Add keyword lists koan

lib/koans/06_keyword_lists.ex

@@ -0,0 +1,62 @@
+defmodule KeywordLists do
+  use Koans
+
+  koan "Like maps, keyword lists are key-value pairs" do
+    kw_list = [foo: "bar"]
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "Keys may be repeated, but only the first is accessed" do
+    kw_list = [foo: "bar", foo: "baz"]
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "You could access a second key by removing the first" do
+    kw_list = [foo: "bar", foo: "baz"]
+    [_|kw_list] = kw_list
+
+    assert kw_list[:foo] == ___
+  end
+
+  koan "Keyword lists just special syntax for lists of two-element tuples" do
+    assert [foo: "bar"] == [{___, ___}]
+  end
+
+  koan "But unlike maps, the keys in keyword lists must be atoms" do
+    not_kw_list = [{"foo", "bar"}]
+
+    assert_raise ArgumentError, fn -> not_kw_list[___] end
+  end
+
+  koan "Conveniently keyword lists can be used for function options" do
+    transform = fn str, opts ->
+      if opts[:upcase], do: String.upcase(str)
+    end
+
+    assert transform.("good", upcase: true) == ___
+  end
+
+  def foo(kw_list), do: kw_list
+
+  koan "Actually function bodies are a sneaky use of keyword lists" do
+    list = foo do
+      :good
+    end
+
+    assert list == [do: ___]
+  end
+
+  def iff(kw_list), do: kw_list
+
+  koan "Turns out our beloved if statements are also using keyword lists" do
+    list = iff do
+      :this
+    else
+      :that
+    end
+
+    assert list == [do: ___, else: ___]
+  end
+end

test/koans/keyword_lists_koans_test.exs

@@ -0,0 +1,19 @@
+defmodule KeywordListsTests do
+  use ExUnit.Case
+  import TestHarness
+
+  test "KeywordLists" do
+    answers = [
+      "bar",
+      "bar",
+      "baz",
+      {:multiple, [:foo, "bar"]},
+      "foo",
+      "GOOD",
+      :good,
+      {:multiple, [:this, :that]},
+    ]
+
+    test_all(KeywordLists, answers)
+  end
+end