Step 0: Life Without Types

Let’s feel what happens when type information is lost.

0-1. The Anything-Goes List

// step0.scala
@main def step0(): Unit =
  // In Scala, you can't omit type parameters.
  // But you can use Any to make a list that holds everything.
  val xs: List[Any] = List(100, "hello", true, 3.14)

  // What do you get when you pull something out?
  val first: Any = xs.head  // It's 100... but the type is Any

  // Try to do arithmetic — compile error
  // val doubled = first * 2
  // [error] value * is not a member of Any
  // [error]    val doubled = first * 2
  // [error]                  ^^^^^^^

  println(first)            // prints 100
  println(first.getClass)   // class java.lang.Integer

Try it:

> scala-cli run step0.scala

Uncomment first * 2 and compile. Read the error. The compiler says Any has no * method.

The insight: The value is 100 at runtime, but the compiler doesn’t know that. The moment type information is lost, the compiler can no longer protect you.