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])
      }
  }