Val and def

class Cons[T](val head: T, val tail: Int)

In the above class definition ‘val’ means, that the class has the fields head and tail. Without ‘val’ those fields are private. So ‘val’ is a way to define a field in a class.

“They are special cases of methods, they can override methods and implement abstract methods and traits. The difference between a val and a def concerns only the initialization. A val is evaluated when the object is first initialized, whereas a def is evaluated each time it is referenced.” (Martin Odersky, coursera course “Functional Programming Principles in Scala”, Polymorphism.