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VL-Lens in Scala
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| import Dependencies._ | |
| lazy val root = (project in file(".")). | |
| settings( | |
| inThisBuild(List( | |
| organization := "com.example", | |
| scalaVersion := "2.11.11", | |
| version := "0.1.0-SNAPSHOT" | |
| )), | |
| name := "vr-lens-demo", | |
| libraryDependencies += scalaTest % Test, | |
| libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.2.20", | |
| scalacOptions ++= Seq( | |
| // "-Ypartial-unification", | |
| "-language:higherKinds" | |
| ) | |
| ) |
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| > console | |
| [info] Starting scala interpreter... | |
| [info] | |
| Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_131). | |
| Type in expressions for evaluation. Or try :help. | |
| scala> import example.lens.demo._ | |
| import example.lens.demo._ | |
| scala> val user = User(999, "aaa") | |
| user: example.lens.demo.User = User(999,aaa) | |
| scala> modify(_name)(user)(_.toUpperCase) | |
| res0: example.lens.demo.User = User(999,AAA) | |
| scala> get(_name)(user) | |
| <console>:16: error: polymorphic expression cannot be instantiated to expected type; | |
| found : [F[_]]example.lens.demo.Lens[F,example.lens.demo.User,String] | |
| required: example.lens.demo.Getter[?,?] | |
| (which expands to) example.lens.demo.Lens[[X]example.lens.demo.Const[?,X],?,?] | |
| get(_name)(user) | |
| ^ | |
| scala> get(_name[({ type F[A] = Const[String, A]})#F])(user) | |
| res2: String = aaa |
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| package example.lens | |
| import scalaz._, Id.Id | |
| //import Scalaz._ | |
| object demo { | |
| case class User(id: Long, name: String) | |
| // type Id[A] = A | |
| // case class Const[A, B](getConst: A) | |
| // trait Functor[F[_]] { | |
| // def map[A, B](fa: F[A])(f: A => B): F[B] | |
| // } | |
| // object Functor { | |
| // def apply[F[_]](implicit F: Functor[F]): Functor[F] = F | |
| // | |
| // implicit val idFuncotr: Functor[Id] = | |
| // new Functor[Id] { | |
| // override def map[A, B](fa: Id[A])(f: A => B): Id[B] = f(fa) | |
| // } | |
| // | |
| // implicit def constFunctor[A]: Functor[({ type F[X] = Const[A, X] })#F] = | |
| // new Functor[({ type F[X] = Const[A, X] })#F] { | |
| // override def map[B, C](fa: Const[A, B])(f: B => C): Const[A, C] = | |
| // Const[A, C](fa.getConst) | |
| // } | |
| // } | |
| trait Lens[F[_], S, A] { | |
| def run(s: S)(f: A => F[A]): F[S] | |
| } | |
| type Setter[S, A] = Lens[Id, S, A] | |
| def modify[S, A](setter: Setter[S, A])(s: S)(f: A => A): S = | |
| setter.run(s)(f) | |
| type Getter[S, A] = Lens[({ type F[X] = Const[A, X] })#F, S, A] | |
| type Getting[R, S, A] = Lens[({ type F[X] = Const[R, X]})#F, S, A] | |
| def get[S, A](getter: Getting[A, S, A])(s: S): A = | |
| getter.run(s)(a => Const(a)).getConst | |
| def _name[F[_]: Functor]: Lens[F, User, String] = new Lens[F, User, String] { | |
| def run(user: User)(f: String => F[String]): F[User] = | |
| Functor[F].map[String, User](f(user.name))(n => user.copy(name = n)) | |
| } | |
| def _nameU(implicit U: Unapply[Functor, Const[String, User]]): Lens[U.M, User, String] = | |
| new Lens[U.M, User, String] { | |
| def run(user: User)(f: String => U.M[String]): U.M[User] = | |
| U.TC.map(f(user.name))(n => user.copy(name = n)) | |
| } | |
| } |
Author
Author
やったぜ（つらい
import scalaz._
object demo {
// ...
def _nameU(implicit U: Unapply[Functor, Const[String, User]]): Lens[U.M, User, String] =
new Lens[U.M, User, String] {
def run(user: User)(f: String => U.M[String]): U.M[User] =
U.TC.map(f(user.name))(n => user.copy(name = n))
}
}scala> get(_nameU)(User(999, "aaa"))
res1: String = aaa
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なぜ型を明示しなきゃいけないのだ、推論してくれ~~~