WIP: Implement a backlinks query for a given Zettel ID

Will be useful for editors to show a "nearby zettels" view.  See
felko/neuron-mode#30 for more details.

neuron/src/app/Neuron/CLI/Types.hs

@@ -135,6 +135,16 @@ commandParser defaultNotesDir today = do
             <|> fmap
               Right
               (fmap (const $ Some $ Q.GraphQuery_Id) $ switch (long "graph" <> help "Get the entire zettelkasten graph as JSON"))
+            <|> fmap
+              Right
+              ( fmap (Some . Q.GraphQuery_BacklinksOf) $
+                  option
+                    zettelIDReader
+                    ( long "backlinks-of"
+                        <> help "Get a backlink graph of ID"
+                        <> metavar "ID"
+                    )
+              )
         )
     searchCommand = do
       searchBy <-

neuron/src/lib/Neuron/Zettelkasten/Query.hs

@@ -45,6 +45,7 @@ runZettelQuery zs = \case
 runGraphQuery :: ZettelGraph -> GraphQuery r -> r
 runGraphQuery g = \case
   GraphQuery_Id -> g
+  GraphQuery_BacklinksOf _ -> g -- FIXME: implement this
 
 zettelQueryResultJson ::
   forall r.
@@ -104,3 +105,5 @@ graphQueryResultJson q r errors =
     resultJson = case q of
       GraphQuery_Id ->
         toJSON r
+      GraphQuery_BacklinksOf _ ->
+        toJSON r

neuron/src/lib/Neuron/Zettelkasten/Query/Graph.hs

@@ -15,12 +15,15 @@ import Data.Dependent.Sum.Orphans ()
 import Data.GADT.Compare.TH
 import Data.GADT.Show.TH
 import Neuron.Zettelkasten.Graph.Type
+import Neuron.Zettelkasten.ID
 import Relude
 
 -- | Like `GraphQuery` but focused on the relationship between zettels.
 data GraphQuery r where
   -- | Query the entire graph.
   GraphQuery_Id :: GraphQuery ZettelGraph
+  -- | Query backlinks.
+  GraphQuery_BacklinksOf :: ZettelID -> GraphQuery ZettelGraph
 
 deriveJSONGADT ''GraphQuery