社内勉強会でusingについて説明

a.scala

// 単純なopen、close

val fileName = "text.txt"

val resource = new BufferedReader(new FileReader(new File(fileName)))
resource.readLine
resource.close

b.scala

// try、catchをちゃんとやる

val resource = new BufferedReader(new FileReader(new File(fileName)))
try {
  resource.readLine
} finally {
  resource.close
}

c.scala

// 似たようなファイルの読み書き処理が増えると野暮ったくなる

val resource = new BufferedReader(new FileReader(new File(fileName)))
try {
  val l = resource.readLine
  l.head
} finally {
  resource.close
}

val resource = new BufferedReader(new FileReader(new File(fileName)))
try {
  val l = resource.readLine
  l.tail
} finally {
  resource.close
}

d.scala

// ファイル処理にかかわるはusingの中に書いて、機能を分離する。

def using[A, R <: {def close()}](resource: R)(func: R => A): A = {
  try {
    func(resource)
  } finally {
    resource.close
  }
}

val in = new BufferedReader(new FileReader(new File(fileName)))

using(in) { resource =>
  val l = resource.readLine
  l.head
}

val in = new BufferedReader(new FileReader(new File(fileName)))

using(in) { resource =>
  val l = resource.readLine
  l.tail
}