Various accumulation examples in Scala

various-accumulation-examples.scala

/** まず、import, case class, val nodeseqのところまで
 * REPLの:pasteコマンドでコピペして、各Exampleも同様にコピペして実行してみてね！
 *
 * Use :paste command in Scala REPL to run example code.
 */

import xml._

case class ColorPair(foregroundOption: Option[String], backgroundOption: Option[String])

/** テストデータ
 *  Foreground, Backgroundのvalueだけを抽出してColorPairをつくりたい。
 *
 * Test data
 * I want to extract and compose values of nodes named "Foreground" and "Background" to build a ColorPair
 */

val nodeseq = NodeSeq.fromSeq(Seq(
  <option name="Foreground" value="586e75"></option>,
  <option name="NoValue"></option>,
  <option name="Background" value="4d1d08"></option>,
  <option name="Irrelevant" value="123456"></option>
))

/** 期待結果
 *  Expected result
 */

ColorPair(Some(586e75),Some(4d1d08))

# Example 1. Accumulation (foldLeft + match)

nodeseq.view
  .map(node => (node.attribute("name").map(_.text), node.attribute("value").map(_.text)))
  .foldLeft(ColorPair(None, None)) { (cp, t) => t._1 match {
    case Some("Foreground") => ColorPair(t._2, cp.backgroundOption)
    case Some("Background") => ColorPair(cp.foregroundOption, t._2)
    case _ => cp
  }
}

// res1: ColorPair = ColorPair(Some(586e75),Some(4d1d08))

# Example 2. Accumulation (foldLeft + partial function)

nodeseq.view
  .map(node => (node.attribute("name").map(_.text), node.attribute("value").map(_.text)))
  .foldLeft(ColorPair(None, None)) {
    case (cp, (Some("Foreground"), foregroundOption)) => ColorPair(foregroundOption, cp.backgroundOption)
    case (cp, (Some("Background"), backgroundOption)) => ColorPair(cp.foregroundOption, backgroundOption)
    case (cp, _) => cp
  }

// res2: ColorPair = ColorPair(Some(586e75),Some(4d1d08))

# Example 3. Use temporary vals 1 (collectFirst)

val nameValuePairs = nodeseq.view
  .map(node => (node.attribute("name").map(_.text), node.attribute("value").map(_.text)) )
val foregroundOption = nameValuePairs.collectFirst {
  case (Some("Foreground"), Some(foreground)) => foreground
}
val backgroundOption = nameValuePairs.collectFirst {
  case (Some("Background"), Some(background)) => background
}
ColorPair(foregroundOption, backgroundOption)

// res3: ColorPair = ColorPair(Some(586e75),Some(4d1d08))

# Example 4. Use temporary vals 2 (find + flatMap)

val nameValuePairs = nodeseq.view
  .map(node => (node.attribute("name").map(_.text), node.attribute("value").map(_.text)) )
val foregroundOption = nameValuePairs.find(_._1 == Some("Foreground")).flatMap(_._2)
val backgroundOption = nameValuePairs.find(_._1 == Some("Background")).flatMap(_._2)
ColorPair(foregroundOption, backgroundOption)

// res4: ColorPair = ColorPair(Some(586e75),Some(4d1d08))