Boolean unapply extractor

gistfile1.scala

// A boolean extractor is an object that returns Boolean from 
// the unapply method rather than Option[_]. 

scala> :paste

object HasVowels {
  def unapply(in: String): Boolean = in.exists("aeiou".toSet)
}

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defined module HasVowels

scala> :paste

// Note that HasVowels() has ()
// This is critical because otherwise the match checks whether
// the input is the HasVowels object.
// The () forces the unapply method to be used for matching

scala> :paste

"hello world" match {
  case HasVowels() => println("Vowel found")
  case _           => println("No Vowels")
}

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Vowel found

scala> :paste

class HasChar(c: Char) {
  def unapply(in: String) = in.contains(c)
}

object HasChar {
  def apply(c: Char) = new HasChar(c)
}

val HasC = HasChar('c')

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defined class HasChar
defined module HasChar
HasC: HasChar = HasChar@45d682e8

scala> :paste

"It actually work!" match {
  case HasC() => println("a 'c' was found")
  case _      => println("no c was found")
}

a 'c' was found

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