two type parameters with the same name

Question

I am wondering why the two type parameters (named \"A\") with the same name (\"A\") is allowed as per the example below. I know this is a POOR naming of type parameters, do

Answer 1

Nested scopes in Scala are free to shadow each others' symbol tables. Types are not the only things you can do this with. For example:

class X[A](a: A) {
  def X[A](a: A) {
    if (a==this.a) {
      val X = Some(this.a)
      X match {
        case Some(a) => "Confused much yet?"
        case _ => "Just because you can do this doesn't mean you should."
      }
    }
  }
}

The principle is that a scope has control over its namespace. This has dangers, if you use it foolishly (e.g. I have used X and a for each of three different things, and A for two--in fact, you could replace every identifier with X except for the one in the Some which has to be lower case). But it also has benefits when writing functional code--you don't have to worry about having to rename some iterating variable or type or whatever just because you happen to place it in a different context.

def example = {
  val a = Array(1,2,3,4,5)
  val sumsq = a.map(i => i*i).sum
  a.map(i => {
    val a = Array.range(1,i)
    val sumsq = a.map(i => i*i).sum  // Cut and paste from above, and works!
    sumsq + i
  }).sum
}

So be aware that you have the power to confuse yourself, and wisely choose to use that power non-confusingly.