如何处理`Reader` monad和`Try`？

Question

我正在阅读这篇关于使用Reader monad进行scala依赖注入的精彩文章.

原始示例运行良好,但我对返回类型做了一点改动UserRepository.get/find.是的User,但我把它改成了Try[User].

然后代码将不会编译,我已经多次尝试,但仍然没有幸运.

import scala.util.Try
import scalaz.Reader

case class User(email: String, supervisorId: Int, firstName: String, lastName: String)

trait UserRepository {
  def get(id: Int): Try[User]

  def find(username: String): Try[User]
}

trait Users {

  def getUser(id: Int) = Reader((userRepository: UserRepository) =>
    userRepository.get(id)
  )

  def findUser(username: String) = Reader((userRepository: UserRepository) =>
    userRepository.find(username)
  )
}

object UserInfo extends Users {

  def userEmail(id: Int) = {
    getUser(id) map (ut => ut.map(_.email))
  }

  def userInfo(username: String) =
    for {
      userTry <- findUser(username)
      user <- userTry     // !!!!!!!! compilation error
      bossTry <- getUser(user.supervisorId)
      boss <- bossTry     // !!!!!!!! compilation error
    } yield Map(
      "fullName" -> s"${user.firstName} ${user.lastName}",
      "email" -> s"${user.email}",
      "boss" -> s"${boss.firstName} ${boss.lastName}"
    )
}

编译错误是:

Error:(34, 12) type mismatch;
 found   : scala.util.Try[Nothing]
 required: scalaz.Kleisli[scalaz.Id.Id,?,?]
      user <- userTry
           ^

和

Error:(36, 12) type mismatch;
 found   : scala.util.Try[scala.collection.immutable.Map[String,String]]
 required: scalaz.Kleisli[scalaz.Id.Id,?,?]
      boss <- bossTry
           ^

我读了文件Kleisli.flatMap(返回类型findUser和getUser是Kleisli),它要求参数类型为:

B => Kleisli[M, A, C]

由于a Try不会是a Kleisli,所以会出现这样的错误.

我不知道如何处理它.我可以scala.util.Try在这里用吗？我怎样才能把它变成一个KLeisli类型？我怎样才能使这个例子有效？

Answer 1

你可以使用ReaderTmonad变换器将Readermonad和Trymonad组合成一个monad,你可以使用a for-reherehe on等.

ReaderT只是一个类型别名Kleisli,您可以使用Kleisli.kleisli而不是Reader.apply构造您的Reader-y计算.请注意,您需要scalaz-contribmonad实例Try(或者您可以编写自己的实例- 它非常简单).

import scala.util.Try
import scalaz._, Scalaz._
import scalaz.contrib.std.utilTry._

case class User(
  email: String,
  supervisorId: Int,
  firstName: String,
  lastName: String
)

trait UserRepository {
  def get(id: Int): Try[User]

  def find(username: String): Try[User]
}

trait Users {
  def getUser(id: Int): ReaderT[Try, UserRepository, User] =
    Kleisli.kleisli(_.get(id))

  def findUser(username: String): ReaderT[Try, UserRepository, User] =
    Kleisli.kleisli(_.find(username))
}

既然已经完成了,UserInfo那就简单得多了(它现在也可以编译!):

object UserInfo extends Users {
  def userEmail(id: Int) = getUser(id).map(_.email)

  def userInfo(
    username: String
  ): ReaderT[Try, UserRepository, Map[String, String]] =
    for {
      user <- findUser(username)
      boss <- getUser(user.supervisorId)
    } yield Map(
      "fullName" -> s"${user.firstName} ${user.lastName}",
      "email" -> s"${user.email}",
      "boss" -> s"${boss.firstName} ${boss.lastName}"
    )
}

我们可以证明它有效:

import scala.util.{ Failure, Success }

val repo = new UserRepository {
  val bar = User("bar@mcfoo.com", 0, "Bar", "McFoo")
  val foo = User("foo@mcbar.com", 0, "Foo", "McBar")

  def get(id: Int) = id match {
    case 0 => Success(bar)
    case 1 => Success(foo)
    case i => Failure(new Exception(s"No user with id $i"))
  }

  def find(username: String) = username match {
    case "bar" => Success(bar)
    case "foo" => Success(foo)
    case other => Failure(new Exception(s"No user with name $other"))
  }
}

然后:

UserInfo.userInfo("foo").run(repo).foreach(println)
Map(fullName -> Foo McBar, email -> foo@mcbar.com, boss -> Bar McFoo)

你运行的方式完全相同Reader,但Try最终会得到一个.