Can I override a scala class method with a method from a trait? - scala

Question

class PasswordCaseClass(val password:String)
trait PasswordTrait { self:PasswordCaseClass =>
override def password = "blue"
}
val o = new PasswordCaseClass("flowers") with PasswordTrait
Is it possible to override PasswordCaseClass's password with what is provided in PasswordTrait? Right now, I receive this error:
e.scala:6: error: overriding value password in class PasswordCase
Class of type String;
method password in trait PasswordTrait of type => java.lang.String needs to be a stable,
immutable value
val o = new PasswordCaseClass("flowers") with PasswordTrait
^
one error found
I would like to be able to have something like this:
class User(val password:String) {
}
trait EncryptedPassword { u:User =>
def password = SomeCriptographyLibrary.encrypt(u.password)
}
val u = new User("random_password") with EncryptedPassword
println(u.password) // see the encrypted version here

Answer 1

You can override a def with a val, but you can't do it the other way around. A val implies a guarantee -- that it's value is stable and immutable -- that a def does not.

Answer 2

This worked for me (with some modifications):
trait PasswordLike {
val password: String
}
class PasswordCaseClass(val password:String) extends PasswordLike
trait PasswordTrait extends PasswordLike {
override val password: String = "blue"
}
and then:
scala> val o = new PasswordCaseClass("flowers") with PasswordTrait
o: PasswordCaseClass with PasswordTrait = $anon$1#c2ccac
scala> o.password
res1: String = blue

Answer 3

You are trying to override the value with the method definition. It simply makes no sense - they have different semantics. Values supposed to be calculated once per object lifecycle (and stored within a final class attribute) and methods can be calculated multiple times. So what you are trying to do is to brake the contract of the class in a number of ways.
Anyway there is also compiler's fault - the error explanation is totally unclear.