Another way around - generic classes and traits

We just saw the effect of genericity and it solved more than one problem, we wrote less code and achieved more. The serveMeal function was a generic one because it takes type parameters, in our case A and B. It performs the intended logic, great! Let's talk about parameterized types. You know the type List, right? Let's take a look at its declaration in the Scala standard library:

sealed abstract class List[+A] extends AbstractSeq[A] 
  with LinearSeq[A] 
  with Product 
  with GenericTraversableTemplate[A, List] 
  with LinearSeqOptimized[A, List[A]] 
  with Serializable 

Okay, the declaration seems far too complex, doesn't it? No, wait, we know what sealed means, we know why we used an abstract class, then the name List, and then a few more declarations for showing inheritance relationships. But there's this thing called [+A] in our declaration. Our job is to find out what this is and why we used it.

From the previous few topics, we gained an...