Scala import problems - error: not found: value

Question

I'm a Haskeller looking into Scala. I'm meeting frustration not with code, but with imports/packages.

I have two files, Test.scala and Lists.scala.

// Lists.scala
package problems

object Lists {
  def last(list: List[Any]): Option[Any] = list match {
    case Nil      => None
    case x :: Nil => Some(x)
    case _ :: xs  => last(xs)
  }
}

And:

// Test.scala
import problems._

object Test extends App {
  println("Starting tests...")
  println(last(List(1,2,3,4,5)))
}

Test.scala does not compile. Running scalac Test.scala Lists.scala yields:

Test.scala:5: error: not found: value last
  println(last(List(1,2,3,4,5))

Yet rewriting last as Lists.last makes it succeed. Doesn't that defeat the point of the import problems._ wildcard? I notice that math functions can be written without a preceeding math. by doing import math._. Why won't this work for my files as well?

Real aim: I just want to be able to make a package, then easily test its functions with println in another file. What's the best way to do that? Can I not do away with the object {...} in Test.scala and just run it with scala, forgoing the compilation process?

Answer 1

Doesn't that defeat the point of the import problems._ wildcard?

No, it does not. By using the wildcard you're bringing all the classes/objects in that package into the scope, but not their contents.

I notice that math functions can be written without a preceding math. by doing import math._ Why won't this work for my files as well?

It will work, but you need a proper import: import problems.Lists._. Otherwise you can place your functions into a package object.