Question

It seems the type of Nil is not polymorphic. How do I correct this function:

scala> def last[A](a:List[A]) : A =   
     | a match {                      
     |   case _ :: tail  => last(tail)
     |   case Nil => Nil[A]           
     | }                              
<console>:8: error: object Nil does not take type parameters.
     case Nil => Nil[A]

UPDATE:

scala> def last[A](a : List[A] ) : Option[A] =
     | a match {                              
     |   case head :: Nil => Some(head)       
     |   case _ :: tail => last(tail)  
     |   case Nil => None                     
     | }                                      

Answer 1

Nil is an object, not a type. So Nil[A] doesn’t make any sense.

An empty list does not have a last element. As such invoking last on Nil should throw an error.

def last[A](l: List[A]): A = l match {                      
  case x :: Nil => x
  case _ :: tail => last(tail)
  case Nil => sys.error("An empty list")
} 

Alternatively you could have last return an Option[A] as shown below:

def last[A](l: List[A]): Option[A] = l match {                      
  case x :: Nil => Some(x)
  case _ :: tail => last(tail)
  case Nil => None
}