feed format registry with supported format handlers + ability to extend

lib/feed_reader/format_registry.ex

@@ -0,0 +1,21 @@
+defmodule FeedReader.FormatRegistry do
+  @moduledoc """
+  Ordered registry of built-in formats.
+
+  Built-ins are tried in RSS, Atom, then RDF order. Per-call custom formats can
+  be appended with `formats/1`; duplicate modules are removed without changing
+  the first occurrence's position.
+  """
+
+  @builtins [FeedReader.Formats.RSS, FeedReader.Formats.Atom, FeedReader.Formats.RDF]
+
+  @doc "Returns the statically ordered built-in format modules."
+  @spec builtins() :: [module()]
+  def builtins, do: @builtins
+
+  @doc "Returns built-ins followed by explicitly supplied formats, in stable order."
+  @spec formats([module()]) :: [module()]
+  def formats(custom_formats) when is_list(custom_formats) do
+    Enum.uniq(@builtins ++ custom_formats)
+  end
+end