# fun fact(n :: Number) -> Number:
#   fun help(shadow n :: Number, acc :: Number) -> Number:
#     if n == 0:
#       acc
#     else:
#       help(n - 1, acc * n)
#     end
#   end
#   help(n, 1)
# where:
#   fact(0) is 1
#   fact(5) is 120
# end

# fun len(l, acc):
#   cases (List) l:
#     | empty => acc
#     | link(_, r) => len(r, 1 + acc)
#   end
# where:
#   len(empty, 0) is 0
#   len(range(0, 10), 0) is 10
# end

data Test:
  | zero
  | one(e :: Test, n :: Number)
  | two(e :: Test, n :: Number)
  | three(e :: Test, n :: Number)
end

fun f<a>(t :: Test) -> Number:
  rec help = lam(shadow t :: Test, acc :: Number) -> Number:
    cases (Test) t:
      | zero => acc
      | one(e, n) =>
        var to-add = n
        when num-modulo(n, 2) == 0:
          to-add := 0 - to-add
        end
        help(e, acc + to-add)
      | two(e, n) =>
        to-add =
          if num-modulo(n, 2) == 0:
            0 - n
          else:
            n
          end
        help(e, acc + to-add)
      | three(e, n) =>
        help(e, acc + if num-modulo(n, 2) == 0: 0 - n else: n end)
    end
  end
  help(t, 0)
where:
  f(zero) is 0
  f(one(two(one(two(three(three(one(two(three(one(three(one(two(three(zero, 1), 2), 3), 4), 5), 6), 7), 8), 9), 10), 11), 12), 13), 14))
    is (1 + 3 + 5 + 7 + 9 + 11 + 13) - (2 + 4 + 6 + 8 + 10 + 12 + 14)

end
#
# fun triangle(n :: Number) -> Number:
#   fun help(shadow n :: Number, acc :: Number) -> Number:
#     if n == 0:
#       acc
#     else:
#       help(n - 1, acc + n)
#     end
#   end
#   help(n, 0)
# where:
#   triangle(0) is 0
#   triangle(10) is 55
# end