core: Description.new, .add and .put accept another Description as data to be added

lib/rdf/description.ex

@@ -29,6 +29,8 @@ defmodule RDF.Description do
     do: %RDF.Description{subject: Triple.convert_subject(subject)}
   def new(subject, predicate, objects),
     do: new(subject) |> add(predicate, objects)
+  def new(subject, description = %RDF.Description{}),
+    do: new(subject) |> add(description)
 
 
   @doc """
@@ -80,6 +82,12 @@ defmodule RDF.Description do
     end
   end
 
+  def add(%RDF.Description{subject: subject, predications: predications},
+          %RDF.Description{predications: other_predications}) do
+    merged_predications = Map.merge predications, other_predications,
+      fn (_, objects, other_objects) -> Map.merge(objects, other_objects) end
+    %RDF.Description{subject: subject, predications: merged_predications}
+  end
 
   @doc """
   Puts objects to a predicate of a `RDF.Description`, overwriting all existing objects.
@@ -115,6 +123,9 @@ defmodule RDF.Description do
       iex> RDF.Description.new({EX.S, EX.P1, EX.O1}) |>
       ...>   RDF.Description.put([{EX.P2, EX.O2}, {EX.S, EX.P2, EX.O3}, {EX.P1, EX.O4}])
       RDF.Description.new([{EX.S, EX.P1, EX.O4}, {EX.S, EX.P2, EX.O2}, {EX.S, EX.P2, EX.O3}])
+      iex> RDF.Description.new({EX.S, EX.P, EX.O1}) |>
+      ...>   RDF.Description.put(RDF.Description.new(EX.S, EX.P, [EX.O1, EX.O2]))
+      RDF.Description.new([{EX.S, EX.P, EX.O1}, {EX.S, EX.P, EX.O2}])
   """
   def put(description, statements)
 
@@ -143,6 +154,13 @@ defmodule RDF.Description do
                             put(desc, predicate, objects) end)
   end
 
+  def put(%RDF.Description{subject: subject, predications: predications},
+          %RDF.Description{predications: other_predications}) do
+    merged_predications = Map.merge predications, other_predications,
+      fn (_, _, other_objects) -> other_objects end
+    %RDF.Description{subject: subject, predications: merged_predications}
+  end
+
 
   @doc """
   Fetches the objects for the given predicate of a Description.

test/unit/description_test.exs

@@ -66,6 +66,13 @@ defmodule RDF.DescriptionTest do
     assert description_includes_predication(desc, {EX.predicate, literal("bar")})
   end
 
+  test "creating a description from another description" do
+    desc1 = Description.new({EX.Other, EX.predicate, EX.Object})
+    desc2 = Description.new(EX.Subject, desc1)
+    assert description_of_subject(desc2, uri(EX.Subject))
+    assert description_includes_predication(desc2, {EX.predicate, uri(EX.Object)})
+  end
+
   describe "add" do
     test "a predicate-object-pair of proper RDF terms" do
       assert Description.add(description, EX.predicate, uri(EX.Object))
@@ -137,6 +144,26 @@ defmodule RDF.DescriptionTest do
       refute description_includes_predication(desc, {EX.predicate, uri(EX.Object3)})
     end
 
+
+    test "another description" do
+      desc =
+        description
+        |> Description.add([{EX.predicate1, EX.Object1},
+                            {EX.predicate2, EX.Object2}])
+        |> Description.add(Description.new({EX.Other, EX.predicate3, EX.Object3}))
+
+      assert description_of_subject(desc, uri(EX.Subject))
+      assert description_includes_predication(desc, {EX.predicate1, uri(EX.Object1)})
+      assert description_includes_predication(desc, {EX.predicate2, uri(EX.Object2)})
+      assert description_includes_predication(desc, {EX.predicate3, uri(EX.Object3)})
+
+      desc = Description.add(desc, Description.new({EX.Other, EX.predicate1, EX.Object4}))
+      assert description_includes_predication(desc, {EX.predicate1, uri(EX.Object1)})
+      assert description_includes_predication(desc, {EX.predicate2, uri(EX.Object2)})
+      assert description_includes_predication(desc, {EX.predicate3, uri(EX.Object3)})
+      assert description_includes_predication(desc, {EX.predicate1, uri(EX.Object4)})
+    end
+
     test "duplicates are ignored" do
       desc = Description.add(description, {EX.predicate, EX.Object})
       assert Description.add(desc, {EX.predicate, EX.Object}) == desc
@@ -177,7 +204,7 @@ defmodule RDF.DescriptionTest do
     assert Enum.count(desc.predications) == 1
   end
 
-  describe "Enumerable implementation" do
+  describe "Enumerable protocol" do
     test "Enum.count" do
       assert Enum.count(Description.new EX.foo) == 0
       assert Enum.count(Description.new {EX.S, EX.p, EX.O}) == 1