lol.exs

defmodule Fun do
  def arity(fun) do
    case :erlang.fun_info(fun, :arity) do
      { :arity, arity } ->
        arity
    end
  end

  def adapt!(fun, 0) do
    fn -> fun.([]) end
  end

  # the max number of arguments for anonymous functions is 20
  Enum.reduce 1 .. 20, [], fn i, args ->
    args = [{ :"arg#{i}", [], nil } | args]

    def adapt!(fun, unquote(i)) do
      fn unquote_splicing(args) -> fun.(unquote(args)) end
    end

    args
  end

  defmacro adapt(arity, do: block) do
    quote do
      fn(var!(arguments)) -> unquote(block) end |> adapt!(unquote(arity))
    end
  end

  def partial(fun, partial) do
    adapt arity(fun) - length(partial) do
      fun |> apply(partial ++ arguments)
    end
  end

  def complement(fun) do
    adapt arity(fun) do
      not apply(fun, arguments)
    end
  end

  def lift(modifier) do
    fn base ->
      adapt arity(base) do
        base |> apply(arguments) |> modifier.()
      end
    end
  end

  def comp(funs) do
    funs |> Enum.reverse |> Enum.reduce arity(funs |> Enum.first), fn
      outer, inner ->
        adapt arity(inner) do
          inner |> apply(arguments) |> outer.()
        end
    end
  end

  def juxt(funs) do
    fn arg ->
      funs |> Enum.map(&(&1.(arg)))
    end
  end

  def separate(list, pred) do
    juxt([&Enum.filter(&1, pred), &Enum.reject(&1, pred)]).(list)
  end

  def always(value, arity) do
    adapt arity do
      value
    end
  end
end