In Scala, what does “extends (A => B)” on a case class mean?

Question

In researching how to do Memoization in Scala, I\'ve found some code I didn\'t grok. I\'ve tried to look this particular \"thing\" up, but don\'t know by what to call it; i.

Answer 1

One more word about this extends (A => B): the extends here is not required, but necessary if the instances of Memo are to be used in higher order functions or situations alike.

Without this extends (A => B), it's totally fine if you use the Memo instance fibonacci in just method calls.

case class Memo[A,B](f: A => B) {
    private val cache = scala.collection.mutable.Map.empty[A, B]
    def apply(x: A):B = cache getOrElseUpdate (x, f(x))
}
val fibonacci: Memo[Int, BigInt] = Memo {
    case 0 => 0
    case 1 => 1
    case n => fibonacci(n-1) + fibonacci(n-2)
}

For example:

Scala> fibonacci(30)
res1: BigInt = 832040

But when you want to use it in higher order functions, you'd have a type mismatch error.

Scala> Range(1, 10).map(fibonacci)
:11: error: type mismatch;
 found   : Memo[Int,BigInt]
 required: Int => ?
              Range(1, 10).map(fibonacci)
                               ^

So the extends here only helps to ID the instance fibonacci to others that it has an apply method and thus can do some jobs.