I'm new in Scala and I'm reading fp in Scala. There is an example code in this book and I type this into Idea:
sealed trait mList[+A]
case object mNil extends mList[Nothing]
case class Cons[+A] (head:A, tail:mList[A]) extends mList[A]
object mList {
def sum(ints: mList[Int] ): Int = ints match {
case mNil => 0
case Cons(h, t) => h + sum(t)
}
def apply[A](as: A*): mList[A] =
if (as.isEmpty) mNil
else Cons(as.head, apply(as.tail:_*))
}
Then I get a warning from Idea that case Cons(h, t) is unreachable.
I'm sure in this book case Nil comes before case Cons. But when I run the code this way I always get sum=0.
So should I swap the orders of the two cases?
You could swap the cases but actually this is not necessary.
You are missing backticks if you'd like to refer to the object (or another existing variable)
def sum(ints: mList[Int]): Int = ints match {
case `mNil` => 0
case Cons(h, t) => h + sum(t)
}
Otherwise without backticks mNil (lower-case) is a new-variable pattern like in case x => ... i.e. it matches everything.
Alternatively you could call the object in upper case. Then you don't need backticks
case object MNil extends mList[Nothing]
def sum(ints: mList[Int]): Int = ints match {
case MNil => 0
case Cons(h, t) => h + sum(t)
}
what is the use back ticks in scala
Need clarification on Scala literal identifiers (backticks)