Question

I’ve got a following situation:

Got a method def f(lst: List[Any]), which does some transformation of the list and returns result of that transformation (all those Any‘s are case classes). What I need to accomplish is when the input list is empty, generate a list, containing one element of the correct type and do that transformation with it.

Is it possible to guarantee on a type level, that some case class has a no-arg constructor? If so, what should Any be replaced with? If no, what is the best way to accomplish this? Maybe I should just change my method to something like def f[T](lst: List[T], default: T)?

Any help appreciated.

Answer 1

Are you looking for something like this?

import scalaz._
import Scalaz._

scala> def f[A : Zero](lst: List[A]) = {
     |   val xs = if(lst.isEmpty) List(mzero[A]) else lst
     |   xs ++ xs // some transformation
     | }
f: [A](lst: List[A])(implicit evidence$1: scalaz.Zero[A])List[A]

scala> f(List.empty[Int])
res1: List[Int] = List(0, 0)

scala> f(List("hello", "world"))
res2: List[java.lang.String] = List(hello, world, hello, world)

If yes, you can refer to this post I wrote on the subject a while ago.