purely_functional_play_framework_application

Purely Functional Play Framework
Application
2018/03/17 ScalaMatsuri 2018
Naoki Aoyama

whoami
Naoki Aoyama
Twitter: @AoiroAoino
GitHub: @aoiroaoino
Working at
Septeni Original,Inc.

膨大なデータ収集から社内ワークフロー効率化までを行い、
広告主様の広告効果を高める広告運用最適化ツール
様々なメディアに対する広告運用やレポート、
クリエイティブ最適化を支援
Scalaを採用し、ドメイン駆動設計(DDD)で開発

グループ会社のコミックスマート社が手掛ける
連載型新作マンガ配信サービス
アプリは900万 DL を超え、オリジナルマンガも100作以上配信。
2017年12月にはアプリ内にアニメ視聴機能を実装し、
オリジナルアニメの配信を開始。
Scala + DDD ほか、 Android 版も Scala で開発。※ iOS 版は Swift

セプテーニ・オリジナルでは
いつでも Scala をやってみたい方を募集しております！
Scala や DDD 未経験でも問題ありません！
入社の研修カリキュラムを通して
Scala と DDD のトレーニングを積むことが出来ます。

</head>
<body>
Let’s get started!

Agenda
Introduction
- Play Framework with Google Guice
- Purely functional separation of program & execution
What’s Tagless Final style?
- Overview
- It’s your neighbor
Tagless Final style practice in Play Framework
- Purely, so useful!
- But, No Silver Bullet
Conclusion

Play Framework with Google Guice
- Play v2.4.0 より Google Guice を用いた DI の仕組みが取り
入れられた
- 大規模なアプリケーションには DI は欠かせない
- 実際、慣れれば便利。※慣れれば
Google Guice is introduce to Play Framework from
v2.4.0. It’s useful once you get used.

Sample Application on Play Framework
- Play Framework v2.6.12
- Scala 2.12.4

package controllers
import javax.inject._
import play.api.mvc._
import models.Greeting
@Singleton
class HomeController @Inject()(cc: ControllerComponents,
@Named("en") greeting: Greeting)
extends AbstractController(cc) {
def index() = Action { implicit request: Request[AnyContent] =>
Ok(views.html.index())
}
def message(name: String) = Action { Ok(greeting.message(name)) }
}

package models
trait Greeting {
def message(name: String): String
}
class EnglishGreeting extends Greeting {
override def message(name: String) = s"Hello, $name"
}
class JapaneseGreeting extends Greeting {
override def message(name: String) = s"こんにちは、$name"
}

import com.google.inject.AbstractModule
import com.google.inject.name.Names
import models._
class Module extends AbstractModule {
override def configure(): Unit = {
bind(classOf[Greeting])
.annotatedWith(Names.named("en"))
.to(classOf[EnglishGreeting])
bind(classOf[Greeting])
.annotatedWith(Names.named("jp"))
.to(classOf[JapaneseGreeting])
}
}

Play Framework with Google Guice
Google Guice is a runtime DI library
- 実行時に依存性を解決する
- Play で主に使われるのは Constructor Injection
Constructor Injection is mostly used in Play
Framework.

全て Google Guice でやる必要はない
そもそもプログラムとインタープリタの分離がしたい。
インターフェースと実装を分けたい、後で入れ替えたい。
そして以下のような性質を持っていると嬉しい
- Compile Time DI
- Composability
- Testability
We want to separate program & execution. Compile
time DI, composability and testability are desired.

全て Google Guice でやる必要はない
純粋関数型プログラミング言語由来のツールを使っておけば全て
解決するんでしょ？
- Reader Monad
- Free Monad
- etc.
I know! Purely functional programming solves all the
things.

Separation of program & execution - Reader Monad
trait UserRepository {
def store(user: User)(implicit session: DBSession): Try[Unit]
def resolveByName(name: String)(implicit session: DBSession): Try[Option[User]]
def resolveAllByNames(name: String*)
(implicit session: DBSession): Try[List[User]]
}
class UserRepositoryOnJDBC extends UserRepository {
def store(user: User)(implicit dbSession: DBSession) = ???
def resolveByName(name: String)(implicit dbSession: DBSession) = ???
def resolveAllByNames(name: String*)(implicit dbSession: DBSession) = ???
}

trait UserRepositoryModule {
def userRepository: UserRepository
}
trait MessageRepositoryModule {
def messageRepository: MessageRepository
}

type Prog[A] = Reader[UserRepositoryModule with MessageRepositoryModule, A]
def getReplyTargetUsers(messageId: Long): Prog[Try[List[User]]] =
Reader { module =>
for {
messageOpt <- module.messageRepository.resolveBy(messageId)
names = messageOpt.fold(List.empty[String])(_.replyTargets)
users <- module.userRepository.resolveAllByNames(names: _*)
} yield users
}

val module = new UserRepositoryModule with MessageRepositoryModule {
override def userRepository = new UserRepositoryOnJDBC()
override def messageRepository = new MessageRepositoryOnJDBC()
}
MessageAPI.getReplyTargetUsers(1).run(module)

- Reader Monad では依存が複数になると扱いにくい。
型が冗長になる
- 引数に一つしか取れないので、Context 組み立てと分解の手
間が発生する
It’s hard to handle with reader monad when it has
multiple dependencies.

Separation of program & execution - Free Monad
type UserRepositoryFree[A] = Free[UserRepositoryOp, A]
sealed trait UserRepositoryOp[A]
object UserRepositoryOp {
case class Store(user: User) extends UserRepositoryOp[Unit]
case class ResolveByName(name: String) extends UserRepositoryOp[Option[User]]
def store(user: User): UserRepositoryFree[Unit] =
Free.liftF[UserRepositoryOp, Unit](Store(user))
def resolveByName(name: String): UserRepositoryFree[Option[User]] =
Free.liftF[UserRepositoryOp, Option[User]](ResolveByName(name))
}

object UserRepository {
UserRepositoryOp.store(user)
UserRepositoryOp.resolveByName(name)
}

object UserRepositoryOnJDBCInterp {
def futureInterp(session: DBSession)
(implicit ec: ExecutionContext): UserRepositoryOp ~> Future =
new (UserRepositoryOp ~> Future) {
def apply[A](op: UserRepositoryOp[A]): Future[A] =
op match {
case UserRepositoryOp.Store(user) =>
Future(...)
case UserRepositoryOp.ResolveByName(name) =>
Future(...)
}
}
}

val dbSession = new DBSession()
UserRepository.resolveByName("John Doe")
.foldMap(futureInterp(dbSession))

Free Monad は
- ボイラープレートが多い
でもコード自動生成に頼るのは微妙
- 実装時に何を処理の最小単位 (≒ 1つの ADT)とするか見極
めが難しい
- 異なる ADT の合成には一苦労
Free Monad requires too many boilerplates,
and composing multiple ADTs is difficult.

＿人人人人人人人人人人＿
＞　Tagless final style　＜
￣Y^Y^Y^Y^Y^Y^Y^Y^Y￣

What’s Tagless Final style
言語内 DSL (Embedded DSL) を作成する手法の一つ
Free Monad と比較して少ない記述量でプログラムと実装の分
離、DSL の合成が実現できる。
One method of creating EDSL. Tagless final has less
boilerplate.

Introduction to Tagless Final
trait UserRepository[F[_]] {
def store(user: User): F[Unit]
def resolveByName(name: String): F[Option[User]]
}

implicit val tryUserRepository: UserRepository[Try] =
new UserRepository[Try] {
def store(user: User): Try[Unit] = ???
def resolveByName(name: String): Try[Option[User]] = ???
}
}

def prog[F[_]](userName: String)
(implicit userRepo: UserRepository[F]): F[Option[User]] =
userRepo.resolveByName(userName)
prog[Try]("John")
// scala.util.Try[Option[User]] = Success(None)

また宇宙からやってきた難しい概念なんでしょう...？
Where are you from?
Isn’t it purely functional Alien?

よくある UserRepository インターフェースと実装例
trait UserRepository[Ctx] {
def store(user: User)(implicit ctx: Ctx): Future[Unit]
def resolveByName(name: String)(implicit ctx: Ctx): Future[Option[User]]
}
// impl
class UserRepositoryOnJDBC()(implicit ec: ExecutionContext)
extends UserRepository[DBSession] {
def store(user: User)(implicit ctx: DBSession): Future[Unit] =
???
def resolveByName(name: String)(implicit ctx: DBSession): Future[Option[User]] =
???
}

Problem 1:
各メソッドが Context を暗黙の値として受け取る
- Context を一つだけとることを前提としている
- 暗黙的な値が複数必要だった場合にはそれらをまとめるコン
テナを用意する必要がある
- メソッド内では受け取った値から自分に必要なものを取り出す
作業が発生する
Each method takes a context as an implicit
parameter.

Problem 1:
class UserRepositoryCtx(ec: ExecutionContext, dbSession: DBSession)
// impl on database
class UserRepositoryOnJDBC() extends UserRepository[UserRepositoryCtx] {
def store(user: User)(implicit ctx: UserRepositoryCtx): Future[Unit] = {
implicit val (ec, s) = (ctx.ec, ctx.dbSession)
??? // update
}
def resolveByName(name: String)
(implicit ctx: UserRepositoryCtx): Future[Option[User]] = {
implicit val (ec, s) = (ctx.ec, ctx.dbSession)
??? // select
}
}

Problem 1:
case class DummyCtx()
// impl by memory
object UserRepositoryOnMemory extends UserRepository[DummyCtx]{
private val db = mutable.Map.empty[UserId, User]
def store(user: User)(implicit ctx: DummyCtx): Future[Unit] =
Future.successful(db.update(user.id, user))
def resolveByName(name: String)(implicit ctx: DummyCtx): Future[Option[User]] =
Future.successful(db.collectFirst {
case (_, u@User(_, n, _)) if n == name => u
})
}

Problem 2:
メソッドの実行結果をどう表現するか
いくつかのパターンが考えられる
- 呼び出し側に責任を押し付け失敗を全て例外で throw
- 例外を型で表現するために Try に包む
- 非同期処理もありうるから Future にしておく
- etc.
How to express the return value as a type.

Problem 2:
メソッドの実行結果をどう表現するか
// It’s simply! but throwable...
def store(user: User)(implicit ctx: Ctx): Unit
def resolveByName(name: String)(implicit ctx: Ctx): Option[User]
}
// It’s not throwable! but blocking only...
def store(user: User)(implicit ctx: Ctx): Try[Unit]
def resolveByName(name: String)(implicit ctx: Ctx): Try[Option[User]]
}
// It’s non blocking! but needs ExecutionContext in Ctx...
}

つまり、どうすればいいのか
UserRepository インターフェース内に実装側の都合を一切持ち
込まないというのが重要
- Context という概念を UserRepository 内に持ち込まない
- Context を暗黙の値にとるような前提の実装をやめる
- メソッドの実行方法や失敗の表現など、まるっと含めて返り値
の型を抽象的に表現する
Do not bring implementation related things to
UserRepository interface.

Remove implicit parameter
}

}
// 1. remove implicit
def store(user: User)(ctx: Ctx): Future[Unit]
def resolveByName(name: String)(ctx: Ctx): Future[Option[User]]
}

}
// 1. remove implicit
def store(user: User)(ctx: Ctx): Future[Unit]
def resolveByName(name: String)(ctx: Ctx): Future[Option[User]]
}
// 2. modify return function
def store(user: User): Ctx => Future[Unit]
def resolveByName(name: String): Ctx => Future[Option[User]]
}

}
// easy to use implicit parameter within impl.
class UserRepositoryOnJDBC(implicit ec: ExecutionContext)
extends UserRepository[DBSession] {
def store(user: User): DBSession => Future[Unit] = {
implicit dbSession =>
???
}
def resolveByName(name: String): DBSession => Future[Option[User]] = {
implicit dbSession =>
???
}
}

Really need Context?
}

}
// 1. create type alias named `Exec` and replace return type
type Exec[R] = Ctx => Future[R]
def store(user: User): Exec[Unit]
def resolveByName(name: String): Exec[Option[User]]
}

}
// 1. create type alias named `Exec` and replace return type
type Exec[R] = Ctx => Future[R]
def store(user: User): Exec[Unit]
def resolveByName(name: String): Exec[Option[User]]
}
// 2. add `F[_]` type parameter and remove `Exec`
}

It’s more abstract interface
}

UserRepository implementation
}

}
type Query[A] = Reader[DBSession, A]
object UserRepositoryOnJDBC {
implicit def queryUserRepository: UserRepository[Query] =
new UserRepository[Query] {
def store(user: User): Query[Unit] =
Reader { implicit dbSession => ??? }
def resolveByName(name: String): Query[Option[User]] =
Reader { implicit dbSession => ??? }
}
}

}
type AsyncQuery[A] = ReaderT[Task, DBSession, A]
implicit def asyncQueryUserRepository: UserRepository[AsyncQuery] =
new UserRepository[AsyncQuery] {
def store(user: User): AsyncQuery[Unit] =
ReaderT { implicit dbSession => Task(???) }
def resolveByName(name: String): AsyncQuery[Option[User]] =
ReaderT { implicit dbSession => Task(???) }
}
}

}
implicit def freeUserRepository: UserRepository[UserRepositoryFree] =
new UserRepository[UserRepositoryFree] {
UserRepositoryOp.store(user)
UserRepositoryOp.resolveByName(name)
}
}

}
type ConnectionIO[A] = Free[ConnectionOp, A]
implicit def doobieUserRepository: UserRepository[ConnectionIO] =
new UserRepository[ConnectionIO] {
def store(user: User): ConnectionIO[Unit] =
sql"...".update.run.map(_ => ())
def resolveByName(name: String): ConnectionIO[Option[User]] =
sql"...".query[User].option
}
}

def apply[F[_]](implicit F: UserRepository[F]): UserRepository[F] = F
}
val dbSession = new DBSession()
type Query[A] = Reader[DBSession, A]
UserRepository[Query].resolveByName("John Doe")
.run(dbSession)
UserRepository[AsyncQuery].resolveByName("John Doe")
.run(dbSession).runAsync
UserRepository[UserRepositoryFree].resolveByName("John Doe")
.foldMap(futureInterp(dbSession))

def apply[F[_]](implicit F: UserRepository[F]): UserRepository[F] = F
}
val xa = Transactor.fromDriverManager[Task](...) // for doobie
type ConnectionIO[A] = Free[ConnectionOp, A]
UserRepository[ConnectionIO].resolveByName("John Doe")
.transact(xa).runAsync

UserController implementation
object UserController {
def getUser[F[_]: Monad: UserRepository](id: Long): F[Result] =
UserRepository[F].resolveBy(id).map {
case Some(user) => Results.Ok(GetUserResponse.from(user).toJson)
case None => Results.NotFound
}
}

UserController implementation
class UserController @Inject()(cc: ControllerComponents, ec: ExecutionContext)
// ...
import UserRepositoryOnJDBC._ // impls
def getUser(id: Long) = Action {
UserController.getUser[Query](id).run(dbSession)
}
def getUserAsync(id: Long) = Action.async {
UserController.getUser[AsyncQuery](id).run(dbSession).runAsync
}
}

UserController testing
class UserRepositoryMock extends UserRepository[Id] {
def store(user: User): Id[Unit] = ???
def resolveBy(id: Long): Id[Option[User]] = ???
def resolveByName(name: String): Id[Option[User]] = ???
}

class UserControllerSpec extends PlaySpec {
"UserController GET" should {
"get user from static controller" in {
implicit val mock = new UserRepositoryMock {
override def resolveBy(id: Long): Option[User] =
if (id == 100)
Some(User(100, "John"))
else
None
}
UserController.getUser[Id](100).header.status mustBe OK
UserController.getUser[Id](999).header.status mustBe NOT_FOUND
}
}
}

object UserController {
// ...
def storeAndGet[F[_]: Monad: UserRepository](name: String): F[Result] =
for {
_ <- UserRepository[F].store(User(Random.nextLong(), name))
user <- UserRepository[F].resolveByName(name)
} yield {
user match {
case Some(u) => Results.Ok(GetUserResponse.from(u).toJson)
case None => Results.InternalServerError
}
}
}

type DB = List[User]
class UserRepositoryStateMock extends UserRepository[State[DB, ?]] {
def store(user: User): State[DB, Unit] = ???
def resolveBy(id: Long): State[DB, Option[User]] = ???
def resolveByName(name: String): State[DB, Option[User]] = ???
}

class UserControllerSpec extends PlaySpec {
"UserController GET" should {
"store and get user from static controller" in {
implicit val mock = new UserRepositoryStateMock {
override def store(user: User): State[DB, Unit] =
State(db => (user :: db, ()))
override def resolveByName(name: String): State[DB, Option[User]] =
State.inspect(_.find(_.name == name))
}
UserController.storeAndGet[State[DB, ?]]("John")
.runEmptyA.value.header.status mustBe OK
}
}

F[_] が異なる場合は for 式で合成できない
object MessageController {
def getUserMessages[F[_]: Monad: MessageRepository: UserRepository](
userId: Long
): F[Result] =
for {
user <- UserRepository[F].resolveBy(userId)
messages <- MessageRepository[F].resolveAllByUserId(userId)
} yield {
user match {
case Some(u) => Results.Ok(GetMessagesResponse.from(u, messages).toJson)
}
}
}

type AsyncCommand[A] = ReaderT[Task, RedisConnection, A]
object MessageRepositoryOnJDBC {
// MessageRepository[AsyncQuery] is not implemented
implicit def redisConnectionMessageRepository: MessageRepository[AsyncCommand] =
new MessageRepository[AsyncCommand] {
def resolveBy(id: Long): AsyncCommand[Option[Message]] = ???
def store(message: Message): AsyncCommand[Unit] = ???
def resolveAllByUserId(userId: Long): AsyncCommand[List[Message]] = ???
}
}

[pfpfap] $ compile
[info] Compiling 1 Scala source to /Users/aoiroaoino/git/pfpf/bbs/target/scala-2.12/classes …
...
[error] /Users/aoiroaoino/git/pfpf/bbs/app/controllers/MessageController.scala:26:50:
could not find implicit value for evidence parameter of type
models.repository.MessageRepository[models.AsyncQuery]
[error] MessageController.getUserMessages[AsyncQuery](userId).run(dbSession).runAsync
[error] ^

いくつかの解決策がある
- Monad を積み上げる
- F[_] を具体的な型にしてから controller 合成する
- 自然変換を使って型を揃える
There are some solutions. Stacking Monads,
composing concrete types, Natural Transformation.

Stacking Monads
class MessageController @Inject()(cc: ControllerComponents, ec: ExecutionContext)
// ...
type SpecialAction[A] = ReaderT[ReaderT[Task, DBSession, ?], RedisConnection, A]
def getUserMessagesSM(userId: Long) = Action.async {
MessageController.getUserMessages[SpecialAction](userId)
.run(redisConnection) // RedisConnection => ReaderT[Task, DBSession, A]
.run(dbSession) // DBSession => Task[A]
.runAsync // CancelableFuture[A]
}
}

Composing concrete types
// ...
def getUserMessages(userId: Long) = Action.async {
(for {
user <- UserRepository[AsyncQuery].resolveBy(userId).run(dbSession)
messages <- MessageRepository[AsyncCommand].resolveAllByUserId(userId)
.run(redisConnection)
} yield {
user match {
case Some(u) => Results.Ok(GetMessagesResponse.from(u, messages).toJson)
}
}).runAsync
}
}

Use natural transformation and mapK()
// ...
type AsyncCommand[A] = ReaderT[Task, RedisConnection, A]
// common type of AsyncQuery and AsyncCommand
type Exec[A] = ReaderT[Task, (DBSession, RedisConnection), A]
MessageController.getUserMessages[Exec](userId)
.run((dbSession, redisConnection)).runAsync
}
}

trait MessageRepository[F[_]] {
def store(message: Message): F[Unit]
def resolveBy(id: Long): F[Option[Message]]
def resolveAllByUserId(userId: Long): F[List[Message]]
def mapK[G[_]](fk: F ~> G): MessageRepository[G] =
new MessageRepository[G] {
def store(message: Message): G[Unit] =
fk(self.store(message))
def resolveBy(id: Long): G[Option[Message]] =
fk(self.resolveBy(id))
def resolveAllByUserId(userId: Long): G[List[Message]] =
fk(self.resolveAllByUserId(userId))
}
}

// ...
val asyncQueryToExec: AsyncQuery ~> Exec =
new (AsyncQuery ~> Exec) {
def apply[A](aq: AsyncQuery[A]): Exec[A] =
ReaderT { case (dbSession, _) => aq.run(dbSessioin) }
}
val asyncCommandToExec: AsyncCommand ~> Exec =
new (AsyncCommand ~> Exec) {
def apply[A](ac: AsyncCommand[A]): Exec[A] =
ReaderT { case (_, redisConn) => ac.run(redisConn) }
}
}

// ...
val asyncQueryToExec: AsyncQuery ~> Exec = ...
val asyncCommandToExec: AsyncCommand ~> Exec = ...
implicit val execUserRepository: UserRepository[Exec] =
UserRepository[AsyncQuery].mapK(asyncQueryToExec)
implicit val execMessageRepository: MessageRepository[Exec] =
MessageRepository[AsyncCommand].mapK(asyncCommandToExec)
}

// ...
val asyncQueryToExec: AsyncQuery ~> Exec = ...
val asyncCommandToExec: AsyncCommand ~> Exec = ...
implicit val execUserRepository: UserRepository[Exec] = ...
implicit val execMessageRepository: MessageRepository[Exec] = ...
MessageController.getUserMessages[Exec](userId)
.run((dbSession, redisConnection)).runAsync
}
}

Conclusion
Google Guice でアプリを全て組み立てる必要はない
- プログラムとインタープリタの分離をする方法はいくつもあ
る。
- Reader や Free もいいけど直接使うと不便さもある。
There are many ways to separate program &
interpreter.

Conclusion
Tagless Final 便利
- 手で書いても苦にならない程度のコード量
- 複数の DSL を比較的簡単に合成できる
- F[_] が異なる場合に一工夫必要。銀の弾丸ではない。
- Play Framework に限らずいろんな場面で使える！
Tagless final is useful because it has less boilerplate.

purely_functional_play_framework_application

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