combine.exs

defmodule Variadic do
  # https://elixirforum.com/t/defining-an-anonymous-function-of-dynamic-arity-not-variadic/38228/7?u=eksperimental

  def spread_combine(h, f, g) do
    {:arity, f_arity} = Function.info(f, :arity)
    {:arity, g_arity} = Function.info(g, :arity)
    args = Macro.generate_arguments(f_arity + g_arity, __MODULE__)
    {f_args, g_args} = Enum.split(args, f_arity)

    fn_ast =
      quote do
        fn unquote_splicing(args) ->
          var!(h).(var!(f).(unquote_splicing(f_args)), var!(g).(unquote_splicing(g_args)))
        end
      end

    {compose_fn, _} = Code.eval_quoted(fn_ast, f: f, g: g, h: h)

    compose_fn
  end
end

defmodule Combine do
  require Variadic

  def run() do
    h = fn x, y -> ["h", x, y] end
    f = fn x, y -> ["f", x, y] end
    g = fn x, y -> ["g", x, y] end

    one = 1
    IO.inspect(Variadic.spread_combine(h, f, g).(one, 2, 3, 4))
    IO.inspect(Variadic.spread_combine(h, f, g).({4}, {3}, {2}, {one}))
  end
end

Combine.run()