16 ReaderWriterState { 17 case (cfg, invocationCount) => cfg.port=p; cfg.portName=n 18 (List("Changing service port to " + cfg.portName), 19 scala.util.Random.nextInt(100), 20 invocationCount) 21 } //> setService: (p: Int, n: String)scalaz.ReaderWriterState[Exercises.rws.Confi 22 //| g,List[String],Int,Int]
23
24 val program: ReaderWriterState[Config, List[String], Int, Int] = for { 25 _ <- log("Start - r: %s, s: %s") 26 res <- invokeService 27 _ <- log("Between - r: %s, s: %s") 28 _ <- setService(8,"Com8") 29 _ <- invokeService 30 _ <- log("Done - r: %s, s: %s") 31 } yield res //> program : scalaz.RWS[Exercises.rws.Config,List[String],Int,Int] = scalaz.I 32 //| ndexedReaderWriterStateT$$anon$5@223191a6
33 val r = program run (Config(443,"Com3"), 0) //> r : scalaz.Id.Id[(List[String], Int, Int)] = (List(Start - r: Config(443,C 34 //| om3), s: 0, Invoking service with port: Com3, Between - r: Config(443,Com3) 35 //| , s: 1, Changing service port to Com8, Invoking service with port: Com8, Do 36 //| ne - r: Config(88,Com8), s: 2),68,2)
37 println(&q |