How do I return a sequence in Swift?

Question

I\'m trying to write an extension for the Matrix example from the book, slightly tweaked to be generic.
I\'m trying to write a method called getRow

Answer 1

Joe Groff suggested to wrap the result in SequenceOf<T>:

extension Matrix {
    func getRow(index: Int) -> SequenceOf<T> {
        return SequenceOf(map(0..self.columns, { self[index, $0] }))
    }
}

Indeed, this works but we had to wrap map result into a helper class which differs from how I do it in C#.

I have to admit I don't yet understand why Sequence and Generator use typealias and aren't generic protocols (like IEnumerable<T> in C#). There is an interesting ongoing discussion about this distinction so I'll leave a few links for a curious mind:

1. Associated Types Considered Weird
2. Associated types vs. type parameters - reason for the former?
3. Abstract Type Members versus Generic Type Parameters in Scala
4. Generics and protocols

Answer 2

I think you are being mislead by the Swift compiler (which is a bit flaky at the moment). The type for your range 0..self.columns is Range<Int>, which is not a Sequence or Collection, so I don't think it can be used via map.

The implementation works for me:

extension Matrix {
  func getRow(index: Int) -> T[] {
    var row = T[]()
    for col in 0..self.columns {
      row.append(self[index, col])
    }
    return row
  }
}