composition.ex

defmodule Composition do
  @identity fn x -> x end

  def compose([]), do: @identity

  def compose([f]) when is_function(f), do: f

  def compose([f, g]) when is_function(f) && is_function(g) do
    fn
      [] -> f.(g.())
      args when is_list(args) -> f.(g.(args))
    end
  end

  def compose(functions) when is_list(functions) do
    reduce(functions, &comp/1)
  end

  def compose_ltr([]), do: @identity

  def compose_ltr([f]) when is_function(f), do: f

  def compose_ltr([f, g]) when is_function(f) && is_function(g) do
    fn
      [] -> g.(f.())
      args when is_list(args) -> g.(f.(args))
    end
  end

  def compose_ltr(functions) when is_list(functions) do
    reduce(functions, &comp/1)
  end
end