Make RDF.Data.equal?/2 implementations commutative

CHANGELOG.md

@@ -9,8 +9,8 @@ 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
+- `RDF.Data.merge/2` and `RDF.Data.equal?/2` are 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

lib/rdf/data.ex

@@ -237,6 +237,14 @@ defimpl RDF.Data, for: RDF.Description do
     RDF.Data.equal?(dataset, description)
   end
 
+  def equal?(description, %_{} = other) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.equal?(other, description)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def equal?(_, _), do: false
 end
 
@@ -331,6 +339,14 @@ defimpl RDF.Data, for: RDF.Graph do
   def equal?(graph, %Dataset{} = dataset),
     do: RDF.Data.equal?(dataset, graph)
 
+  def equal?(graph, %_{} = other) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.equal?(other, graph)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def equal?(_, _), do: false
 end
 
@@ -433,5 +449,13 @@ defimpl RDF.Data, for: RDF.Dataset do
     )
   end
 
+  def equal?(dataset, %_{} = other) do
+    if RDF.Data.impl_for(other) do
+      RDF.Data.equal?(other, dataset)
+    else
+      raise ArgumentError, "no RDF.Data implementation found for #{inspect(other)}"
+    end
+  end
+
   def equal?(_, _), do: false
 end

test/support/external_data_impl.ex

@@ -27,6 +27,8 @@ defmodule External do
     def values(_external, _opts \\ []), do: nil
     def map(_external, _fun), do: nil
 
-    def equal?(_, _), do: false
+    def equal?(%External{}, data) do
+      RDF.Data.equal?(data, RDF.graph(External.data()))
+    end
   end
 end

test/unit/data_test.exs

@@ -626,7 +626,8 @@ defmodule RDF.DataTest do
     end
   end
 
-  test "external RDF.Data protocol implementations" do
+  describe "external RDF.Data protocol implementations" do
+    test "merge/2" do
       description = EX.S |> EX.p(EX.O2)
 
       assert RDF.Data.merge(description, %External{}) ==
@@ -642,4 +643,11 @@ defmodule RDF.DataTest do
       assert RDF.Data.merge(dataset, %External{}) ==
                Dataset.add(dataset, External.data())
     end
+
+    test "equal/2" do
+      assert EX.S |> EX.p(42) |> RDF.Data.equal?(%External{}) == true
+      assert {EX.S, EX.p(), 42} |> RDF.graph() |> RDF.Data.equal?(%External{}) == true
+      assert {EX.S, EX.p(), 42} |> RDF.dataset() |> RDF.Data.equal?(%External{}) == true
+    end
+  end
 end