Where does the type erasure happen and what is a safe way to stay away from this?

Question

def decode(l: List[(Int, Symbol)]) : List[Symbol] = {
    def foo(r : Int, s: Symbol):List[Symbol] = r match {
      case 0 => Nil
      case x: Int => s::foo(r-1,s)
    }

    l match {
      case Nil => Nil
      case h::tail => foo(h._1 , h._2 ) :+ decode(tail)
    }

}

Here I get a compilation error "scala: type mismatch; found:List[Object] required: List[Symbol] case h::tail => foo(h._1 : Int, h._2 : Symbol) :+ decode(tail) "

Answer 1

It is not a type erasure, simple type mismatch:

foo(h._1 , h._2 ) :+ decode(tail)

Result of foo is List[Symbol], result of decode is List[Symbol]. Now you're trying to put List inside List, no surprize that compiler thinks that the only way to store List and Symbols inside List is to give that later list Object (Any) type.

Most likely you wanted to simply combine two lists:

foo(h._1 , h._2 ) ++ decode(tail)