Instantiating objects on both sides of assignment operator in Scala; how does it work

Question

I want to understand the mechanism behind the following line:

 val List(x) = Seq(1 to 10)

What is the name of this mechanism? Is this the same as typecasting, or is there something else going on? (Tested in Scala 2.11.12.)

Answer 1

The mechanism is called Pattern Matching.

Here is the official documentation: https://docs.scala-lang.org/tour/pattern-matching.html

This works also in for comprehensions:

for{
 People(name, firstName) <- peoples
} yield s"$firstName $name"

To your example:

val List(x) = Seq(1 to 10)

x is the content of that List - in your case Range 1 to 10 (You have a list with one element).

If you have really a list with more than one element, that would throw an exception

val List(x) = (1 to 10).toList // -> ERROR: undefined

So the correct pattern matching would be:

val x::xs = (1 to 10).toList

Now x is the first element (head) and xs the rest (tail).