lets.ex

defmodule ExUnit.Lets do
  defmacro __using__([]) do
    quote do
      import ExUnit.Lets
    end
  end

  defmacro let(name, expr) do
    if match?([do: _], expr) do
      expr = expr[:do]
    end

    quote do
      def unquote(name)() do
        dict = Process.get(:_ex_unit_lets, HashDict.new)

        unless HashDict.has_key?(dict, unquote(name)) do
          dict = HashDict.put(dict, unquote(name), unquote(expr))
        end
        Process.put(:_ex_unit_lets, dict)
        HashDict.get(dict, unquote(name))
      end
    end
  end
end

lets_test.exs

Code.require_file "../test_helper.exs", __DIR__

defmodule ExUnit.LetsTest do
  import ExUnit.CaptureIO

  use ExUnit.Case, async: true
  use ExUnit.Lets

  let :foo, "foo"
  let :bar do
    IO.puts("let :bar evaluated")
    "bar"
  end
  let :baz, do: "baz"

  test "getting let value" do
    assert foo == "foo"
    assert bar == "bar"
    assert baz == "baz"
  end

  test "evaluates each let at most once" do
    assert capture_io(fn ->
      bar
      bar
    end) == "let :bar evaluated\n"
  end

  test "new test evaluates let again" do
    assert capture_io(fn ->
      bar
      bar
    end) == "let :bar evaluated\n"
  end
end