Nulls in Scala ...why is this possible?

Question

I was coding in Scala and doing some quick refactoring in Intellij, when I stumbled upon the following piece of weirdness...

package misc

/**
 * Created by abimbola on 05/10/15.
 */
object WTF extends App {

  val name: String = name
  println(s"Value is: $name")
}

I then noticed that the compiler didn't complain, so I decided to attempt to run this and I got a very interesting output

Value is: null
Process finished with exit code 0

Can anyone tell me why this works?

EDIT:

1. First problem, the value name is assigned a reference to itself even though it does not exist yet; why exactly does the Scala compiler not explode with errors???

2. Why is the value of the assignment null?

Answer 1

1.) Why does the compiler not explode

Here is a reduced example. This compiles because through given type a default value can be inferred:

class Example { val x: Int = x }

scalac Example.scala 
Example.scala:1: warning: value x in class Example does nothing other than call itself recursively
class Example { val x: Int = x }

This does not compile because no default value can be inferred:

class ExampleDoesNotCompile { def x = x }

scalac ExampleDoesNotCompile.scala 
ExampleDoesNotCompile.scala:1: error: recursive method x needs result type
class ExampleDoesNotCompile { def x = x }

1.1 What happens here

My interpretation. So beware: The uniform access principle kicks in. The assignment to the val x calls the accessor x() which returns the unitialized value of x. So x is set to the default value.

class Example { val x: Int = x }
                             ^
[[syntax trees at end of                   cleanup]] // Example.scala
package <empty> {
  class Example extends Object {
    private[this] val x: Int = _;
    <stable> <accessor> def x(): Int = Example.this.x;
    def <init>(): Example = {
      Example.super.<init>();
      Example.this.x = Example.this.x();
      ()
    }
  }
}                            ^

2.) Why the value is null

The default values are determined by the environment Scala is compiled to.

In the example you have given it looks like you run on the JVM. The default value for Object here is null.

So when you do not provide a value the default value is used as a fallback.

Default values JVM:

byte  0
short 0
int   0
long  0L
float 0.0f
double    0.0d
char  '\u0000'
boolean   false
Object    null // String are objects.

Also the default value is a valid value for given type: Here is an example in the REPL:

scala> val x : Int = 0
x: Int = 0

scala> val x : Int = null
<console>:10: error: an expression of type Null is ineligible for implicit conversion
val x : Int = null
                   ^
scala> val x : String = null
x: String = null