record_matching.ex

defmodule RecordHelper.DynGenerator do
  defmacro __using__(_) do
    generate_macros
  end

  defp generate_macros do
    Enum.map(1..20, fn(i) -> def_macro(args(i)) end)
  end

  defp def_macro(args) do
    quote do 
      defmacro fields(unquote_splicing([quote(do: record) | args])) do
        define_fields(record, [unquote_splicing(args)])
      end
    end
  end

  defp args(n) do
    Enum.map(1..n, fn(i) -> {binary_to_atom("arg#{i}"), [], nil} end)
  end
end

defmodule RecordHelper do
  use RecordHelper.DynGenerator
  
  defp define_fields(record, args) do
    quote do
      unquote(record)[unquote(Enum.map(args, function(:field, 1)))]
    end
  end

  defp field({field, pattern}) do
    quote do
      {unquote(field), unquote(pattern)}
    end
  end

  defp field(name), do: field({elem(name, 0), name})
end

defrecord MyRecord, [:a, :b]

defmodule Test do
  import RecordHelper

  def myfun(fields(MyRecord, a, {:b, nil})) do
    IO.puts "a=#{a}"
  end

  def myfun(fields(MyRecord, a, b)) do
    IO.puts "a=#{a}, b=#{b}"
  end

  def run do
    myfun(MyRecord.new(a: 1, b: 2))
    myfun(MyRecord.new(a: 1))
  end
end

Test.run