CODEWITHSUNDEEP
scalagenericstypeclass
Michael K
Michael K

Reputation: 2288

Generic way to check if range contains value in Scala

I'd like to write a generic class that holds the endpoints of a range, but the generic version kicks back a compilation error: value >= is not a member of type parameter A

final case class MinMax[A <: Comparable[A]](min: A, max: A) {
  def contains[B <: Comparable[A]](v: B): Boolean = {
    (min <= v) && (max >= v)
  }
}

The specific version works as expected: 

final case class MinMax(min: Int, max: Int) {
  def contains(v: Int): Boolean = {
    (min <= v) && (max >= v)
  }
}

MinMax(1, 3).contains(2) // true
MinMax(1, 3).contains(5) // false

Upvotes: 4

Views: 559

Answers (1)

Luis Miguel Mej&#237;a Su&#225;rez
Luis Miguel Mej&#237;a Su&#225;rez

Reputation: 22850

You were too close.

In Scala we have Ordering, which is a typeclass, to represent types that can be compared for equality and less than & greater than.

Thus, your code can be written like this:

// Works for any type A, as long as the compiler can prove that the exists an order for that type.
final case class MinMax[A](min: A, max: A)(implicit ord: Ordering[A]) {
  import ord._ // This is want brings into scope operators like <= & >=

  def contains(v: A): Boolean =
    (min <= v) && (max >= v)
}

Upvotes: 8

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