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Cats（3）－ freeK-Free编程更轻松，Free programming with freeK
在上一节我们讨论了通过Coproduct来实现DSL组合：用一些功能简单的基础DSL组合成符合大型多复杂功能应用的DSL。但是我们发现：cats在处理多层递归Coproduct结构时会出现编译问题。再就是Free编程是一个繁复的工作，容易出错，造成编程效率的低下。由于Free编程目前是函数式编程的主要方式（我个人认为），我们必须克服Free编程的效率问题。通过尝试，发现freeK可以作为一个很好的Free编程工具。freeK是个开源的泛函组件库，我们会在这次讨论里用freeK来完成上次讨论中以失败暂停的多层Coproduct Free程序。我们先试试Interact和Login两个混合DSL例子：
1 object ADTs { 2 sealed trait Interact[+A] 3 object Interact { 4 case class Ask(prompt: String) extends Interact[String] 5 case class Tell(msg: String) extends Interact[Unit] 6 } 7 sealed trait Login[+A] 8 object Login { 9 case class Authenticate(uid: String, pwd: String) extends Login[Boolean]10 }11 }12 object DSLs {13 import ADTs._14 import Interact._15 import Login._16 type PRG = Interact :|: Login :|: NilDSL17 val PRG = DSL.Make[PRG]18 19 val authenticDSL: Free[PRG.Cop, Boolean] =20 for {21 uid <- Ask("Enter your user id:").freek[PRG]22 pwd <- Ask("Enter password:").freek[PRG]23 auth <- Authenticate(uid,pwd).freek[PRG]24 } yield auth25 }从ADT到DSL设计，用freeK使代码简单了很多。我们不需要再对ADT进行Inject和Free.liftF升格了，但必须在没条语句后附加.freek[PRG]。本来可以通过隐式转换来避免这样的重复代码，但scalac会在编译时产生一些怪异现象。这个PRG就是freeK的Coproduct结构管理方法，PRG.Cop就是当前的Coproduct。freeK是用:|:符号来连接DSL的，替代了我们之前繁复的Inject操作。
功能实现方面有什么变化吗？
1 object IMPLs { 2 import ADTs._ 3 import Interact._ 4 import Login._ 5 val idInteract = new (Interact ~> Id) { 6 def apply[A](ia: Interact[A]): Id[A] = ia match { 7 case Ask(p) => {println(p); scala.io.StdIn.readLine} 8 case Tell(m) => println(m) 9 }10 }11 val idLogin = new (Login ~> Id) {12 def apply[A](la: Login[A]): Id[A] = la match {13 case Authenticate(u,p) => (u,p) match {14 case ("Tiger","123") => true15 case _ => false16 }17 }18 }19 val interactLogin = idInteract :&: idLogin20 }这部分没有什么变化。freeK用:&:符号替换了or操作符。
那我们又该如何运行用freeK编制的程序呢？
1 object freeKDemo extends App {2 import FreeKModules._3 import DSLs._4 import IMPLs._5 val r0 = authenticDSL.foldMap(interactLogin.nat)6 val r = authenticDSL.interpret(interactLogin)7 println(r0)8 println(r)9 }interactLogin.nat就是以前的G[A]～>Id，所以我们依然可以用cats提供的foldMap来运算。不过freeK提供了更先进的interpret函数。它的特点是不要求Coproduct结构的构建顺序，我们无须再特别注意用inject构建Coproduct时的先后顺序了。也就是说:|:和:&:符号的左右元素可以不分，这将大大提高编程效率。
我们还是按上次的功能设计用Reader来进行用户密码验证功能的依赖注入。依赖界面定义如下：
1 object Dependencies {2 trait PasswordControl {3 val mapPasswords: Map[String,String]4 def matchUserPassword(uid: String, pwd: String): Boolean5 }6 }我们需要把Interact和Login都对应到Reader：
1 import Dependencies._ 2 type ReaderContext[A] = Reader[PasswordControl,A] 3 object readerInteract extends (Interact ~> ReaderContext) { 4 def apply[A](ia: Interact[A]): ReaderContext[A] = ia match { 5 case Ask(p) => Reader {pc => {println(p); scala.io.StdIn.readLine}} 6 case Tell(m) => Reader {_ => println(m)} 7 } 8 } 9 object readerLogin extends (Login ~> ReaderContext) {10 def apply[A](la: Login[A]): ReaderContext[A] = la match {11 case Authenticate(u,p) => Reader {pc => pc.matchUserPassword(u,p)}12 }13 }14 val userInteractLogin = readerLogin :&: readerInteract注意在上面我故意调换了:&:符号两边对象来证明interpret函数是不依赖Coproduct顺序的。
运算时我们需要构建一个测试的PasswordControl实例，然后把它传入Reader.run函数：
1 object freeKDemo extends App { 2 import FreeKModules._ 3 import DSLs._ 4 import IMPLs._ 5 // val r0 = authenticDSL.foldMap(interactLogin.nat) 6 // val r = authenticDSL.interpret(interactLogin) 7 import Dependencies._ 8 object UserPasswords extends PasswordControl { 9 override val mapPasswords: Map[String, String] = Map(10 "Tiger" -> "123",11 "John" -> "456"12 )13 override def matchUserPassword(uid: String, pwd: String): Boolean =14 mapPasswords.getOrElse(uid,pwd+"!") == pwd15 }16 17 interactLoginDSL.interpret(userInteractLogin).run(UserPasswords)18 }
测试运行正常。现在我们要尝试三个独立DSL的组合了。先增加一个用户权限验证DSL：
1 sealed trait Auth[+A]2 object Auth {3 case class Authorize(uid: String) extends Auth[Boolean]4 }
假如这个用户权限验证也是通过依赖注入的，我们先调整一下依赖界面：
1 object Dependencies { 2 trait PasswordControl { 3 val mapPasswords: Map[String,String] 4 def matchUserPassword(uid: String, pswd: String): Boolean 5 } 6 trait AccessControl { 7 val mapAccesses: Map[String, Boolean] 8 def grandAccess(uid: String): Boolean 9 }10 trait Authenticator extends PasswordControl with AccessControl11 }我们用Authenticator来代表包括PasswordControl,AccessControl的所有外部依赖。这样我们就需要把Reader的传入对象改变成Authenticator：
1 import Dependencies._2 type ReaderContext[A] = Reader[Authenticator,A]首先我们把增加的Auth语法与前两个语法构成的Coproduct再集合，然后进行集合三种语法的DSL编程：
1 import Auth._ 2 type PRG3 = Auth :|: PRG //Interact :|: Login :|: NilDSL 3 val PRG3 = DSL.Make[PRG3] 4 val authorizeDSL: Free[PRG3.Cop, Unit] = 5 for { 6 uid <- Ask("Enter your User ID:").freek[PRG3] 7 pwd <- Ask("Enter your Password:").freek[PRG3] 8 auth <- Authenticate(uid,pwd).freek[PRG3] 9 perm <- if (auth) Authorize(uid).freek[PRG3]10 else Free.pure[PRG3.Cop,Boolean](false)11 _ <- if (perm) Tell(s"Hello $uid, access granted!").freek[PRG3]12 else Tell(s"Sorry $uid, access denied!").freek[PRG3]13 } yield()这个程序的功能具体实现方式如下：
1 val readerAuth = new (Auth ~> ReaderContext) {2 def apply[A](aa: Auth[A]): ReaderContext[A] = aa match {3 case Authorize(u) => Reader {ac => ac.grandAccess(u)}4 }5 }6 val userAuth = readerAuth :&: userInteractLogin下面是测试数据制作以及运算：
1 import Dependencies._ 2 object AuthControl extends Authenticator { 3 override val mapPasswords = Map( 4 "Tiger" -> "1234", 5 "John" -> "0000" 6 ) 7 override def matchUserPassword(uid: String, pswd: String) = 8 mapPasswords.getOrElse(uid, pswd+"!") == pswd 9 10 override val mapAccesses = Map (11 "Tiger" -> true,12 "John" -> false13 )14 override def grandAccess(uid: String) =15 mapAccesses.getOrElse(uid, false)16 }17 18 // interactLoginDSL.interpret(userInteractLogin).run(AuthControl)19 authorizeDSL.interpret(userAuth).run(AuthControl)测试运行结果：
1 Enter your User ID: 2 Tiger 3 Enter your Password: 4 1234 5 Hello Tiger, access granted! 6 7 Process finished with exit code 0 8 ... 9 Enter your User ID:10 John11 Enter your Password:12 000013 Sorry John, access denied!14 15 Process finished with exit code 0结果正是我们所预期的。在这次示范中我没费什么功夫就顺利的完成了一个三种语法DSL的编程示范。这说明freeK确实是个满意的Free编程工具。这次讨论的示范代码如下：
1 import cats.free.Free 2 import cats.{Id, ~>} 3 import cats.data.Reader 4 import demo.app.FreeKModules.ADTs.Auth.Authorize 5 import freek._ 6 object FreeKModules { 7 object ADTs { 8 sealed trait Interact[+A] 9 object Interact { 10 case class Ask(prompt: String) extends Interact[String] 11 case class Tell(msg: String) extends Interact[Unit] 12 } 13 sealed trait Login[+A] 14 object Login { 15 case class Authenticate(uid: String, pwd: String) extends Login[Boolean] 16 } 17 sealed trait Auth[+A] 18 object Auth { 19 case class Authorize(uid: String) extends Auth[Boolean] 20 } 21 } 22 object DSLs { 23 import ADTs._ 24 import Interact._ 25 import Login._ 26 type PRG = Interact :|: Login :|: NilDSL 27 val PRG = DSL.Make[PRG] 28 29 val authenticDSL: Free[PRG.Cop, Boolean] = 30 for { 31 uid <- Ask("Enter your user id:").freek[PRG] 32 pwd <- Ask("Enter password:").freek[PRG] 33 auth <- Authenticate(uid,pwd).freek[PRG] 34 } yield auth 35 36 val interactLoginDSL: Free[PRG.Cop, Unit] = 37 for { 38 uid <- Ask("Enter your user id:").freek[PRG] 39 pwd <- Ask("Enter password:").freek[PRG] 40 auth <- Authenticate(uid,pwd).freek[PRG] 41 _ <- if (auth) Tell(s"Hello $uid, welcome to the zoo!").freek[PRG] 42 else Tell(s"Sorry, Who is $uid?").freek[PRG] 43 } yield () 44 45 import Auth._ 46 type PRG3 = Auth :|: PRG //Interact :|: Login :|: NilDSL 47 val PRG3 = DSL.Make[PRG3] 48 val authorizeDSL: Free[PRG3.Cop, Unit] = 49 for { 50 uid <- Ask("Enter your User ID:").freek[PRG3] 51 pwd <- Ask("Enter your Password:").freek[PRG3] 52 auth <- Authenticate(uid,pwd).freek[PRG3] 53 perm <- if (auth) Authorize(uid).freek[PRG3] 54 else Free.pure[PRG3.Cop,Boolean](false) 55 _ <- if (perm) Tell(s"Hello $uid, access granted!").freek[PRG3] 56 else Tell(s"Sorry $uid, access denied!").freek[PRG3] 57 } yield() 58 59 } 60 object IMPLs { 61 import ADTs._ 62 import Interact._ 63 import Login._ 64 val idInteract = new (Interact ~> Id) { 65 def apply[A](ia: Interact[A]): Id[A] = ia match { 66 case Ask(p) => {println(p); scala.io.StdIn.readLine} 67 case Tell(m) => println(m) 68 } 69 } 70 val idLogin = new (Login ~> Id) { 71 def apply[A](la: Login[A]): Id[A] = la match { 72 case Authenticate(u,p) => (u,p) match { 73 case ("Tiger","123") => true 74 case _ => false 75 } 76 } 77 } 78 val interactLogin = idInteract :&: idLogin 79 import Dependencies._ 80 type ReaderContext[A] = Reader[Authenticator,A] 81 object readerInteract extends (Interact ~> ReaderContext) { 82 def apply[A](ia: Interact[A]): ReaderContext[A] = ia match { 83 case Ask(p) => Reader {pc => {println(p); scala.io.StdIn.readLine}} 84 case Tell(m) => Reader {_ => println(m)} 85 } 86 } 87 object readerLogin extends (Login ~> ReaderContext) { 88 def apply[A](la: Login[A]): ReaderContext[A] = la match { 89 case Authenticate(u,p) => Reader {pc => pc.matchUserPassword(u,p)} 90 } 91 } 92 val userInteractLogin = readerLogin :&: readerInteract 93 94 val readerAuth = new (Auth ~> ReaderContext) { 95 def apply[A](aa: Auth[A]): ReaderContext[A] = aa match { 96 case Authorize(u) => Reader {ac => ac.grandAccess(u)} 97 } 98 } 99 val userAuth = readerAuth :&: userInteractLogin100 }101 102 }103 object Dependencies {104 trait PasswordControl {105 val mapPasswords: Map[String,String]106 def matchUserPassword(uid: String, pswd: String): Boolean107 }108 trait AccessControl {109 val mapAccesses: Map[String, Boolean]110 def grandAccess(uid: String): Boolean111 }112 trait Authenticator extends PasswordControl with AccessControl113 }114 115 object freeKDemo extends App {116 import FreeKModules._117 import DSLs._118 import IMPLs._119 // val r0 = authenticDSL.foldMap(interactLogin.nat)120 // val r = authenticDSL.interpret(interactLogin)121 import Dependencies._122 object AuthControl extends Authenticator {123 override val mapPasswords = Map(124 "Tiger" -> "1234",125 "John" -> "0000"126 )127 override def matchUserPassword(uid: String, pswd: String) =128 mapPasswords.getOrElse(uid, pswd+"!") == pswd129 130 override val mapAccesses = Map (131 "Tiger" -> true,132 "John" -> false133 )134 override def grandAccess(uid: String) =135 mapAccesses.getOrElse(uid, false)136 }137 138 // interactLoginDSL.interpret(userInteractLogin).run(AuthControl)139 authorizeDSL.interpret(userAuth).run(AuthControl)140 }
Cats（3）－ freeK-Free编程更轻松，Free programming with freeK
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