parse/1 accepts anything that implements the String.Chars protocol

lib/xml_query.ex

@@ -10,7 +10,7 @@ defmodule XmlQuery do
   alias XmlQuery.Xmerl
   require XmlQuery.Xmerl
 
-  @type xml() :: xml_binary() | xml_document() | xml_element() | XmlQuery.Element.t()
+  @type xml() :: xml_binary() | xml_document() | xml_element() | XmlQuery.Element.t() | String.Chars.t()
   @type xml_attribute() :: Xmerl.xml_attribute()
   @type xml_binary() :: binary()
   @type xml_document() :: Xmerl.xml_document()
@@ -114,6 +114,9 @@ defmodule XmlQuery do
     into(doc)
   end
 
+  def parse(%_{} = xml),
+    do: xml |> implements!(String.Chars) |> to_string() |> parse()
+
   @doc """
   Returns `xml` as a prettified string.
 
@@ -190,4 +193,10 @@ defmodule XmlQuery do
     #{hint}
     """
   end
+
+  defp implements!(x, protocol) do
+    if protocol.impl_for(x) == nil,
+      do: raise("Expected #{inspect(x)} to implement protocol #{inspect(protocol)}"),
+      else: x
+  end
 end

test/support/etc/test_node.ex

@@ -0,0 +1,13 @@
+defmodule Test.Etc.TestNode do
+  @moduledoc """
+  A struct that implements `String.Chars`, for use in `XmlQueryTest`.
+
+  This needs to be in its own `.ex` file rather than defined inside a test,
+  because `defimpl` doesn't work in `.exs` files (see https://hexdocs.pm/elixir/Protocol.html#module-consolidation)
+  """
+  defstruct [:contents]
+
+  defimpl String.Chars do
+    def to_string(test_node), do: "<test-node>#{test_node.contents}</test-node>"
+  end
+end

test/xml_query_test.exs

@@ -205,6 +205,20 @@ defmodule XmlQueryTest do
                |> Xq.parse()
     end
 
+    test "can parse any struct that implements String.Chars" do
+      %Test.Etc.TestNode{contents: "<foo>bar</foo>"}
+      |> Xq.parse()
+      |> Xq.pretty()
+      |> assert_eq(
+        """
+        <test-node>
+          <foo>bar</foo>
+        </test-node>
+        """,
+        :squish
+      )
+    end
+
     test "wrap XML element records in Xq.Element" do
       xml =
         {:xmlElement, :root, :root, [], {:xmlNamespace, [], []}, [], 1, [],