List and Tuples in Scala

Question

From the book \'Programming in Scala\' by Martin Odersky:

Another useful container object is the tuple. Like lists, tuples are immutable, but unlike lis

Answer 1

But I can have:

val oneTwoThreee = List(1, 2, "Third Element") //same as:List.apply(1,2,3)
for (i <- 0 to 2) {
  println(oneTwoThreee.apply((i)))
}

And its output is:

1 
2
Third Element

So List in Scala can have different types of elements.

No, it can't. The type of your list is List[Any], so all elements are of the same type: Any.

If you type your code into the Scala REPL, it will tell you at each step what the types are:

scala> val oneTwoThreee = List(1, 2, "Third Element") //same as:List.apply(1,2,3)
oneTwoThreee: List[Any] = List(1, 2, Third Element)
              ↑↑↑↑↑↑↑↑↑

You can also always ask the Scala REPL for the type:

scala> :type oneTwoThreee
List[Any]

Any is a very general, and thus very useless type, since it doesn't have any "interesting" methods. In fact, you are doing pretty much the only thing you can do with an Any: representing it as a String. That's why you're not noticing the problem, you accidentally happened to pick the only thing that works.

Try multiplying the first and second element of your list:

oneTwoThreee(0) * oneTwoThreee(1)
// error: value * is not a member of Any
       oneTwoThreee(0) * oneTwoThreee(1)
                       ^

You may be wondering why you can’t access the elements of a tuple like the elements of a list, for example, with “pair(0)”. The reason is that a list’s apply method always returns the same type, but each element of a tuple may be a different type: But as above code shows, List.apply() can return different types.

No, it can't. Again let's just ask the Scala REPL what the types are:

oneTwoThreee(0)
//=> res: Any = 1
//        ↑↑↑

oneTwoThreee(1)
//=> res: Any = 2
//        ↑↑↑

oneTwoThreee(2)
//=> res: Any = Third Element
//        ↑↑↑

As you can see, the type is always the same: Any.