From 80179ab3a1f11eafd94ff005316da78f9593dd40 Mon Sep 17 00:00:00 2001
From: Andreas Abel <andreas.abel@ifi.lmu.de>
Date: Fri, 2 Feb 2024 12:08:59 +0100
Subject: [PATCH] Add `flattenSCC1 :: SCC a -> NonEmpty a`

This gives a more precise type to the existing `flattenSCC :: SCC a -> [a]`.
Closes #985.
---
 containers/src/Data/Graph.hs | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/containers/src/Data/Graph.hs b/containers/src/Data/Graph.hs
index 33983ffee..3ce605e89 100644
--- a/containers/src/Data/Graph.hs
+++ b/containers/src/Data/Graph.hs
@@ -246,9 +246,18 @@ flattenSCCs :: [SCC a] -> [a]
 flattenSCCs = concatMap flattenSCC
 
 -- | The vertices of a strongly connected component.
+--
+-- @flattenSCC = 'Data.List.NonEmpty.toList' . 'flattenSCC1'@.
+--
+-- This function is retained for backward compatibility,
+-- 'flattenSCC1' has the more precise type.
 flattenSCC :: SCC vertex -> [vertex]
-flattenSCC (AcyclicSCC v) = [v]
-flattenSCC (NECyclicSCC vs) = NE.toList vs
+flattenSCC = NE.toList . flattenSCC1
+
+-- | The vertices of a strongly connected component.
+flattenSCC1 :: SCC vertex -> NonEmpty vertex
+flattenSCC1 (AcyclicSCC v) = NE.singleton v
+flattenSCC1 (NECyclicSCC vs) = vs
 
 -- | \(O((V+E) \log V)\). The strongly connected components of a directed graph,
 -- reverse topologically sorted.