Question

Basically, I would like to be able to build a custom extractor without having to store it in a variable prior to using it.

This isn’t a real example of how I would use it, it would more likely be used in the case of a regular expression or some other string pattern like construct, but hopefully it explains what I’m looking for:

def someExtractorBuilder(arg:Boolean) = new {
  def unapply(s:String):Option[String] = if(arg) Some(s) else None
}

//I would like to be able to use something like this 
val {someExtractorBuilder(true)}(result) = "test"
"test" match {case {someExtractorBuilder(true)}(result) => result }

//instead I would have to do this:
val customExtractor = someExtractorBuilder(true)
val customExtractor(result) = "test"
"test" match {case customExtractor(result) => result}

When just doing a single custom extractor it doesn’t make much difference, but if you were building a large list of extractors for a case statement, it could make things more difficult to read by separating all of the extractors from their usage.

I expect that the answer is no you can’t do this, but I thought I’d ask around first 😀

Answer 1

Nope.

8.1.7 Extractor Patterns

An extractor pattern x (p 1 , . . . ,
p n ) where n ≥ 0 is of the same
syntactic form as a constructor
pattern. However, instead of a case
class, the stable identifier x denotes
an object which has a member method
named unapply or unapplySeq that
matches the pattern.