Scala Polymorphic Function Type Mismatch

Question

When trying to run a polymorphic function that abstracts over the type of the array, I get a type mismatch with the following error message:

Type mismatch: expected: (Int) => Boolean, actual: Int

  val listIntegers = List(55, 50, 25, 23, 21, 16, 15)

  def abstractSearchKey[A](a: List[A], p: A => Boolean): Int = {
    def loop(n: Int): Int ={
      if (n > a.length) -1
      else if (p(a(n))) n 
      else loop(n+1)
    }
    loop(0)
  }

  abstractSearchKey[Int](listIntegers, 25)

I'm confused about this error message because this exercise is based on Chiusano's Functional Programming with Scala. In fact, on page 24, the authors state:

...the p function must accept a value of type A (since it's a function of type A => Boolean)

Any advice or feedback on this matter would be greatly appreciated!

nicodp nicodp · Accepted Answer · 2017-12-29T00:51:26

The problem is that you are expecting a function (a predicate of type Int => Bool )as the second argument of abstractSearchKey, and you are pasing 25 which is an Int.

Try passing a lambda function like x => x > 20, for example, to be able to cut the loop using this predicate:

abstractSearchKey[Int](listIntegers, x => x > 20)

Edit: SergGr from the comments just pointed out that you might want this function: x => x == 25 instead, so it might be:

abstractSearchKey[Int](listIntegers, x => x == 25)

Scala Polymorphic Function Type Mismatch

1 Answers