Add support for N-Triples-star encoding

lib/rdf/serializations/ntriples_encoder.ex

@@ -57,6 +57,10 @@ defmodule RDF.NTriples.Encoder do
     to_string(bnode)
   end
 
+  def term({s, p, o}) do
+    "<< #{term(s)} #{term(p)} #{term(o)} >>"
+  end
+
   @spec iolist_statement(Triple.t()) :: iolist
   def iolist_statement({subject, predicate, object}) do
     [iolist_term(subject), " ", iolist_term(predicate), " ", iolist_term(object), " .\n"]

test/unit/ntriples_star_encoder_test.exs

@@ -0,0 +1,25 @@
+defmodule RDF.Star.NTriples.EncoderTest do
+  use RDF.Test.Case
+
+  alias RDF.NTriples
+
+  test "annotations of triples on subject position" do
+    assert NTriples.Encoder.encode!(graph_with_annotation()) ==
+             """
+             << <http://example.com/S> <http://example.com/P> "Foo" >> <http://example.com/ap> <http://example.com/ao> .
+             """
+  end
+
+  test "annotations of triples on object position" do
+        assert NTriples.Encoder.encode!(
+                 Graph.new([
+                   statement(),
+                   {EX.AS, EX.ap(), statement()},
+                 ])
+               ) ==
+                 """
+                 <http://example.com/AS> <http://example.com/ap> << <http://example.com/S> <http://example.com/P> "Foo" >> .
+                 <http://example.com/S> <http://example.com/P> "Foo" .
+                 """
+  end
+end