chained/pipeline assertions in elixir

elixir-chained-assertions.exs

defmodule TestHelper do
  defmacro __using__(_opts) do
    quote do
      import ExUnit.Assertions, only: [assert: 1]

      # we can name this whatever we'd like,
      # but "is" makes sense to me in most cases
      # 👇
      def is(result, expectation) do
        assert result == expectation
        result # 👈 allows us to continue chaining assertions in a pipeline
      end

      # this one allows us to make more complex assertions
      # e.g., asserting that a nested key is of a particular value
      def has(result, assertion) when is_function(assertion) do
        assert assertion.(result) == true
        result
      end
    end
  end
end

# test/adder_test.exs

defmodule AdderTest do
  use TestHelper # 👈 included here

  test "add" do
    1..3
    |> Enum.map(& &1 * 2)
    |> Adder.add(1)
    |> is([3, 5, 7]) # 👈 used here
    |> has(&List.last(&1) == 7) # 👈 and here (chained)
  end
end