pub fn foldr(xs: List(a), f: fn(a, b) -> b, zero: b) -> b {
  when xs is {
    [] -> zero
    [x, ..rest] -> f(x, foldr(rest, f, zero))
  }
}

pub fn concat(left: List(a), right: List(a)) -> List(a) {
  foldr(left, fn(x, xs) { [x, ..xs] }, right)
}

test concat_1() {
  concat([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
}