PR: More List optimizations

[jskripsky]

This PR optimizes a few simple List functions by using pattern matching instead of calling other functions. It also fixes the doc of List.tail and adds examples.

Here is the PR branch.

New Code

data.List.unsnoc : [a] -> Optional ([a], a)
data.List.unsnoc = cases
  xs :+ x -> Some (xs, x)
  [] -> None

data.List.last : [a] -> Optional a
data.List.last = cases
  _ :+ x -> Some x
  [] -> None

data.List.uncons : [a] -> Optional (a, [a])
data.List.uncons = cases
  x +: xs -> Some (x, xs)
  [] -> None

data.List.tail : [a] -> Optional [a]
data.List.tail = cases
  _ +: xs -> Some xs
  [] -> None

Old Code

data.List.unsnoc : [a] -> Optional ([a], a)
data.List.unsnoc as =
  i = List.size (List.drop 1 as)
  match List.at i as with
    None   -> None
    Some a -> Some (List.take i as, a)

data.List.last : [a] -> Optional a
data.List.last xs =
  use Nat -
  List.at (List.size xs - 1) xs

data.List.uncons : [a] -> Optional (a, [a])
data.List.uncons as = match List.at 0 as with
  None   -> None
  Some a -> Some (a, List.drop 1 as)

data.List.tail : [a] -> Optional [a]
data.List.tail xs = Optional.map at2 (List.uncons xs)

[runarorama]

These changes are merged into main. Thank you! 🌈⭐