Scala tasks and questions

In this topic, I will collect different questions and small tasks of Scala.
Solutions or explanations will be on any page. All related by # - numbers.

#1. Why method generates an error at compile time:
error: missing parameter type for expanded function

<console>:11: error: missing parameter type for expanded function
The argument types of an anonymous function must be fully known. (SLS 8.5)
Expected type was: Int
       def recursiveSum(xs :Int*) :Int = {
                                                              ^

For example, you write a method like this, to calculate sum recursively.

def recursiveSum(xs :Int*) :Int = {
  case xs :Seq[Int] if xs.isEmpty => 0
  case xs :Seq[Int] if xs.nonEmpty => xs.head + recursiveSum(xs.tail: _*)
}

#2. How to make a chain of PartialFunctions, manually and with fold, and use it to filter data.
     And the short explanation about P.F. and their methods.

val resultSeqPers :Seq[Person] = sourceSeqPers collect CombinFilters 

,where CombinFilters is a combination of seq of P.F.

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