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@chara06ken(Hara Kenta)

がばがばscala独学 - トレイト(2)

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はじめに

下記のテキストを通して、基本的な書き方を学びつつ、個人的に気になったことを検証していくメモ代わりです。
https://dwango.github.io/scala_text/basic.html
本当にありがとう、dwango様・・・

トレイト

  • ざっくりまとめ
    • 菱形継承問題を解決するために
      • (trait->class) 継承先classにてoverride -> 継承の意味合いが薄れる
      • (trait->trait) 継承先traitにてoverride -> 線形化

トレイトの様々な機能

菱形継承問題

  • 菱形継承の例

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p() : Unit }
trait TraitB extends TraitA { def p() : Unit = println("TraitB") }
trait TraitC extends TraitA { def p() : Unit = println("TraitC") }

class ClassA extends TraitB with TraitC

val c = new ClassA
c.p()

// Exiting paste mode, now interpreting.

<console>:17: error: class ClassA inherits conflicting members:
  method p in trait TraitB of type ()Unit  and
  method p in trait TraitC of type ()Unit
(Note: this can be resolved by declaring an override in class ClassA.)
              class ClassA extends TraitB with TraitC
                    ^
    • conflictingとしてerrorとなる

  • 解決手法１：override

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit }
trait TraitB extends TraitA { def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { def p(): Unit = println("TraitC") }

class ClassA extends TraitB with TraitC { override def p(): Unit = println("ClassA") }

val c = new ClassA
c.p()

// Exiting paste mode, now interpreting.

ClassA
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA
c: ClassA = ClassA@302d20c7

  • 解決手法２：親元のメソッドを利用したoverride

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit }
trait TraitB extends TraitA { def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { def p(): Unit = println("TraitC") }

class ClassA extends TraitB with TraitC { override def p(): Unit = super[TraitB].p() }

val c = new ClassA
c.p()

// Exiting paste mode, now interpreting.

TraitB
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA
c: ClassA = ClassA@6587f054

---
scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit = println("TraitA")}
trait TraitB extends TraitA { def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { def p(): Unit = println("TraitC") }

class ClassA extends TraitB with TraitC { override def p(): Unit = super[TraitA].p() }

val c = new ClassA
c.p()

// Exiting paste mode, now interpreting.

<console>:16: error: TraitA does not name a parent class of class ClassA
              class ClassA extends TraitB with TraitC { override def p(): Unit = super[TraitA].p() }
                                                                                 ^

  • 双方のメソッドを呼び出す場合

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit}
trait TraitB extends TraitA { def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { def p(): Unit = println("TraitC") }

class ClassA extends TraitB with TraitC {
  override def p(): Unit = {
    super[TraitB].p()
    super[TraitC].p()
  }
}

val c = new ClassA
c.p()

// Exiting paste mode, now interpreting.

TraitB
TraitC
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA
c: ClassA = ClassA@5362e897
    • 継承関係が複雑になった場合にすべてを明示的に呼ぶのは大変
    • コンストラクタのように必ず呼び出されるメソッドも有り

線形化

  • 菱形継承問題の以下を解決する

    • 継承関係が複雑になった場合にすべてを明示的に呼ぶのは大変
    • コンストラクタのように必ず呼び出されるメソッドも有り

  • 線形化の例

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit}
trait TraitB extends TraitA { override def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { override def p(): Unit = println("TraitC") }

class ClassA extends TraitB with TraitC
(new ClassA).p

// Exiting paste mode, now interpreting.

TraitC
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA
    • TraitB/TraitC 各々のメソッドに対してoverrideを記載

  • extends と with での継承を逆にすると？

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit}
trait TraitB extends TraitA { override def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { override def p(): Unit = println("TraitC") }

class ClassA extends TraitC with TraitB
(new ClassA).p

// Exiting paste mode, now interpreting.

TraitB
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA

  • 3つ以上継承した場合は？

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit}
trait TraitB extends TraitA { override def p(): Unit = println("TraitB") }
trait TraitC extends TraitA { override def p(): Unit = println("TraitC") }
trait TraitD extends TraitA { override def p(): Unit = println("TraitD") }

class ClassA extends TraitB with TraitC with TraitD
(new ClassA).p

// Exiting paste mode, now interpreting.

TraitD
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined trait TraitD
defined class ClassA
    • 最後に継承したメソッドで上書き

  • super により線形化された親トレイトを使う

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait TraitA { def p(): Unit = println("TraitA")}
trait TraitB extends TraitA {
  override def p(): Unit = {
    super.p()
    println("TraitB")
  }
}
trait TraitC extends TraitA {
  override def p(): Unit = {
    super.p()
    println("TraitC")
  }
}

class ClassA extends TraitB
class ClassB extends TraitC
class ClassC extends TraitB with TraitC
class ClassD extends TraitC with TraitB
println("=== ClassA ===")
(new ClassA).p
println("=== ClassB ===")
(new ClassB).p
println("=== ClassC ===")
(new ClassC).p
println("=== ClassD ===")
(new ClassD).p

// Exiting paste mode, now interpreting.

=== ClassA ===
TraitA
TraitB
=== ClassB ===
TraitA
TraitC
=== ClassC ===
TraitA
TraitB
TraitC
=== ClassD ===
TraitA
TraitC
TraitB
defined trait TraitA
defined trait TraitB
defined trait TraitC
defined class ClassA
defined class ClassB
defined class ClassC
defined class ClassD

自分型

  • Scalaにはクラスやトレイトの中で自分自身の型にアノテーションを記述することができる機能
  • 名称/表記
    • 自分型アノテーション（self type annotations）
    • 自分型（self types）

  • 自分型使用例

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit = println("Greeter") }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

// Exiting paste mode, now interpreting.

defined trait Greeter
defined trait Robot

  • 先を読まず使ってみた？

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit = println("Greeter") }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}
class c extends Robot
(new c).start
(new c).start.toString

// Exiting paste mode, now interpreting.

<console>:16: error: illegal inheritance;
 self-type c does not conform to Robot's selftype Robot with Greeter
              class c extends Robot
                              ^
    • 直接の継承はできない！？

  • オブジェクトを実際に作るためには greet メソッドを実装したトレイトが必要

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit = println("Greeter") }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
class c extends Robot with HelloGreeter

// Exiting paste mode, now interpreting.

<console>:17: error: overriding method greet in trait Greeter of type => Unit;
 method greet needs `override' modifier
       trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
                                                ^
    • overtideしろと出る。なんで！？
      • def greet: Unit = println("Greeter") 単純に親traitで宣言していたためっぽい
    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
class c extends Robot with HelloGreeter
(new c).start

// Exiting paste mode, now interpreting.

Hello!
defined trait Greeter
defined trait Robot
defined trait HelloGreeter
defined class c

  • 直接継承した場合の見え方の違い

    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit }

trait Robot1 {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

trait Robot2 extends Greeter{
  def start: Unit = greet
  override final def toString = "Robot"
}

trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
class c1 extends Robot1 with HelloGreeter
class c2 extends Robot2 with HelloGreeter
(new c1).start
(new c2).start
(new c1).greet
(new c2).greet

// Exiting paste mode, now interpreting.

Hello!
Hello!
Hello!
Hello!
defined trait Greeter
defined trait Robot1
defined trait Robot2
defined trait HelloGreeter
defined class c1
defined class c2
    • 違いが出ないぞ！？
      • 通常は (new c1).greet が失敗するはず
    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
val r:Robot = new Robot with HelloGreeter
r.greet

// Exiting paste mode, now interpreting.

<console>:19: error: value greet is not a member of Robot
              r.greet
                ^
    • Robot traitを直接読みに行った場合のみ出るらしい。何故？
    scala> :paste
// Entering paste mode (ctrl-D to finish)

trait Greeter { def greet: Unit }

trait Robot {
  self: Greeter =>
  def start: Unit = greet
  override final def toString = "Robot"
}

trait HelloGreeter extends Greeter { def greet: Unit = println("Hello!") }
val r:Robot = new Robot with HelloGreeter
r.start

// Exiting paste mode, now interpreting.

Hello!
defined trait Greeter
defined trait Robot
defined trait HelloGreeter
r: Robot = Robot
    • startは読み出せるので、定義が間違っているわけではない

  • val r:Robot = new Robot with HelloGreeter で何が生成されている？

    • 内部的にinner classを持ち合わせているらしい（要調査）
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