Question

I’m completely new to Scala. Right now I’m attempting port a parser I wrote in Standard ML to Scala and having an issue with the following code:

abstract class Token
case class Zero extends Token
case class At extends Token
//...

object Tokenizer {
  def tokenize(seq : List[Char]) : List[Token] = seq match {
    case List() => error("Empty input")
    case '0' :: rest => Zero :: tokenize(rest)
    case '@' :: rest => At :: tokenize(rest)
    //...
  }  
}

In SML I wouldn’t have to declare the return type of the tokenize() method but it seems Scala needs it and it is somehow not happy with the type I have provided (it complains Zero, At are invalid types and that they should be of type Token instead). Note that I also want to patten match the token list at a later point in time during the parsing phase.

I did some searching on the web and on stackoverflow itself to see if a similar question has been raised before (it looked so trivial) but somehow I couldn’t find anything. I’m pretty sure I’ve got something basic wrong, please feel free to enlighten me 🙂

Answer 1

If you want to create new instances of Zero and At case classes, then you should use apply factory method to instantiate them (or new keyword: new Zero), like this (in Scala Zero() would be equal to Zero.apply()):

case '0' :: rest => Zero() :: tokenize(rest)

If you write just Zero (and not Zero()) then you are using companion object of Zero class, that was created automatically by compiler.