From af5cc26d7fef8b0f8e10858ec6d70a34b2171ebe Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Mon, 16 May 2022 22:03:03 +0200
Subject: [PATCH] Make RDF.Data.merge/3 implementations commutative

---
 CHANGELOG.md                       |  4 ++++
 lib/rdf/data.ex                    | 26 ++++++++++++++++++++++--
 test/support/external_data_impl.ex | 32 ++++++++++++++++++++++++++++++
 test/unit/data_test.exs            | 17 ++++++++++++++++
 4 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 test/support/external_data_impl.ex

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6f2e956..b56d222 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 ### Changed
 
+- `RDF.Data.merge/3` is now commutative, i.e. structs which implement the
+  `RDF.Data` protocol can be given also as the second argument
+  (previously custom structs with `RDF.Data` protocol implementations always
+  had to be given as the first argument)
 - several performance improvements
 
 
diff --git a/lib/rdf/data.ex b/lib/rdf/data.ex
index c9f21ad..7dbbf45 100644
--- a/lib/rdf/data.ex
+++ b/lib/rdf/data.ex
@@ -3,8 +3,6 @@ defprotocol RDF.Data do
   An abstraction over the different data structures for collections of RDF statements.
   """
 
-  @type t :: RDF.Description.t() | RDF.Graph.t() | RDF.Dataset.t()
-
   @doc """
   Adds statements to a RDF data structure.
 
@@ -166,6 +164,14 @@ defimpl RDF.Data, for: RDF.Description do
   def merge(description, %Dataset{} = dataset, opts),
     do: RDF.Data.merge(dataset, description, opts)
 
+  def merge(description, %_{} = other, opts) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.merge(other, description, opts)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def delete(description, input, opts \\ [])
 
   def delete(
@@ -272,6 +278,14 @@ defimpl RDF.Data, for: RDF.Graph do
   def merge(graph, %Dataset{} = dataset, opts),
     do: RDF.Data.merge(dataset, graph, opts)
 
+  def merge(graph, %_{} = other, opts) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.merge(other, graph, opts)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def delete(graph, input, opts \\ [])
 
   def delete(%Graph{name: name} = graph, %Graph{name: other_name}, _opts)
@@ -340,6 +354,14 @@ defimpl RDF.Data, for: RDF.Dataset do
   def merge(dataset, %Dataset{} = other_dataset, opts),
     do: Dataset.add(dataset, other_dataset, opts)
 
+  def merge(dataset, %_{} = other, opts) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.merge(other, dataset, opts)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def delete(dataset, input, opts \\ [])
 
   def delete(%Dataset{name: name} = dataset, %Dataset{name: other_name}, _opts)
diff --git a/test/support/external_data_impl.ex b/test/support/external_data_impl.ex
new file mode 100644
index 0000000..2ba8ac3
--- /dev/null
+++ b/test/support/external_data_impl.ex
@@ -0,0 +1,32 @@
+defmodule External do
+  defstruct []
+
+  import RDF.Sigils
+  def data, do: RDF.triple(~I<http://example.com/S>, ~I<http://example.com/p>, 42)
+
+  defimpl RDF.Data do
+    def merge(%External{}, data, opts) do
+      RDF.Data.merge(data, External.data(), opts)
+    end
+
+    def delete(_external, _, _opts), do: nil
+    def pop(_external), do: nil
+    def empty?(_external), do: false
+    def include?(_external, _, _opts \\ []), do: false
+    def describes?(_external, _), do: false
+    def description(_external, _), do: nil
+    def descriptions(_external), do: []
+    def statements(_external), do: []
+    def subjects(_external), do: []
+    def predicates(_external), do: []
+    def objects(_external), do: []
+    def resources(_external), do: []
+    def subject_count(_external), do: 0
+    def statement_count(_external), do: 0
+
+    def values(_external, _opts \\ []), do: nil
+    def map(_external, _fun), do: nil
+
+    def equal?(_, _), do: false
+  end
+end
diff --git a/test/unit/data_test.exs b/test/unit/data_test.exs
index a20d17e..be99e91 100644
--- a/test/unit/data_test.exs
+++ b/test/unit/data_test.exs
@@ -625,4 +625,21 @@ defmodule RDF.DataTest do
       refute RDF.Data.equal?(dataset, graph)
     end
   end
+
+  test "external RDF.Data protocol implementations" do
+    description = EX.S |> EX.p(EX.O2)
+
+    assert RDF.Data.merge(description, %External{}) ==
+             Description.add(description, External.data())
+
+    graph = RDF.graph({EX.S, EX.p(), EX.O2})
+
+    assert RDF.Data.merge(graph, %External{}) ==
+             Graph.add(graph, External.data())
+
+    dataset = RDF.dataset({EX.S, EX.p(), EX.O2})
+
+    assert RDF.Data.merge(dataset, %External{}) ==
+             Dataset.add(dataset, External.data())
+  end
 end