Filter a list by pattern matching in Elixir

filter_matching.exs

defmodule MyEnum do
  defmacro filter_matching(collection, pattern) do
    quote do
      Enum.filter unquote(collection), fn
        unquote(pattern) -> true
        _ -> false
      end
    end
  end
end

require MyEnum

MyEnum.filter_matching [a: 1, b: 2, a: 3], {:a, _}
#=> [a: 1, a: 3]

One other approach is to just write two functions, one that matches the true case, and another that matches the false case and just pass that function. Especially helpful if you want to negate some condition such as nil in a parameter.

  zip_rates
  |> Enum.filter(&zip_nil/1)

  def zip_nil { _zip, nil } do
    false
  end

  def zip_nil { _zip, _price } do
    true
  end