Fix insertionSort function in TypeClasses.scala

[rvilla87]

You have to add result type in order to call insertionSort as stated in "Calling Parameterized Sort".

[FRosner]

Does the code compile now after your change? Can you please include the original error message and an example of the code that caused it?

[rvilla87]

Yes, I will explain it in detail.

The first code of insertionSort works correctly but when trying to apply the changes stated in "Parameterization of Sort" it throws this error:

recursive method insertionSort needs result type
    case y :: ys => insert(y, insertionSort(ys)(lessThan))

The modified code that returns this error is:

def insertionSort[T](xs: List[T])(lessThan: (T, T) => Boolean) = {
    def insert(y: T, ys: List[T]): List[T] =
    ys match {
      case List() => y :: List()
      case z :: zs =>
        if (lessThan(y, z)) y :: z :: zs
        else z :: insert(y, zs)
    }

  xs match {
    case List() => List()
    case y :: ys => insert(y, insertionSort(ys)(lessThan))
  }
}

Now, if we apply the fix it works correctly (adding : List[T] ):

def insertionSort[T](xs: List[T])(lessThan: (T, T) => Boolean): List[T] = {
    def insert(y: T, ys: List[T]): List[T] =
    ys match {
      case List() => y :: List()
      case z :: zs =>
        if (lessThan(y, z)) y :: z :: zs
        else z :: insert(y, zs)
    }

  xs match {
    case List() => List()
    case y :: ys => insert(y, insertionSort(ys)(lessThan))
  }
}

We can call it correctly as stated in the exercise:
insertionSort(nums)((x: Int, y: Int) => x < y)

[FRosner]

Thank you!