Change RDF.Graph.description/2 fallback value

CHANGELOG.md

@@ -23,6 +23,9 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 ### Changed
 
+- `RDF.Graph.description/2` is no longer an alias for `RDF.Graph.get/2`, but
+  has a different behaviour now: it will return an empty description when no
+  description for the requested subject exists in the graph
 - The inspect string of `RDF.Description` now includes the subject separately, so 
   it can be seen also when the description is empty.
 

VERSION

@@ -1 +1 @@
-0.11.1-pre
+0.12.0-pre

lib/rdf/data.ex

@@ -78,7 +78,7 @@ defprotocol RDF.Data do
   def predicates(data)
 
   @doc """
-  Returns the  set of all resources used in the objects within the statements of a RDF data structure.
+  Returns the set of all resources used in the objects within the statements of a RDF data structure.
   """
   def objects(data)
 
@@ -189,10 +189,10 @@ defimpl RDF.Data, for: RDF.Description do
     do: Description.describes?(description, subject)
 
   def description(%Description{subject: subject} = description, s) do
-    with ^subject <- Statement.coerce_subject(s) do
+    if match?(^subject, Statement.coerce_subject(s)) do
       description
     else
-      _ -> Description.new(s)
+      Description.new(s)
     end
   end
 
@@ -286,11 +286,9 @@ defimpl RDF.Data, for: RDF.Graph do
 
   def include?(graph, input, opts \\ []), do: Graph.include?(graph, input, opts)
 
-  def describes?(graph, subject),
-    do: Graph.describes?(graph, subject)
+  def describes?(graph, subject), do: Graph.describes?(graph, subject)
 
-  def description(graph, subject),
-    do: Graph.description(graph, subject) || Description.new(subject)
+  def description(graph, subject), do: Graph.description(graph, subject)
 
   def descriptions(graph), do: Graph.descriptions(graph)
 

lib/rdf/graph.ex

@@ -620,22 +620,25 @@ defmodule RDF.Graph do
   end
 
   @doc """
-  Gets the description of the given subject.
+  Gets the description of the given `subject` in the given `graph`.
 
-  When the subject can not be found the optionally given default value or `nil` is returned.
+  When the subject can not be found the optionally given default value or
+  `nil` is returned.
 
   ## Examples
 
       iex> RDF.Graph.new([{EX.S1, EX.P1, EX.O1}, {EX.S2, EX.P2, EX.O2}])
       ...> |> RDF.Graph.get(EX.S1)
       RDF.Description.new(EX.S1, init: {EX.P1, EX.O1})
-      iex> RDF.Graph.new() |> RDF.Graph.get(EX.Foo)
+
+      iex> RDF.Graph.get(RDF.Graph.new(), EX.Foo)
       nil
-      iex> RDF.Graph.new() |> RDF.Graph.get(EX.Foo, :bar)
+
+      iex> RDF.Graph.get(RDF.Graph.new(), EX.Foo, :bar)
       :bar
 
   """
-  @spec get(t, Statement.coercible_subject(), Description.t() | nil) :: Description.t() | nil
+  @spec get(t, Statement.coercible_subject(), any) :: Description.t() | any
   def get(%__MODULE__{} = graph, subject, default \\ nil) do
     case fetch(graph, subject) do
       {:ok, value} -> value
@@ -643,7 +646,29 @@ defmodule RDF.Graph do
     end
   end
 
-  defdelegate description(graph, subject), to: __MODULE__, as: :get
+  @doc """
+  Returns the description of the given `subject` in the given `graph`.
+
+  As opposed to `get/3` this function returns an empty `RDF.Description` when
+  the subject does not exist in the given `graph`.
+
+  ## Examples
+
+      iex> RDF.Graph.new([{EX.S1, EX.P1, EX.O1}, {EX.S2, EX.P2, EX.O2}])
+      ...> |> RDF.Graph.description(EX.S1)
+      RDF.Description.new(EX.S1, init: {EX.P1, EX.O1})
+
+      iex> RDF.Graph.description(RDF.Graph.new(), EX.Foo)
+      RDF.Description.new(EX.Foo)
+
+  """
+  @spec description(t, Statement.coercible_subject()) :: Description.t()
+  def description(%__MODULE__{} = graph, subject) do
+    case fetch(graph, subject) do
+      {:ok, value} -> value
+      :error -> Description.new(subject)
+    end
+  end
 
   @doc """
   All `RDF.Description`s within a `RDF.Graph`.

lib/rdf/list.ex

@@ -209,7 +209,7 @@ defmodule RDF.List do
 
     defp do_reduce(%RDF.List{head: head, graph: graph}, {:cont, acc}, fun) do
       with description when not is_nil(description) <-
-             Graph.description(graph, head),
+             Graph.get(graph, head),
            [_] <- Description.get(description, RDF.first()),
            [rest] <- Description.get(description, RDF.rest()),
            acc = fun.(description, acc) do