[extra-01] add custom date range graphql type

This type allow graphql to work natively with postgres date range and
reduce the amount of work to convert between representations.

test/wabanex_web/schema/types/custom/date_range_test.exs

@@ -0,0 +1,48 @@
+defmodule WabanexWeb.Schema.Types.Custom.DateRangeTest do
+  use ExUnit.Case, async: true
+
+  alias Absinthe.{Blueprint.Input, Type}
+  alias PgRanges.DateRange
+  alias WabanexWeb.Schema.Types
+
+  defmodule TestSchema do
+    use Absinthe.Schema
+
+    import_types Types.Custom.DateRange
+
+    query do
+    end
+  end
+
+  defp serialize(type, value) do
+    TestSchema.__absinthe_type__(type)
+    |> Type.Scalar.serialize(value)
+  end
+
+  defp parse(type, value) do
+    TestSchema.__absinthe_type__(type)
+    |> Type.Scalar.parse(value)
+  end
+
+  describe ":date_range" do
+    test "serialize a list of dates as a list of ISO8601 date strings" do
+      assert "1978-12-15,1980-02-13" == serialize(:date_range, [~D[1978-12-15], ~D[1980-02-13]])
+      assert "1978-12-15," == serialize(:date_range, [~D[1978-12-15], nil])
+    end
+
+    test "serialize a postgres range as a list of ISO8601 date strings" do
+      assert "1978-12-15,1980-02-13" ==
+               serialize(:date_range, DateRange.new(~D[1978-12-15], ~D[1980-02-13]))
+
+      assert "1978-12-15," == serialize(:date_range, DateRange.new(~D[1978-12-15], nil))
+    end
+
+    test "can be parsed from a string of ISO8601" do
+      assert {:ok, DateRange.new(~D[1978-12-15], ~D[1980-02-13])} ==
+               parse(:date_range, %Input.String{value: "1978-12-15,1980-02-13"})
+
+      assert {:ok, DateRange.new(~D[1978-12-15], nil)} ==
+               parse(:date_range, %Input.String{value: "1978-12-15"})
+    end
+  end
+end