How to implement flatten as an extension on an Array without type casting?
arrays, generics, swift
Solution
You can't extend a specific type of a generic type in Swift:
extension Array<Int> {
}
error: non-nominal type 'Array' cannot be extended
But you can write a function that takes a specific type of array. If you want to flatten an array of arrays (`Array<T[]>`, `T[][]` or Array>) your functions signature would look like this:
func flatten<T> (array: Array<T[]>) -> T[]
It takes an array of arrays of `T` and returns an array of `T`. You can then use your approach with `reduce`:
func flatten<T> (array: Array<T[]>) -> T[] {
return array.reduce(T[](), +)
}
Problem
``` extension Array { func flatten<T>() -> T[] { let xs = (self as Any) as Array<Array<T>> return xs.reduce(T[](), +) } } ``` It works, but I'm not sure if all those casts are required. Is there a better way? For comparison, here is the implementation in one of the swift extension libraries. I'm not sure if they have it all figured out too -- their implementation begins with the following comment: // There's still some work to do here