CVN-143 Add recursive removal of empty triples (#739)

* CVN-143 Add recursive removal of empty triples

* Fix typo in comment

---------

Co-authored-by: Pasquale Pinto <[email protected]>
Co-authored-by: Rob van den Bogaard <[email protected]>

modules/mod_ginger_rdf/models/m_rdf_triple.erl

@@ -7,7 +7,9 @@
     insert/2,
     m_find_value/3,
     m_to_list/2,
-    m_value/2
+    m_value/2,
+
+    remove_empty/1
 ]).
 
 -include_lib("zotonic.hrl").
@@ -127,3 +129,41 @@ create_predicate(Uri, Context) ->
         {error, Reason} ->
             {error, Reason}
     end.
+
+
+% Removes all empty triples, recursively, from an 'rdf_resource', a 'triple' or
+% a list of either; returns any other value unaltered.
+% Note: the type of the output is always the same as the input's.
+remove_empty(List) when is_list(List) ->
+    % First remove the empty triples from the resources/triples in the list:
+    NewList = lists:map(fun remove_empty/1, List),
+    % Then filter the items in the list themselves:
+    [Item || Item <- NewList, not is_empty(Item)];
+remove_empty(Triple) when is_record(Triple, triple) ->
+    NewObject = remove_empty(Triple#triple.object),
+    Triple#triple{object=NewObject};
+remove_empty(Resource) when is_record(Resource, rdf_resource) ->
+    NewTriples = remove_empty(Resource#rdf_resource.triples),
+    Resource#rdf_resource{triples=NewTriples};
+remove_empty(Other) ->
+    Other.
+
+% A triple is considered empty if it doesn't add any new information, aka iff:
+% 1. its object is empty (see 'is_empty_object')
+% 2. there are no 'subject_props' nor 'object_props'
+-spec is_empty(rdf:triple()) -> boolean().
+is_empty(#triple{object = Object, subject_props = [], object_props = []}) ->
+    is_empty_object(Object);
+is_empty(_) ->
+    false.
+
+% A triple's object is considered empty if it doesn't add any new information, aka iff:
+% 1. it's an empty URI/binary value
+% 2. it's an 'rdf_value' whose 'value' is itself an empty object
+-spec is_empty_object(ginger_uri:uri() | #rdf_value{} | #rdf_resource{}) -> boolean().
+is_empty_object(Value) when is_binary(Value) ->
+    z_utils:is_empty(Value);
+is_empty_object(#rdf_value{value = Value}) ->
+    is_empty_object(Value);
+is_empty_object(_) ->
+    false.