What concrete datatype is created when I use the Seq[T] constructor?

Question

Scala allows me to instantiate an object that implements the Seq trait directly using this syntax:

val numbers: Seq[Int] = Seq(1, 2, 3)
val names: Seq[String] = Seq("Alice", "Bob", "Charles")

1. Since Seq is a trait and not a concrete implementation, what data type underlies numbers and names?
2. What's the best way I could have figured this out for myself?
3. Is it idiomatic to create a trait object directly?

Thanks!

Answer 1

List is the default implementation. You can easily test this in the Scala REPL:

scala> val numbers: Seq[Int] = Seq(1, 2, 3)
numbers: Seq[Int] = List(1, 2, 3)

scala> val names: Seq[String] = Seq("Alice", "Bob", "Charles")
names: Seq[String] = List(Alice, Bob, Charles)

As far as what is better (creating List or Seq), I think it's about preference. Both options are available, and as far as I know the compiler does not complain when choosing one over the other. I personally always use Seq.