Add RDF.Description.update/4

CHANGELOG.md

@@ -9,6 +9,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 ### Added
 
+- `RDF.Description.update/4` updates the objects of a predicate in a description 
+  with a custom update function
 - `RDF.Description.take/2` creates a description from another one by limiting 
   its statements to a set of predicates
 - `RDF.Graph.take/3` creates a graph from another one by limiting 

lib/rdf/description.ex

@@ -367,13 +367,49 @@ defmodule RDF.Description do
     |> List.first
   end
 
+  @doc """
+  Updates the objects of the `predicate` in `description` with the given function.
 
-#  def update(description = %RDF.Description{}, predicate, initial \\ [], fun) do
-#    triple_predicate = coerce_predicate(predicate)
-#    description.predicates
-#    |> Map.update(triple_predicate, initial, fn objects ->
-#    end)
-#  end
+  If `predicate` is present in `description` with `objects` as value,
+  `fun` is invoked with argument `objects` and its result is used as the new
+  list of objects of `predicate`. If `predicate` is not present in `description`,
+  `initial` is inserted as the `objects` of `predicate`. The initial value will
+  not be passed through the update function.
+
+  The initial value and the returned objects by the update function will automatically
+  coerced to proper RDF object values before added.
+
+  ## Examples
+
+      iex> RDF.Description.new({EX.S, EX.p, EX.O}) |>
+      ...> RDF.Description.update(EX.p, fn objects -> [EX.O2 | objects] end)
+      RDF.Description.new([{EX.S, EX.p, EX.O}, {EX.S, EX.p, EX.O2}])
+      iex> RDF.Description.new(EX.S) |>
+      ...> RDF.Description.update(EX.p, EX.O, fn _ -> EX.O2 end)
+      RDF.Description.new({EX.S, EX.p, EX.O})
+
+  """
+  def update(description = %RDF.Description{}, predicate, initial \\ nil, fun) do
+    predicate = coerce_predicate(predicate)
+
+    case get(description, predicate) do
+      nil ->
+        if initial do
+          put(description, predicate, initial)
+        else
+          description
+        end
+
+      objects ->
+        objects
+        |> fun.()
+        |> List.wrap()
+        |> case do
+             []      -> delete_predicates(description, predicate)
+             objects -> put(description, predicate, objects)
+           end
+    end
+  end
 
 
   @doc """

test/unit/description_test.exs

@@ -317,6 +317,42 @@ defmodule RDF.DescriptionTest do
     end
   end
 
+  describe "update/4" do
+    test "list values returned from the update function become new coerced objects of the predicate" do
+      assert Description.new(EX.S, EX.P, [EX.O1, EX.O2])
+             |> Description.update(EX.P,
+                  fn [_object | other] -> [EX.O3 | other] end) ==
+               Description.new(EX.S, EX.P, [EX.O3, EX.O2])
+    end
+
+    test "single values returned from the update function becomes new object of the predicate" do
+      assert Description.new(EX.S, EX.P, [EX.O1, EX.O2])
+             |> Description.update(EX.P, fn _ -> EX.O3 end) ==
+               Description.new(EX.S, EX.P, EX.O3)
+    end
+
+    test "returning an empty list or nil from the update function causes a removal of the predications" do
+      description = EX.S
+                    |> EX.p(EX.O1, EX.O2)
+      assert description
+             |> Description.update(EX.p, fn _ -> [] end) ==
+               Description.new(EX.S, {EX.p, []})
+      assert description
+             |> Description.update(EX.p, fn _ -> nil end) ==
+               Description.new(EX.S, {EX.p, []})
+    end
+
+    test "when the property is not present the initial object value is added for the predicate and the update function not called" do
+      fun = fn _ -> raise "should not be called" end
+      assert Description.new(EX.S)
+             |> Description.update(EX.P, EX.O, fun) ==
+               Description.new(EX.S, EX.P, EX.O)
+
+      assert Description.new(EX.S)
+             |> Description.update(EX.P, fun) ==
+               Description.new(EX.S)
+    end
+  end
 
   test "pop" do
     assert Description.pop(Description.new(EX.S)) == {nil, Description.new(EX.S)}