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Cats（1）－从Free开始，Free cats

cats是scala的一个新的函数式编程工具库，其设计原理基本继承了scalaz：大家都是haskell typeclass的scala版实现。当然，cats在scalaz的基础上从实现细节、库组织结构和调用方式上进行了一些优化，所以对用户来说：cats的基础数据类型、数据结构在功能上与scalaz是大致相同的，可能有一些语法上的变化。与scalaz著名抽象、复杂的语法表现形式相比，cats的语法可能更形象、简单直白。在scalaz的学习过程中，我们了解到所谓函数式编程就是monadic Programming：即用monad这样的数据类型来构建程序。而实际可行的monadic programming就是用Free-Monad编程了。因为Free-Monad程序是真正可运行的，或者说是可以实现安全运行的，因为它可以保证在固定的堆栈内实现无限运算。我们知道：函数式编程模式的运行方式以递归算法为主，flatMap函数本身就是一种递归算法。这就预示着monadic programming很容易造成堆栈溢出问题（StackOverflowError）。当我们把普通的泛函类型F[A]升格成Free-Monad后就能充分利用Free-Monad安全运算能力来构建实际可运行的程序了。由于我们在前面已经详细的了解了scalaz的大部分typeclass，包括Free，对cats的讨论就从Free开始，聚焦在cats.Free编程模式方面。同时，我们可以在使用cats.Free的过程中对cats的其它数据类型进行补充了解。

cats.Free的类型款式如下：

sealed abstract class Free[S[_], A] extends Product with Serializable {...}

S是个高阶类，就是一种函数式运算。值得注意的是：现在S不需要是个Functor了。因为Free的一个实例Suspend类型是这样的：

/** Suspend the computation with the given suspension. */
  private final case class Suspend[S[_], A](a: S[A]) extends Free[S, A]

我们不需要map就可以把F[A]升格成Free

/**
   * Suspend a value within a functor lifting it to a Free.
   */
  def liftF[F[_], A](value: F[A]): Free[F, A] = Suspend(value)

我们在scalaz.Free的讨论中并没能详尽地分析在什么情况下S[_]必须是个Functor。下面我们需要用一些篇幅来解析。

Free程序的特点是算式（description）／算法（implementation）关注分离（separation of concern）：我们用一组数据类型来模拟一种编程语句ADT（algebraic data type），这一组ADT就形成了一种定制的编程语言DSL（domain specific language）。Free的编程部分就是用DSL来描述程序功能（description of purpose），即算式了。算法即用DSL描述的功能的具体实现，可以有多种的功能实现方式。我们先看个简单的DSL：

 1 import cats.free._
 2 import cats.Functor
 3 object catsFree {
 4   object ADTs {
 5     sealed trait Interact[+A]
 6     object Interact {
 7       case class Ask(prompt: String) extends Interact[String]
 8       case class Tell(msg: String) extends Interact[Unit]
 9       
10       def ask(prompt: String): Free[Interact,String] = Free.liftF(Ask(prompt))
11       def tell(msg: String): Free[Interact,Unit] = Free.liftF(Tell(msg))
12 
13 
14       implicit object interactFunctor extends Functor[Interact]  {
15         def map[A,B](ia: Interact[A])(f: A => B): Interact[B] = ???
16       /*   ia match {
17            case Ask(p) => ???
18            case Tell(m) => ???
19         } */
20       }  
21     }
22   }
23   object DSLs {
24     import ADTs._
25     import Interact._
26     val prg: Free[Interact,Unit] = for {
27       first <- ask("What's your first name?")
28       last <- ask("What's your last name?")
29       _ <- tell(s"Hello $first $last")
30     } yield()
31   }

在这个例子里Interact并不是一个Functor，因为我们无法获取Interact Functor实例的map函数。先让我们分析一下Functor的map：

1      implicit object interactFunctor extends Functor[Interact]  {
2         def map[A,B](ia: Interact[A])(f: A => B): Interact[B] = ia match {
3            case Ask(p) => ???
4            case Tell(m) => ???
5         }
6       }

map的作用是用一个函数A => B把F[A]转成F[B]。也就是把语句状态从F[A]转成F[B]，但在Interact的情况里F[B]已经是明确的Interact[Unit]和Interact[String]两种状态，而map的f是A => B，在上面的示范里我们该如何施用f来获取这个Interact[B]呢？从上面的示范里我们观察可以得出Ask和Tell这两个ADT纯粹是为了模拟ask和tell这两个函数。ask和tell分别返回Free版本的String,Unit结果。可以说：Interact并没有转换到下一个状态的要求。那么假如我们把ADT调整成下面这样呢：

 1       sealed trait FunInteract[NS]
 2       object FunInteract {
 3         case class FunAsk[NS](prompt: String, onInput: String =>  NS) extends FunInteract[NS]
 4         case class FunTell[NS](msg: String, ns: NS) extends FunInteract[NS]
 5         
 6         def funAsk(prompt: String): Free[FunInteract,String] = Free.liftF(FunAsk(prompt,identity))
 7         def funAskInt(prompt: String): Free[FunInteract,Int] = Free.liftF(FunAsk(prompt,_.toInt))
 8         def funTell(msg: String): Free[FunInteract,Unit] = Free.liftF(FunTell(msg,()))
 9         
10         implicit object funInteract extends Functor[FunInteract] {
11            def map[A,NS](fa: FunInteract[A])(f: A => NS) = fa match {
12               case FunAsk(prompt,input) => FunAsk(prompt,input andThen f)
13               case FunTell(msg,ns) => FunTell(msg,f(ns))
14            }
15         }
16       }

现在这两个ADT是有类型参数NS的了：FunAsk[NS],FunTell[NS]。NS代表了ADT当前类型，如FunAsk[Int]、FunTell[String]...，现在这两个ADT都通过类型参数NS变成了可map的对象了，如FunAsk[String] >>> FunAsk[String], FunAsk[String] >>> FunAsk[Int]...。所以我们可以很顺利的实现object funInteract的map函数。但是，一个有趣的现象是：为了实现这种状态转换，如果ADT需要返回操作结果，就必须具备一个引领状态转换的机制，如FunAsk类型里的onInput: String => NS：它代表funAsk函数返回的结果可以指向下一个状态。新增函数funAskInt是个很好的示范：通过返回的String结果将状态转换到FunAsk[Int]状态。函数funTell不返回结果，所以FunTell没有状态转换机制。scalaz旧版本Free.Suspend的类型款式是：Suspend[F[Free,A]]，这是一个递归类型，内部的Free代表下一个状态。由于我们必须用F.map才能取出下一个状态，所以F必须是个Functor。我们应该注意到如果ADT是Functor的话会造成Free程序的冗余代码。既然cats.Free对F[A]没有设置Functor门槛，那么我们应该尽量避免使用Functor。

得出对ADT类型要求结论后，我们接着示范cats的Free编程。下面是Free程序的功能实现interpret部分（implementation）：

1     import ADTs._
2     object iconsole extends (Interact ~> Id) {
3       def apply[A](ia: Interact[A]): Id[A] = ia match {
4          case Ask(p) => {println(p); readLine}
5          case Tell(m) => println(m)
6       }
7     }
8   }

DSL程序的功能实现就是把ADT F[A]对应到实际的指令集G[A]，在Free编程里用NaturalTransformation ～>来实现。注意G[A]必须是个Monad。在上面的例子里对应关系是：Interact～>Id，代表直接对应到运算指令println和readLine。我们也可以实现另一个版本：

 1     type Prompt = String
 2     type Reply = String
 3     type Message = String
 4     type Tester[A] = Map[Prompt,Reply] => (List[Message],A)
 5     object tester extends (Interact ~> Tester) {
 6       def apply[A](ia: Interact[A]): Tester[A] = ia match {
 7         case Ask(p) => { m => (List(), m(p)) }
 8         case Tell(m) => { _ => (List(m), ()) }
 9       }
10     }
11     import cats.Monad
12     implicit val testerMonad = new Monad[Tester] {
13       override def pure[A](a: A): Tester[A] = _ => (List(),a)
14       override def flatMap[A,B](ta: Tester[A])(f: A => Tester[B]): Tester[B] = m => {
15         val (o1,a1) = ta(m)
16         val (o2,a2) = f(a1)(m)
17         (o1 ++ o2, a2)
18       }
19       override def tailRecM[A,B](a: A)(f: A => Tester[Either[A,B]]): Tester[B] =
20          defaultTailRecM(a)(f)
21     }
22   }

上面是个模拟测试：我们用个Map[K,V]来模拟互动，K模拟问prompt，V模拟获取回答Input。测试方式是个Function1，输入测试数据Map，在List[Message]里返回所有Tell产生的信息。上面提到过Tester[A]必须是个Monad，所以我们实现了Tester的Monad实例testMonad。实际上 m=>(List,a)就是个writer函数。所谓的Writer就是包嵌一个对值pair(L,V)的Monad，L代表Log，V代表运算值。Writer的特性就是log所有V的运算过程。我们又可以用Writer来实现这个tester：

 1    import cats.data.WriterT
 2     type WF[A] = Map[Prompt,Reply] => A
 3     type WriterTester[A] = WriterT[WF,List[Message],A]
 4     def testerToWriter[A](f: Map[Prompt,Reply] => (List[Message],A)) =
 5     WriterT[WF,List[Message],A](f)
 6     object testWriter extends (Interact ~> WriterTester) {
 7       import Interact._
 8       def apply[A](ia: Interact[A]): WriterTester[A] = ia match {
 9         case Ask(p) => testerToWriter(m => (List(),m(p)))
10         case Tell(m) => testerToWriter(_ => (List(m),()))
11       }
12     }

如果我们用Writer来实现Interact，实际上就是把Ask和Tell都升格成Writer类型。

我们再来看看在cats里是如何运算Free DSL程序的。相对scalaz而言，cats的运算函数简单的多，就一个foldMap，我们来看看它的定义：

/**
   * Catamorphism for `Free`.
   *
   * Run to completion, mapping the suspension with the given
   * transformation at each step and accumulating into the monad `M`.
   *
   * This method uses `tailRecM` to provide stack-safety.
   */
  final def foldMap[M[_]](f: FunctionK[S, M])(implicit M: Monad[M], r: RecursiveTailRecM[M]): M[A] =
    r.sameType(M).tailRecM(this)(_.step match {
      case Pure(a) => M.pure(Right(a))
      case Suspend(sa) => M.map(f(sa))(Right(_))
      case FlatMapped(c, g) => M.map(c.foldMap(f))(cc => Left(g(cc)))
    })

除了要求M是个Monad之外，cats还要求M的RecursiveTailRecM隐式实例。那么什么是RecursiveTailRecM呢：

/**
 * This is a marker type that promises that the method
 * .tailRecM for this type is stack-safe for arbitrary recursion.
 */
trait RecursiveTailRecM[F[_]] extends Serializable {
  /*
   * you can call RecursiveTailRecM[F].sameType(Monad[F]).tailRec
   * to have a static check that the types agree
   * for safer usage of tailRecM
   */
  final def sameType[M[_[_]]](m: M[F]): M[F] = m
}

我们用RecursiveTailRecM来保证这个Monad类型与tailRecM是匹配的，这是一种运算安全措施，所以在foldMap函数里r.sameType(M).tailRecM保证了tailRecM不会造成StackOverflowError。cats.Free里还有一种不需要类型安全检验的函数foldMapUnsafe：

/**
   * Same as foldMap but without a guarantee of stack safety. If the recursion is shallow
   * enough, this will work
   */
  final def foldMapUnsafe[M[_]](f: FunctionK[S, M])(implicit M: Monad[M]): M[A] =
    foldMap[M](f)(M, RecursiveTailRecM.create)

这个函数不需要RecursiveTailRecM。下面我们选择能保证运算安全的方法来运算tester：首先我们需要Tester类型的Monad和RecursiveTailRecM实例：

 1     import cats.Monad
 2     implicit val testerMonad = new Monad[Tester] with RecursiveTailRecM[Tester]{
 3       override def pure[A](a: A): Tester[A] = _ => (List(),a)
 4       override def flatMap[A,B](ta: Tester[A])(f: A => Tester[B]): Tester[B] = m => {
 5         val (o1,a1) = ta(m)
 6         val (o2,a2) = f(a1)(m)
 7         (o1 ++ o2, a2)
 8       }
 9       override def tailRecM[A,B](a: A)(f: A => Tester[Either[A,B]]): Tester[B] =
10         defaultTailRecM(a)(f)
11     }

然后我们制造一些测试数据：

1   val testData = Map("What's your first name?" -> "Tiger",
2   "What's your last name?" -> "Chan")             //> testData  : scala.collection.immutable.Map[String,String] = Map(What's your first name? -> Tiger, What's your last name? -> Chan)

测试运算：

1 import ADTs._,DSLs._,IMPLs._
2    val testData = Map("What's your first name?" -> "Tiger",
3   "What's your last name?" -> "Chan")    //> testData  : scala.collection.immutable.Map[String,String] = Map(What's your first name? -> Tiger, What's your last name? -> Chan)
4   val prgRunner = prg.foldMap(tester)    //> prgRunner  : demo.ws.catsFree.IMPLs.Tester[Unit] = <function1>
5   prgRunner(testData)                    //> res0: (List[demo.ws.catsFree.IMPLs.Message], Unit) = (List(Hello Tiger Chan),())

那么如果运算testWriter呢？我们先取得WriterT的Monad实例：

1    implicit val testWriterMonad =  WriterT.catsDataMonadWriterForWriterT[WF,List[Message]]

然后构建一个RecursiveTailRecM实例后再用同样的测试数据来运算：

1  implicit val testWriterRecT = new RecursiveTailRecM[WriterTester]{}
2            //> testWriterRecT  : cats.RecursiveTailRecM[demo.ws.catsFree.IMPLs.WriterTester] = demo.ws.catsFree$$anonfun$main$1$$anon$2@6093dd95
3   val prgRunner = prg.foldMap(testWriter)         //> prgRunner  : demo.ws.catsFree.IMPLs.WriterTester[Unit] = WriterT(<function1>)
4   prgRunner.run(testData)._1.map(println)         //> Hello Tiger Chan
5                                                   //| res0: List[Unit] = List(())

运算结果一致。

我们再示范一下cats官方文件里关于free monad例子：模拟一个KVStore的put,get,delete功能。ADT设计如下：

1   object ADTs {
2     sealed trait KVStoreA[+A]
3     case class Put[T](key: String, value: T) extends KVStoreA[Unit]
4     case class Get[T](key: String) extends KVStoreA[Option[T]]
5     case class Del(key: String) extends KVStoreA[Unit]
6   }

对应的模拟功能函数设计如下：

 1     type KVStore[A] = Free[KVStoreA,A]
 2     object KVStoreA {
 3       def put[T](key: String, value: T): KVStore[Unit] =
 4         Free.liftF[KVStoreA,Unit](Put[T](key,value))
 5       def get[T](key: String): KVStore[Option[T]] =
 6         Free.liftF[KVStoreA,Option[T]](Get[T](key))
 7       def del(key: String): KVStore[Unit] =
 8         Free.liftF[KVStoreA,Unit](Del(key))
 9       def mod[T](key: String, f: T => T): KVStore[Unit] =
10         for {
11           opt <- get[T](key)
12           _ <- opt.map {t => put[T](key,f(t))}.getOrElse(Free.pure(()))
13         } yield()
14     }

注意一下mod函数：它是由基础函数get和put组合而成。我们要求所有在for内的类型为Free[KVStoreA,?]，所以当f函数施用后如果opt变成None时就返回结果Free.pure(())，它的类型是：Free[Nothing,Unit]，Nothing是KVStoreA的子类。

现在我们可以用这个DSL来编制KVS程序了：

 1  object DSLs {
 2     import ADTs._
 3     import KVStoreA._
 4     def prg: KVStore[Option[Int]] =
 5     for {
 6       _ <- put[Int]("wild-cats", 2)
 7       _ <- mod[Int]("wild-cats", (_ + 12))
 8       _ <- put[Int]("tame-cats", 5)
 9       n <- get[Int]("wild-cats")
10       _ <- del("tame-cats")
11     } yield n
12   }

我们可以通过State数据结纯代码（pure code）方式来实现用immutable map的KVStore：

 1  object IMPLs {
 2     import ADTs._
 3     import cats.{~>}
 4     import cats.data.State
 5    
 6     type KVStoreState[A] = State[Map[String, Any], A]
 7     val kvsToState: KVStoreA ~> KVStoreState = new (KVStoreA ~> KVStoreState) {
 8       def apply[A](fa: KVStoreA[A]): KVStoreState[A] =
 9         fa match {
10           case Put(key, value) => State { (s:Map[String, Any]) =>
11              (s.updated(key, value),()) }
12           case Get(key) => State { (s:Map[String, Any]) =>
13             (s,s.get(key).asInstanceOf[A]) }
14           case Del(key) => State { (s:Map[String, Any]) =>
15               (s - key, (())) }
16         }
17     }
18   }

我们把KVStoreA ADT模拟成对State结构的S转换（mutation），返回State{S=>(S,A)}。KVStoreState[A]类型的S参数为immutable.Map[String, Any]，所以我们在S转换操作时用immutable map的操作函数来构建新的map返回，典型的pure code。我们来运算一下KVStoreA程序：

1   import ADTs._,DSLs._,IMPLs._
2   val prgRunner = prg.foldMap(kvsToState)    //> prgRunner  : demo.ws.catsFreeKVS.IMPLs.KVStoreState[Option[Int]] = cats.data.StateT@2cfb4a64
3   prgRunner.run(Map.empty).value       //> res0: (Map[String,Any], Option[Int]) = (Map(wild-cats -> 14),Some(14))

但是难道不需要Monad、RecursiveTailRecM实例了吗？实际上cats已经提供了State的Monad和RecursiveTailRecM实例：

1   import cats.{Monad,RecursiveTailRecM}
2   implicitly[Monad[KVStoreState]]      //> res1: cats.Monad[demo.ws.catsFreeKVS.IMPLs.KVStoreState] = cats.data.StateT Instances$$anon$2@71bbf57e
3   implicitly[RecursiveTailRecM[KVStoreState]]     //> res2: cats.RecursiveTailRecM[demo.ws.catsFreeKVS.IMPLs.KVStoreState] = cats.RecursiveTailRecM$$anon$1@7f13d6e

在cats的StateT.scala里可以找到这段代码：

private[data] sealed trait StateTInstances2 {
  implicit def catsDataMonadForStateT[F[_], S](implicit F0: Monad[F]): Monad[StateT[F, S, ?]] =
    new StateTMonad[F, S] { implicit def F = F0 }

  implicit def catsDataRecursiveTailRecMForStateT[F[_]: RecursiveTailRecM, S]: RecursiveTailRecM[StateT[F, S, ?]] = RecursiveTailRecM.create[StateT[F, S, ?]]

  implicit def catsDataSemigroupKForStateT[F[_], S](implicit F0: Monad[F], G0: SemigroupK[F]): SemigroupK[StateT[F, S, ?]] =
    new StateTSemigroupK[F, S] { implicit def F = F0; implicit def G = G0 }
}

我把上面两个示范的源代码提供在下面：

Interact:

  1 import cats.free._
  2 import cats.{Functor, RecursiveTailRecM}
  3 object catsFree {
  4   object ADTs {
  5     sealed trait Interact[+A]
  6     object Interact {
  7       case class Ask(prompt: String) extends Interact[String]
  8       case class Tell(msg: String) extends Interact[Unit]
  9 
 10       def ask(prompt: String): Free[Interact,String] = Free.liftF(Ask(prompt))
 11       def tell(msg: String): Free[Interact,Unit] = Free.liftF(Tell(msg))
 12 
 13 
 14       implicit object interactFunctor extends Functor[Interact]  {
 15         def map[A,B](ia: Interact[A])(f: A => B): Interact[B] = ???
 16         /*   ia match {
 17              case Ask(p) => ???
 18              case Tell(m) => ???
 19           } */
 20       }
 21 
 22       sealed trait FunInteract[NS]
 23       object FunInteract {
 24         case class FunAsk[NS](prompt: String, onInput: String =>  NS) extends FunInteract[NS]
 25         case class FunTell[NS](msg: String, ns: NS) extends FunInteract[NS]
 26 
 27         def funAsk(prompt: String): Free[FunInteract,String] = Free.liftF(FunAsk(prompt,identity))
 28         def funAskInt(prompt: String): Free[FunInteract,Int] = Free.liftF(FunAsk(prompt,_.toInt))
 29         def funTell(msg: String): Free[FunInteract,Unit] = Free.liftF(FunTell(msg,()))
 30 
 31         implicit object funInteract extends Functor[FunInteract] {
 32           def map[A,NS](fa: FunInteract[A])(f: A => NS) = fa match {
 33             case FunAsk(prompt,input) => FunAsk(prompt,input andThen f)
 34             case FunTell(msg,ns) => FunTell(msg,f(ns))
 35           }
 36         }
 37       }
 38     }
 39   }
 40   object DSLs {
 41     import ADTs._
 42     import Interact._
 43     val prg: Free[Interact,Unit] = for {
 44       first <- ask("What's your first name?")
 45       last <- ask("What's your last name?")
 46       _ <- tell(s"Hello $first $last")
 47     } yield()
 48   }
 49   object IMPLs {
 50     import cats.{Id,~>}
 51     import ADTs._
 52     import Interact._
 53     object iconsole extends (Interact ~> Id) {
 54       def apply[A](ia: Interact[A]): Id[A] = ia match {
 55         case Ask(p) => {println(p); readLine}
 56         case Tell(m) => println(m)
 57       }
 58     }
 59 
 60     type Prompt = String
 61     type Reply = String
 62     type Message = String
 63     type Tester[A] = Map[Prompt,Reply] => (List[Message],A)
 64     object tester extends (Interact ~> Tester) {
 65       def apply[A](ia: Interact[A]): Tester[A] = ia match {
 66         case Ask(p) => { m => (List(), m(p)) }
 67         case Tell(m) => { _ => (List(m), ()) }
 68       }
 69     }
 70     import cats.Monad
 71     implicit val testerMonad = new Monad[Tester] with RecursiveTailRecM[Tester]{
 72       override def pure[A](a: A): Tester[A] = _ => (List(),a)
 73       override def flatMap[A,B](ta: Tester[A])(f: A => Tester[B]): Tester[B] = m => {
 74         val (o1,a1) = ta(m)
 75         val (o2,a2) = f(a1)(m)
 76         (o1 ++ o2, a2)
 77       }
 78       override def tailRecM[A,B](a: A)(f: A => Tester[Either[A,B]]): Tester[B] =
 79         defaultTailRecM(a)(f)
 80     }
 81     import cats.data.WriterT
 82     import cats.instances.all._
 83     type WF[A] = Map[Prompt,Reply] => A
 84     type WriterTester[A] = WriterT[WF,List[Message],A]
 85     def testerToWriter[A](f: Map[Prompt,Reply] => (List[Message],A)) =
 86       WriterT[WF,List[Message],A](f)
 87     implicit val testWriterMonad =  WriterT.catsDataMonadWriterForWriterT[WF,List[Message]]
 88     object testWriter extends (Interact ~> WriterTester) {
 89       import Interact._
 90       def apply[A](ia: Interact[A]): WriterTester[A] = ia match {
 91         case Ask(p) => testerToWriter(m => (List(),m(p)))
 92         case Tell(m) => testerToWriter(_ => (List(m),()))
 93       }
 94     }
 95   }
 96 
 97   import ADTs._,DSLs._,IMPLs._
 98    val testData = Map("What's your first name?" -> "Tiger",
 99   "What's your last name?" -> "Chan")
100   //val prgRunner = prg.foldMap(tester)
101   //prgRunner(testData)
102   implicit val testWriterRecT = new RecursiveTailRecM[WriterTester]{}
103   val prgRunner = prg.foldMap(testWriter)
104   prgRunner.run(testData)._1.map(println)
105 }

KVStore：

 1 import cats.free._
 2 import cats.instances.all._
 3 object catsFreeKVS {
 4   object ADTs {
 5     sealed trait KVStoreA[+A]
 6     case class Put[T](key: String, value: T) extends KVStoreA[Unit]
 7     case class Get[T](key: String) extends KVStoreA[Option[T]]
 8     case class Del(key: String) extends KVStoreA[Unit]
 9     type KVStore[A] = Free[KVStoreA,A]
10     object KVStoreA {
11       def put[T](key: String, value: T): KVStore[Unit] =
12         Free.liftF[KVStoreA,Unit](Put[T](key,value))
13       def get[T](key: String): KVStore[Option[T]] =
14         Free.liftF[KVStoreA,Option[T]](Get[T](key))
15       def del(key: String): KVStore[Unit] =
16         Free.liftF[KVStoreA,Unit](Del(key))
17       def mod[T](key: String, f: T => T): KVStore[Unit] =
18         for {
19           opt <- get[T](key)
20           _ <- opt.map {t => put[T](key,f(t))}.getOrElse(Free.pure(()))
21         } yield()
22     }
23   }
24   object DSLs {
25     import ADTs._
26     import KVStoreA._
27     def prg: KVStore[Option[Int]] =
28     for {
29       _ <- put[Int]("wild-cats", 2)
30       _ <- mod[Int]("wild-cats", (_ + 12))
31       _ <- put[Int]("tame-cats", 5)
32       n <- get[Int]("wild-cats")
33       _ <- del("tame-cats")
34     } yield n
35   }
36   object IMPLs {
37     import ADTs._
38     import cats.{~>}
39     import cats.data.State
40    
41     type KVStoreState[A] = State[Map[String, Any], A]
42     val kvsToState: KVStoreA ~> KVStoreState = new (KVStoreA ~> KVStoreState) {
43       def apply[A](fa: KVStoreA[A]): KVStoreState[A] =
44         fa match {
45           case Put(key, value) => State { (s:Map[String, Any]) =>
46              (s.updated(key, value),()) }
47           case Get(key) => State { (s:Map[String, Any]) =>
48             (s,s.get(key).asInstanceOf[A]) }
49           case Del(key) => State { (s:Map[String, Any]) =>
50               (s - key, (())) }
51         }
52     }
53   }
54   import ADTs._,DSLs._,IMPLs._
55   val prgRunner = prg.foldMap(kvsToState)
56   prgRunner.run(Map.empty).value
57   
58   import cats.{Monad,RecursiveTailRecM}
59   implicitly[Monad[KVStoreState]]
60   implicitly[RecursiveTailRecM[KVStoreState]]
61 }

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原文地址：https://www.cnblogs.com/tiger-xc/p/5844574.html

Cats（1）－ 从Free开始，Free cats

Cats（1）－从Free开始，Free cats