Let RDF.Turtle.Encoder use base IRI from the graph when none provided

CHANGELOG.md

@@ -18,6 +18,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 ### Changed
 
 - `RDF.Turtle.Decoder` saves the base IRI now
+- `RDF.Turtle.Encoder` now takes the base IRI to be serialized from the graph when 
+  no base IRI is given with the `base` option 
 
 
 [Compare v0.6.1...HEAD](https://github.com/marcelotto/rdf-ex/compare/v0.6.1...HEAD)

lib/rdf/serializations/turtle_encoder.ex

@@ -28,7 +28,7 @@ defmodule RDF.Turtle.Encoder do
 
   @impl RDF.Serialization.Encoder
   def encode(data, opts \\ []) do
-    with base         = Keyword.get(opts, :base) |> init_base(),
+    with base         = Keyword.get(opts, :base) |> base_iri(data) |> init_base(),
          prefixes     = Keyword.get(opts, :prefixes) |> prefixes(data) |> init_prefixes(),
          {:ok, state} = State.start_link(data, base, prefixes) do
       try do
@@ -45,6 +45,9 @@ defmodule RDF.Turtle.Encoder do
     end
   end
 
+  defp base_iri(nil, %RDF.Graph{base_iri: base_iri}), do: base_iri
+  defp base_iri(base_iri, _), do: RDF.iri(base_iri)
+
   defp init_base(nil), do: nil
 
   defp init_base(base) do

test/unit/turtle_encoder_test.exs

@@ -85,6 +85,26 @@ defmodule RDF.Turtle.EncoderTest do
                """
     end
 
+    test "when no base IRI is given, the base IRI from the given graph is used" do
+      assert Turtle.Encoder.encode!(Graph.new([{EX.S1, EX.p1, EX.O1}], prefixes: %{},
+               base_iri: EX.__base_iri__)) ==
+               """
+               @base <#{to_string(EX.__base_iri__)}> .
+               <S1>
+                   <p1> <O1> .
+               """
+    end
+
+    test "when a base IRI is given, it has used instead of the base IRI of the given graph" do
+      assert Turtle.Encoder.encode!(Graph.new([{EX.S1, EX.p1, EX.O1}], prefixes: %{},
+               base_iri: EX.other), base: EX.__base_iri__) ==
+               """
+               @base <#{to_string(EX.__base_iri__)}> .
+               <S1>
+                   <p1> <O1> .
+               """
+    end
+
     test "when no prefixes are given and no prefixes are in the given graph the default_prefixes are used" do
       assert Turtle.Encoder.encode!(Graph.new({EX.S, EX.p, XSD.string})) ==
                """