Tour of Scala

Polymorphic Methods


Methods in Scala can be parameterized by type as well as value. The syntax is similar to that of generic classes. Type parameters are declared within a pair of brackets while value parameters are enclosed in a pair of parentheses.

Here is an example:

def listOfDuplicates[A](x: A, length: Int): List[A] = {
    if (length < 1)
        x :: listOfDuplicates(x, length - 1)
println(listOfDuplicates[Int](3, 4))  // List(3, 3, 3, 3)
println(listOfDuplicates("La", 8))  // List(La, La, La, La, La, La, La, La)

The method listOfDuplicates takes a type parameter A and values parameters x and n. In this case, value x is of type A. If length < 1 we return an empty list. Otherwise we prepend x to the the list of duplicates returned by the recursive call to listOfDuplicates. (note: :: means prepend an element on the left to a sequence on the right).

When we call listOfDuplicates with [Int] as the type parameter, the first argument must be an int and the return type will be List[Int]. However, you don’t always need to explicitly provide the the type parameter because the compiler can often figure it out based on the type of value argument ("La" is a String). In fact, if calling this method from Java it is impossible to provide the type parameter.