Improve renderLoop

src/Gargantext/Core/NodeStory/Types.hs

@@ -12,6 +12,7 @@ Portability : POSIX
 {-# LANGUAGE ConstraintKinds   #-}
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE TemplateHaskell   #-}
+{-# LANGUAGE ViewPatterns #-}
 
 module Gargantext.Core.NodeStory.Types
   ( HasNodeStory
@@ -203,19 +204,27 @@ data BuildForestError
 
 instance ToHumanFriendlyError BuildForestError where
   mkHumanFriendly (BFE_loop_detected visited)
-    = "Loop detected in ngrams: " <> renderLoop visited
+    = "Loop detected in terms: " <> renderLoop visited
 
 renderLoop :: Set VisitedNode -> T.Text
-renderLoop = T.intercalate " -> " . map (unNgramsTerm . _vn_term) . Set.toAscList
+renderLoop (sortBy (comparing _vn_position) . Set.toList -> visited) = case visited of
+  [] -> mempty
+  (x : _) ->
+    let cycleWithoutRecursiveKnot = T.intercalate " -> " . map (unNgramsTerm . _vn_term) $ visited
+    -- Pretty print the first visited node last, so that the user can "see" the full recursive knot.
+    in cycleWithoutRecursiveKnot <> " -> " <> (unNgramsTerm . _vn_term $ x)
 
 -- | Keeps track of the relative order in which visited a node, to be able to print cycles.
 data VisitedNode =
   VN { _vn_position :: !Int, _vn_term :: !NgramsTerm }
   deriving (Show)
 
+-- /NOTA BENE/: It's important to use this custom instance for the loop detector
+-- to work correctly. If we stop comparing on the terms the loop detector .. will loop.
 instance Eq VisitedNode where
   (VN _ t1) == (VN _ t2) = t1 == t2
 
+-- /NOTA BENE/: Same proviso as for the 'Eq' instance.
 instance Ord VisitedNode where
   compare (VN _ t1) (VN _ t2) = t1 `compare` t2