[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.

lib/wabanex_web/schema/types/custom/date_range.ex

@@ -0,0 +1,46 @@
+defmodule WabanexWeb.Schema.Types.Custom.DateRange do
+  use Absinthe.Schema.Notation
+
+  alias Absinthe.Blueprint.Input
+
+  scalar :date_range, name: "DateRange" do
+    description("""
+      The `DateRange` scalar represents a range/period of date. The `DateRange`
+      appears as a string of two ISO8601 formatted dates separated by comma.
+    """)
+
+    serialize &serialize_range/1
+    parse &parse_range/1
+  end
+
+  @spec serialize_range(PgRanges.DateRange.t()) :: String.t()
+  @spec serialize_range(list(String.t())) :: String.t()
+  defp serialize_range(%PgRanges.DateRange{lower: start_range, upper: end_range}),
+    do: serialize_range([start_range, end_range])
+
+  defp serialize_range([start_range, nil]), do: "#{Date.to_iso8601(start_range)},"
+
+  defp serialize_range([start_range, end_range]),
+    do: "#{Date.to_iso8601(start_range)},#{Date.to_iso8601(end_range)}"
+
+  @spec parse_range(Input.String.t()) :: {:ok, PgRanges.DateRange.t()} | :error
+  @spec parse_range(Input.Null.t()) :: {:ok, nil}
+  defp parse_range(%Input.String{value: value}) do
+    value
+    |> String.split(",")
+    |> Enum.map(&Date.from_iso8601/1)
+    |> case do
+      [{:error, _}] -> :error
+      [{:error, _}, _] -> :error
+      [_, {:error, _}] -> :error
+      result -> {:ok, result |> Enum.map(fn {:ok, value} -> value end) |> new()}
+    end
+  end
+
+  defp parse_range(%Input.Null{}), do: {:ok, nil}
+  defp parse_range(_), do: :error
+
+  defp new([start_date]), do: new([start_date, nil])
+  defp new([start_date, end_date]), do: PgRanges.DateRange.new(start_date, end_date)
+  defp new([]), do: new([nil, nil])
+end