core: add error handling for RDF.Namespace term resolution

lib/rdf/namespace.ex

@@ -46,15 +46,28 @@ defmodule RDF.Namespace do
 
   def resolve_term(expr)
 
-  def resolve_term(uri = %URI{}), do: uri
+  def resolve_term(uri = %URI{}),
+    do: uri
+  def resolve_term(namespaced_term) when is_atom(namespaced_term),
+    do: namespaced_term |> to_string() |> do_resolve_term()
 
-  def resolve_term(namespaced_atom) when is_atom(namespaced_atom) do
+  defp do_resolve_term("Elixir." <> _ = namespaced_term) do
     {term, namespace} =
-      namespaced_atom
+      namespaced_term
       |> Module.split
       |> List.pop_at(-1)
     {term, namespace} = {String.to_atom(term), Module.concat(namespace)}
-    namespace.__resolve_term__(term)
+    if Keyword.has_key?(namespace.__info__(:functions), :__resolve_term__) do
+      namespace.__resolve_term__(term)
+    else
+      raise RDF.Namespace.UndefinedTermError,
+        "#{namespaced_term} is not a term on a RDF.Namespace"
+    end
+  end
+
+  defp do_resolve_term(namespaced_term) do
+    raise RDF.Namespace.UndefinedTermError,
+      "#{namespaced_term} is not a term on a RDF.Namespace"
   end
 
 end

test/unit/vocabulary_namespace_test.exs

@@ -384,11 +384,14 @@ defmodule RDF.Vocabulary.NamespaceTest do
   end
 
 
-  @tag skip: "TODO: Can we make RDF.uri(:foo) an undefined function call with guards or in another way?"
   test "resolving an unqualified term raises an error" do
     assert_raise RDF.Namespace.UndefinedTermError, fn -> RDF.uri(:foo) end
   end
 
+  test "resolving an non-RDF.Namespace module" do
+    assert_raise RDF.Namespace.UndefinedTermError, fn -> RDF.uri(ExUnit.Test) end
+  end
+
 
   describe "term resolution in a strict vocab namespace" do
     alias TestNS.{ExampleFromGraph, ExampleFromNTriplesFile, StrictExampleFromTerms}