make_a_choice.ex

defmodule Something do
  defstruct [
    material: nil
  ]
  
  def get_material(%Something{material: material}) when material |> is_function do
    material.()
  end
  def get_material(%Something{material: material}) do
    material
  end
end

show_in_iex.exs

iex(1)> defmodule Something do
...(1)>   defstruct [
...(1)>     material: nil
...(1)>   ]
...(1)>
...(1)>   def get_material(%Something{material: material}) when material |> is_function do
...(1)>     material.()
...(1)>   end
...(1)>   def get_material(%Something{material: material}) do
...(1)>     material
...(1)>   end
...(1)> end
{:module, Something,
 <<70, 79, 82, 49, 0, 0, 10, 36, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 251,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, ...>>, {:get_material, 1}}
iex(2)> %Something{material: "glass"}
%Something{material: "glass"}
iex(3)> %Something{material: "glass"} |> Something.get_material
"glass"
iex(4)> %Something{material: fn -> "metal" end} |> Something.get_material
"metal"
iex(5)>