Getting Type Mismatch error in function as parameter in scala

Question

I am facing some issues of type mismatch in scala when I am calling the in_shuffle method from n_shuffle by passing a function as a parameter.

  def in_shuffle[T](original: List[T], restrict_till:Int= -1):List[T]={

    require(original.size % 2 == 0, "In shuffle requires even number of elements")
    def shuffle(t: (List[T], List[T])): List[T] =
      t._2 zip t._1 flatMap { case (a, b) => List(a, b) }

    def midpoint(l: List[T]): Int = l.size / 2

    @annotation.tailrec
    def loop(current: List[T], restrict_till:Int, count:Int=0): List[T] = {
      if (original == current || restrict_till == count) current
      else{
        val mid         = midpoint(current)
        val shuffled_ls = shuffle(current.splitAt(mid))
        loop(shuffled_ls, restrict_till, count+1)
      }
    }
    loop(shuffle(original.splitAt(midpoint(original))), restrict_till, 1)
  }

def n_shuffle[T](f: (List[T], Int) => List[T], list:List[T], n:Int):List[T]={
  println("Inside Sub-function")
  f(list, n)
}

Here is how i'm calling n_shuffle in main

print( n_shuffle(in_shuffle, (1 to 8).toList, 2) )

Error I am getting is

Error:(161, 22) type mismatch;
 found   : (List[Nothing], Int) => List[Nothing]
 required: (List[Int], Int) => List[Int]
    print( n_shuffle(in_shuffle, (1 to 8).toList, 2) )

Any help will highly be appreciated. Thanks

Answer 1

Try multiple parameter lists to aid type inference

def n_shuffle[T](list: List[T], n: Int)(f: (List[T], Int) => List[T]): List[T]
n_shuffle((1 to 8).toList, 2)(in_shuffle)

or provide explicit type annotation

n_shuffle(in_shuffle[Int], (1 to 8).toList, 2)
n_shuffle[Int](in_shuffle, (1 to 8).toList, 2)

The reason compiler is unable to infer type of the first parameter in

def n_shuffle[T](f: (List[T], Int) => List[T], list: List[T], n: Int)

is because it would get it from (1 to 8).toList however that is the second argument.

Answer 2

It seems that the inference in this case does not work as expected. Forcing the type in in_shuffle[Int] method did the trick.

Try this instead.

print(n_shuffle(in_shuffle[Int], (1 to 8).toList, 2))