After posting, I realised that my Elixir is ambiguous. It should be:
def solve(n, queens, r) when r == n, do: [{Enum.reverse(queens)}]
def solve(n, queens, r) do
0..(n-1)
|> Stream.filter(fn f → safe?(queens, {f, r}) end)
|> Stream.flat_map(fn f → solve(n, [{f, r} | queens], r+1) end)
end
