Fix detection of vocabulary namespaces

lib/rdf/prefix_map.ex

@@ -41,7 +41,7 @@ defmodule RDF.PrefixMap do
     do: normalize({prefix, IRI.new(namespace)})
 
   defp normalize({prefix, namespace}) when is_atom(namespace) do
-    if function_exported?(namespace, :__base_iri__, 0) do
+    if RDF.Vocabulary.Namespace.vocabulary_namespace?(namespace) do
       normalize({prefix, apply(namespace, :__base_iri__, [])})
     else
       raise ArgumentError,

lib/rdf/vocabulary_namespace.ex

@@ -572,4 +572,9 @@ defmodule RDF.Vocabulary.Namespace do
   def term_to_iri(base_iri, term),
     do: RDF.iri(base_iri <> term)
 
+  @doc false
+  def vocabulary_namespace?(name) do
+    Code.ensure_loaded?(name) && function_exported?(name, :__base_iri__, 0)
+  end
+
 end

test/unit/prefix_map_test.exs

@@ -70,12 +70,22 @@ defmodule RDF.PrefixMapTest do
       assert PrefixMap.add(@example1, :ex, EX) == {:ok, @example4}
     end
 
+    test "when the IRI namespace is given as a RDF.Vocabulary.Namespace which is not loaded yet" do
+      assert {:ok, prefix_map} = PrefixMap.new |> PrefixMap.add(:rdfs, RDF.NS.RDFS)
+      assert PrefixMap.has_prefix?(prefix_map, :rdfs)
+    end
+
     test "when the IRI namespace is given as an atom" do
       assert_raise ArgumentError, "Invalid prefix mapping for :ex, :foo is not a vocabulary namespace", fn ->
         PrefixMap.add(@example1, :ex, :foo)
       end
     end
 
+    @tag skip: "TODO: "
+    test "when a default namespace is given" do
+      assert PrefixMap.add(@example1, :_, @ex_ns2) == {:ok, @example5}
+    end
+
     test "when a mapping of the given prefix to the same namespace already exists" do
       assert PrefixMap.add(@example2, :ex2, "http://example.com/bar#") == {:ok, @example2}
     end