An enum declares a finite set of named data constructors. Unlike Java or C enumerations, Scala enum cases may carry arbitrary arguments, so an enum can describe the complete shape of a data type. section 3.1
A list as an enum
A singly linked list has exactly two forms: Nil for an empty list and Cons for an element followed by another list. The recursive case makes the structure explicit: `enum List[+A]: case Nil; case Cons(head: A, tail: List[A])`. The type parameter permits List[Int], List[Double], or List[String]. section 3.1
Listing 3.1 Singly linked lists
package fpinscala.datastructures
enum List[+A]: #1
case Nil #2
case Cons(head: A, tail: List[A]) #3
object List: #4
def apply[A](as: A*): List[A] = #5
if as.isEmpty then Nil
else Cons(as.head, apply(as.tail*))
Inspect the complete Scala definition, including the recursive constructors, covariant type parameter, and companion-object apply method. section 3.1
Use enum cases to make the possible shapes of a value visible:
- `Nil` constructs an empty list, while `Cons(1, Nil)` constructs a one-element list. section 3.1
- A tree can use `Leaf(value)` and `Branch(left, right)` to represent values and recursive subtrees. section 3.4
- Pattern matching can destructure enum cases, support recursive algorithms, and be checked for exhaustiveness. section 3.6
- Sealed-trait hierarchies provide an alternative representation, especially for Scala 2; enum cases state the constructors more directly. section 3.4
