Tic Tac Toe game in Elixirc

tic_tac_toe.ex

defmodule TicTacToe do
  require Integer

  defmodule X do end
  defmodule Y do end

  defmodule Grid do
    defstruct turns: 0, cells: Enum.map(1..9, fn(_) -> nil end)
  end

  def place(%Grid{turns: turns}, X, _) when Integer.odd?(turns) do
    {:error, "out of turn token"}
  end

  def place(grid, token, position) do
    if nil?(Enum.fetch!(grid.cells, position)) do
      grid = %Grid{
        grid | turns: + 1, cells: List.replace_at(grid.cells, position, token)
      }

      {:ok, grid}
    else
      {:error, "cell is occupied"}
    end
  end

  def winner(grid) do
    rows = Enum.chunk(grid.cells, 3)
    columns = Enum.map(0..2,
      fn(i) -> Enum.slice(grid.cells, i+3, 1) end
    )
    IO.inspect columns
    diagonals = []

    winner_in_cells(rows) || winner_in_cells(columns) || winner_in_cells(diagonals)
  end

  def winner_in_cells(cell_groups) do
    match = Enum.find(
      cell_groups,
      fn(cells) ->
        result = Enum.uniq(cells)
        Enum.count(result) == 1 && List.first(result) != nil
      end
    )

    if match do
      List.first(match)
    else
      nil
    end
  end
end

tic_tac_toe_test.exs

defmodule TicTacToe do
  require Integer

  defmodule X do end
  defmodule Y do end

  defmodule Grid do
    defstruct turns: 0, cells: Enum.map(1..9, fn(_) -> nil end)
  end

  def place(%Grid{turns: turns}, X, _) when Integer.odd?(turns) do
    {:error, "out of turn token"}
  end

  def place(grid, token, position) do
    if nil?(Enum.fetch!(grid.cells, position)) do
      grid = %Grid{
        grid | turns: + 1, cells: List.replace_at(grid.cells, position, token)
      }

      {:ok, grid}
    else
      {:error, "cell is occupied"}
    end
  end

  def winner(grid) do
    rows = Enum.chunk(grid.cells, 3)
    columns = Enum.map(0..2,
      fn(i) -> Enum.slice(grid.cells, i+3, 1) end
    )
    IO.inspect columns
    diagonals = []

    winner_in_cells(rows) || winner_in_cells(columns) || winner_in_cells(diagonals)
  end

  def winner_in_cells(cell_groups) do
    match = Enum.find(
      cell_groups,
      fn(cells) ->
        result = Enum.uniq(cells)
        Enum.count(result) == 1 && List.first(result) != nil
      end
    )

    if match do
      List.first(match)
    else
      nil
    end
  end
end