extension Array {
          func take(_ n: Int) -> [Element] {
             self[0..<n]
          }
      }

“Long-term storage of ArraySlice instances is discouraged. A slice holds a reference to the entire storage of a larger array, not just to the portion it presents, even after the original array’s lifetime ends. Long-term storage of a slice may therefore prolong the lifetime of elements that are no longer otherwise accessible, which can appear to be memory and object leakage.”

https://developer.apple.com/documentation/swift/arrayslice

Would it be acceptable to put these implementations in a Functional library?

However, this has nothing to do with generics.If you're willing to sacrifice performance, you could easily write

  extension Array {
      func take(_ n: Int) -> [Element] {
          Array(self[0..<n])
      }
  }

That one and the need for generic associatedTypes trip me up all the time.

I think about it every time I see new feature announcements like today's. The opportunity cost of the wasted effort, the new thing that will probably make things harder for everyone. Possibly including those who try to fulfill the generics manifesto.