Boolean Extractors

As discussed in other topics there are several ways to create custom extractors for objects. See

• Extractors Introduction
• Extract Sequences
• Operator Style Extractors

There is one more custom extractor that can be defined. Simple boolean extractors. They are used as follows:

1. scala> "hello world" match {                   
2.      | case HasVowels() => println("Vowel found")
3.      | case _ => println("No Vowels")            
4.      | }
5. Vowel found

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

Examples:

1. scala> object HasVowels{ def unapply(in:String):Boolean = in.exists( "aeiou" contains _ ) }
2. defined module HasVowels
4. // Note that HasVowels() has ().
5. // This is critical because otherwise the match checks whether
6. // the input is the HasVowels object.
7. // The () forces the unapply method to be used for matching
8. scala> "hello world" match {
9.      | case HasVowels() => println("Vowel found")
10.      | case _ => println("No Vowels")
11.      | }
12. Vowel found
14. // Don't forget the ()
15. scala> "kkkkkk" match {    
16.      | case HasVowels() => println("Vowel found")
17.      | case _ => println("No Vowels")
18.      | }
19. No Vowels
21. scala> class HasChar(c:Char) {           
22.      | def unapply(in:String) = in.contains(c)
23.      | }
24. defined class HasChar
26. scala> val HasC = new HasChar('c')
27. HasC: HasChar = HasChar@3f2f529b
29. // Don't forget the () it is required here as well
30. scala> "It actually works!" match {
31.      | case HasC() => println("a c was found")
32.      | case _ => println("no c found")  
33.      | }
34. a c was found
36. // Don't forget the ()
37. scala> "hello world" match { 
38.      | case HasC() => println("a c was found")
39.      | case _ => println("no c found")       
40.      | }
41. no c found