Scala
Refactoring
Tomer Gabel, Scala Matsuri 2016
for Fun and Profit
楽しく役立つ Scala リファクタリング

Agenda
• For the next 40
minutes, we’ll:
– Look at examples
– Discuss patterns
– … and anti-patterns
– Showcase refactoring
techniques
例を通してパターンとアンチパターンを議論し、
リファクタリングのテクニックを説明する

Our Victim
• … is ScalaChess
– Provides a full domain
model for chess
– Good test coverage
– High quality code
– MIT license
– Integrated in lichess.org* ScalaChess is an open-source project by Thibault Duplessis
チェスのドメインモデルを備えるScalaChessを例にする
高品質でlichess.orgと統合されたMITライセンスのOSS

THE LAY OF THE
LAND前線の状況

Stringly Typed
“Used to describe an implementation
that needlessly relies on strings when
programmer & refactor friendly options
are available.”
-- Coding
Horror
アンチパターン：Stringly Typed
型付けできる所で不必要に文字列に頼った実装のこと

Stringly Typed
• Examples:
– Carrying unparsed data around
– Using empty strings instead of Options
case class Person(name: String, location: String)
def nearest(to: Person, all: List[Person]): Person = {
val geo: Point = Point.parse(to.location)
all.minBy(p => geo.distanceTo(Point.parse(p.location)))
}
1. Inefficient (space/time)
2. Error handling all over the place
3. What’s with all the boilerplate?
Stringly Typed の例1:
パースしてないデータを持ち回す

Stringly Typed
• Examples:
– Carrying unparsed data around
– Using empty strings instead of Options
case class Person(name: String, location: Point)
def nearest(to: Person, all: List[Person]): Person =
all.minBy(p => to.location distanceTo p.location)
1. Efficient (only parsed once)
2. Sane error handling
3. Zero boilerplate!
パース後のデータを使うことで効率的で、
エラー処理が容易に、お決まりの処理も不要になる

Stringly Typed
• Examples:
– Carrying unparsed data around
– Using empty strings instead of Options
case class Person(firstName: String, lastName: String)
def render(p: Person): String =
s"""
|<div id='first-name'>${p.firstName}</div>
|<div id='last-name'>${p.lastName}</div>
""".stripMargin
1. Nothing enforces emptiness check!
2. Scala has a great type for these :-)
Stringly Typed の例2:
Option の代わりに空文字を使う

REAL-WORLD EXAMPLE TIME!

Collective Abuse
• Scala has a massive
collection library
• Loads of built-ins too
– Case classes
– Functions and partials
– Tuples, tuples, tuples
• Fairly easy to abuse
Scalaにはたくさんのコレクション、caseクラス、関数、
タプルがある。これらは、実は簡単に濫用できてしまう

Collective Abuse
• Common anti-patterns:
– Too many inline steps
– Tuple overload
val actors: List[(Int, String, Double)] = // ...
def bestActor(query: String) =
actors.filter(_._2 contains query)
.sortBy(-_._3)
.map(_._1)
.headOption
1. What does this even do?!
2. How does data flow here?
アンチパターン：
一行に処理を詰め込み過ぎる

Collective Abuse
• Common anti-patterns:
– Too many inline steps
– Tuple overload
val actors: List[(Int, String, Double)] = // ...
def bestActor(query: String) = {
val matching = actors.filter(_._2 contains query)
val bestByScore = matching.sortBy(-_._3).headOption
bestByScore.map(_._1)
} Name intermediate steps!
中間状態に名前を付けよう！

Collective Abuse
• Common anti-patterns:
– Too many inline steps
– Tuple overload
val actors: List[(Int, String, Double)] = // ...
def bestActor(query: String) =
actors.filter(_._2 contains query)
.sortBy(-_._3)
.map(_._1)
.headOption
What’s with all these
underscores?
アンチパターン：
タプルの使いすぎ

Collective Abuse
• Common anti-patterns:
– Too many inline steps
– Tuple overload
case class Actor(id: Int, name: String, score: Double)
def bestActor(query: String, actors: List[Actor]) =
actors.filter(_.name contains query)
.sortBy(-_.score)
.map(_.id)
.headOption
Scala classes are cheap.
Use them.
Scala ではcaseクラスを簡単に定義できる
どんどん使おう

REAL-WORLD EXAMPLE TIME!

Questions?
tomer@tomergabel.com
@tomerg
http://il.linkedin.com/in/tomergabel
WE’RE DONE
HERE!