feat: persisting elements in filesystem

lib/bds/frontmatter.ex

@@ -0,0 +1,29 @@
+defmodule BDS.Frontmatter do
+  @moduledoc false
+
+  def serialize_document(fields, body) when is_list(fields) do
+    frontmatter =
+      fields
+      |> Enum.flat_map(&serialize_field/1)
+      |> Enum.join("\n")
+
+    ["---", frontmatter, "---", body || "", ""]
+    |> Enum.join("\n")
+  end
+
+  defp serialize_field({_key, nil}), do: []
+  defp serialize_field({_key, ""}), do: []
+  defp serialize_field({_key, false}), do: []
+
+  defp serialize_field({key, true}) do
+    ["#{key}: true"]
+  end
+
+  defp serialize_field({key, values}) when is_list(values) do
+    ["#{key}:" | Enum.map(values, &"  - #{&1}")]
+  end
+
+  defp serialize_field({key, value}) do
+    ["#{key}: #{value}"]
+  end
+end