Let RDF.Turtle.Encoder use other prefixes when none provided

CHANGELOG.md

@@ -26,6 +26,11 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 - when `RDF.Graph.add` and `RDF.Graph.put` are called with another graph, its
   prefixes are merged 
 - `RDF.Turtle.Decoder` saves the prefixes now
+- `RDF.Turtle.Encoder` now takes the prefixes to be serialized in the following 
+  order of precedence:
+	- from the `prefixes` option (as before)
+	- from the `prefixes` field of the given graph
+	- from the `RDF.default_prefixes`
 - drop support for OTP < 20, since prefixes can consist of UTF characters which
   are not supported in atoms on these versions  	
 

lib/rdf/serializations/turtle_encoder.ex

@@ -29,7 +29,7 @@ defmodule RDF.Turtle.Encoder do
   @impl RDF.Serialization.Encoder
   def encode(data, opts \\ []) do
     with base         = Keyword.get(opts, :base) |> init_base(),
-         prefixes     = Keyword.get(opts, :prefixes, %{}) |> init_prefixes(),
+         prefixes     = Keyword.get(opts, :prefixes) |> prefixes(data) |> init_prefixes(),
          {:ok, state} = State.start_link(data, base, prefixes) do
       try do
         State.preprocess(state)
@@ -58,11 +58,13 @@ defmodule RDF.Turtle.Encoder do
     end
   end
 
-  defp init_prefixes(nil), do: %{}
+  defp prefixes(nil, %RDF.Graph{prefixes: prefixes}) when not is_nil(prefixes), do: prefixes
+  defp prefixes(nil, _), do: RDF.default_prefixes()
+  defp prefixes(prefixes, _), do: RDF.PrefixMap.new(prefixes)
 
   defp init_prefixes(prefixes) do
     Enum.reduce prefixes, %{}, fn {prefix, iri}, reverse ->
-      Map.put(reverse, RDF.iri(iri), to_string(prefix))
+      Map.put(reverse, iri, to_string(prefix))
     end
   end
 

test/unit/turtle_encoder_test.exs

@@ -19,16 +19,16 @@ defmodule RDF.Turtle.EncoderTest do
 
   describe "serializing a graph" do
     test "an empty graph is serialized to an empty string" do
-      assert Turtle.Encoder.encode!(Graph.new) == ""
+      assert Turtle.Encoder.encode!(Graph.new, prefixes: %{}) == ""
     end
 
     test "statements with IRIs only" do
-      assert Turtle.Encoder.encode!(Graph.new [
+      assert Turtle.Encoder.encode!(Graph.new([
                 {EX.S1, EX.p1, EX.O1},
                 {EX.S1, EX.p1, EX.O2},
                 {EX.S1, EX.p2, EX.O3},
                 {EX.S2, EX.p3, EX.O4},
-              ]) ==
+              ]), prefixes: %{}) ==
               """
               <http://example.org/#S1>
                   <http://example.org/#p1> <http://example.org/#O1>, <http://example.org/#O2> ;
@@ -62,6 +62,41 @@ defmodule RDF.Turtle.EncoderTest do
               """
     end
 
+    test "when no prefixes are given, the prefixes from the given graph are used" do
+      assert Turtle.Encoder.encode!(Graph.new([
+               {EX.S1, EX.p1, EX.O1},
+               {EX.S1, EX.p1, EX.O2},
+               {EX.S1, EX.p2, XSD.integer},
+               {EX.S2, EX.p3, EX.O4},
+             ], prefixes: %{
+               "": EX.__base_iri__,
+               xsd: XSD.__base_iri__
+             })) ==
+               """
+               @prefix : <#{to_string(EX.__base_iri__)}> .
+               @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
+
+               :S1
+                   :p1 :O1, :O2 ;
+                   :p2 xsd:integer .
+
+               :S2
+                   :p3 :O4 .
+               """
+    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})) ==
+               """
+               @prefix rdf: <#{to_string(RDF.__base_iri__)}> .
+               @prefix rdfs: <#{to_string(RDFS.__base_iri__)}> .
+               @prefix xsd: <#{to_string(XSD.__base_iri__)}> .
+
+               <http://example.org/#S>
+                   <http://example.org/#p> xsd:string .
+               """
+    end
+
     test "statements with empty prefixed names" do
       assert Turtle.Encoder.encode!(Graph.new({EX.S, EX.p, EX.O}),
               prefixes: %{"" => EX.__base_iri__}) ==
@@ -74,11 +109,11 @@ defmodule RDF.Turtle.EncoderTest do
     end
 
     test "statements with literals" do
-      assert Turtle.Encoder.encode!(Graph.new [
+      assert Turtle.Encoder.encode!(Graph.new([
                 {EX.S1, EX.p1, ~L"foo"},
                 {EX.S1, EX.p1, ~L"foo"en},
                 {EX.S2, EX.p2, RDF.literal("strange things", datatype: EX.custom)},
-              ]) ==
+              ]), prefixes: %{}) ==
               """
               <http://example.org/#S1>
                   <http://example.org/#p1> "foo"@en, "foo" .
@@ -89,10 +124,10 @@ defmodule RDF.Turtle.EncoderTest do
     end
 
     test "statements with blank nodes" do
-      assert Turtle.Encoder.encode!(Graph.new [
+      assert Turtle.Encoder.encode!(Graph.new([
                 {EX.S1, EX.p1, [RDF.bnode(1), RDF.bnode("foo"), RDF.bnode(:bar)]},
                 {EX.S2, EX.p1, [RDF.bnode(1), RDF.bnode("foo"), RDF.bnode(:bar)]},
-              ]) ==
+              ]), prefixes: %{}) ==
               """
               <http://example.org/#S1>
                   <http://example.org/#p1> _:1, _:bar, _:foo .