Build anonymous functions with a variable length of arguments

variadic.exs

defmodule Variadic do
  @moduledoc """
  Solution for https://elixirforum.com/t/defining-an-anonymous-function-of-dynamic-arity-not-variadic/38228
  """

  defmacro spread_combine(h, f, g) do
    quote bind_quoted: [h: h, f: f, g: g, module: __CALLER__.module],
          location: :keep 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)

      # The trick here is in order to bypass the limitation of using unquote_splicing to pass arguments to special forms,
      # we build the AST "by hand" and evaluate it.
      Variadic.deffn args do
        h.(apply(f, f_args), apply(g, g_args))
      end
    end
  end

  @doc false
  # Builds the Kernel.SpecialForms.fn/1 AST
  def deffn(args, do: body) do
    fn_ast =
      {:fn, [],
       [
         {:->, [], [args, body]}
       ]}

    Code.eval_quoted(fn_ast) |> elem(0)
  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()