5. Lists, tuples and destructuring

The built-in List has literals, ++ for concatenation, len, range and for-in. List patterns destructure head and tail:

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fn describe(xs: List[Int]) -> String =
  match xs {
    [] -> "empty"
    [x] -> "just ${x}"
    [first, ..rest] -> "${first}, and ${len(rest)} more"
  }

pub fn main() -> Unit !io = {
  println(describe([]))
  println(describe([9]))
  println(describe([1, 2, 3]))
}

empty
just 9
1, and 2 more

A tuple packs a fixed number of values of different types; let destructures one directly:

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fn divmod(a: Int, b: Int) -> (Int, Int) = (a / b, a % b)

pub fn main() -> Unit !io = {
  let (q, r) = divmod(17, 5)
  println("${q} remainder ${r}")
}

3 remainder 2

See also: spec.en.md §2.2, §4.11, §5.1