Fix handling of empty descriptions in Turtle encoder

CHANGELOG.md

@@ -31,6 +31,11 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 - When triples with an empty object list where added to an `RDF.Graph`, it 
   included empty descriptions, which lead to inconsistent behaviour 
   (for example it would be counted in `RDF.Graph.subject_count/1`).
+- When an `RDF.Graph` contained empty descriptions these were rendered by 
+  the `RDF.Turtle.Encoder` to a subject without predicates and objects, i.e.
+  invalid Turtle. This actually shouldn't happen and is either caused by 
+  misuse or a bug. So instead, a `RDF.Graph.EmptyDescriptionError` with a 
+  detailed message will be raised now when this case is detected.
 
 
 [Compare v0.11.0...HEAD](https://github.com/rdf-elixir/rdf-ex/compare/v0.11.0...HEAD)

lib/rdf/exceptions.ex

@@ -22,14 +22,6 @@ defmodule RDF.Triple.InvalidPredicateError do
   end
 end
 
-defmodule RDF.XSD.Datatype.Mismatch do
-  defexception [:value, :expected_type]
-
-  def message(%{value: value, expected_type: expected_type}) do
-    "'#{inspect(value)}' is not a #{expected_type}"
-  end
-end
-
 defmodule RDF.Quad.InvalidGraphContextError do
   defexception [:graph_context]
 
@@ -38,6 +30,31 @@ defmodule RDF.Quad.InvalidGraphContextError do
   end
 end
 
+defmodule RDF.Graph.EmptyDescriptionError do
+  defexception [:subject]
+
+  def message(%{subject: subject}) do
+    """
+    RDF.Graph with empty description about '#{inspect(subject)}' detected.
+    Empty descriptions in a graph lead to inconsistent behaviour. The RDF.Graph API
+    should ensure that this never happens. So this probably happened by changing the
+    contents of the RDF.Graph struct directly, which is strongly discouraged.
+    You should always use the RDF.Graph API to change the content of a graph.
+    If this happened while using the RDF.Graph API, this is a bug.
+    Please report this at https://github.com/rdf-elixir/rdf-ex/issues and describe the
+    circumstances how this happened.
+    """
+  end
+end
+
+defmodule RDF.XSD.Datatype.Mismatch do
+  defexception [:value, :expected_type]
+
+  def message(%{value: value, expected_type: expected_type}) do
+    "'#{inspect(value)}' is not a #{expected_type}"
+  end
+end
+
 defmodule RDF.Namespace.InvalidVocabBaseIRIError do
   defexception [:message]
 end

lib/rdf/serializations/turtle_encoder.ex

@@ -232,6 +232,9 @@ defmodule RDF.Turtle.Encoder do
   defp description_order(%{subject: s1}, %{subject: s2}), do: to_string(s1) < to_string(s2)
 
   defp description_statements(description, state, nesting) do
+    if Description.empty?(description) do
+      raise Graph.EmptyDescriptionError, subject: description.subject
+    else
       with %BlankNode{} <- description.subject,
            ref_count when ref_count < 2 <- State.bnode_ref_counter(state, description.subject) do
         unrefed_bnode_subject_term(description, ref_count, state, nesting)
@@ -239,6 +242,7 @@ defmodule RDF.Turtle.Encoder do
         _ -> full_description_statements(description, state, nesting)
       end
     end
+  end
 
   defp full_description_statements(subject, description, state, nesting) do
     nesting = nesting + @indentation

test/unit/turtle_encoder_test.exs

@@ -416,15 +416,38 @@ defmodule RDF.Turtle.EncoderTest do
                  ] .
                """
     end
+
+    test "serializing a pathological graph with an empty description" do
+      description = RDF.description(EX.S)
+      graph = %Graph{Graph.new() | descriptions: %{description.subject => description}}
+
+      assert_raise Graph.EmptyDescriptionError, fn ->
+        Turtle.Encoder.encode!(graph)
+      end
+    end
   end
 
-  test "serializing a description" do
+  describe "serializing a description" do
+    test "a non-empty description" do
       description = EX.S |> EX.p(EX.O)
 
       assert Turtle.Encoder.encode!(description) ==
                description |> Graph.new() |> Turtle.Encoder.encode!()
     end
 
+    test "an empty description" do
+      description = RDF.description(EX.S)
+
+      assert Turtle.Encoder.encode!(description) |> String.trim() ==
+               """
+               @prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
+               @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
+               @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
+               """
+               |> String.trim()
+    end
+  end
+
   describe "prefixed_name/2" do
     setup do
       {:ok,