Problem with `fgl` in Haskell

example.hs

#! /usr/bin/env nix-shell
#! nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [fgl])" -i "runghc -Wall"

{-# LANGUAGE ScopedTypeVariables #-}

import qualified Data.Graph.Inductive.Graph as G
import qualified Data.Graph.Inductive.NodeMap as GM
import Data.Graph.Inductive.PatriciaTree (Gr)

type Graph = Gr String Integer

mkGraph :: [(String, [(Integer, String)])] -> (GM.NodeMap String, Graph, [G.LNode String])
mkGraph = foldr f (GM.new, G.empty, [])
  where
    f (from, children) (nodemap, graph, lnodes) =
      let (newG, newM, labelledNodes) = GM.insMapNodes nodemap (from : map snd children) graph
          edges = map (\(edgeLabel, to) -> (from, to, edgeLabel)) children
          newG' = GM.insMapEdges newM edges newG
       in (newM, newG', lnodes ++ labelledNodes)

main :: IO ()
main = do
  let (manualGraph :: Gr String Integer) =
        G.mkGraph
          [(0, "muted yellow"), (1, "shiny gold"), (2, "faded blue"), (3, "bright white")]
          [(0, 1, 2), (0, 2, 9), (3, 1, 1)]
      (nodemap, manualGraph2 :: Gr String Integer, labelledNodes) =
        mkGraph
          [ ("bright white", [(1, "shiny gold")]),
            ("muted yellow", [(2, "shiny gold"), (9, "faded blue")])
          ]
  print $ "Graphs: manual, manual2"
  print manualGraph
  print manualGraph2
  print ""
  print "G.inn manual, manual2"
  print "In both graphs two nodes should link to the 'shiny gold' (1) node,"
  print "but only the one with G.mkGraph has two edges."
  print $ G.inn manualGraph 1
  print $ G.inn manualGraph2 1
  print ""
  print "Edges"
  print $ G.edges manualGraph
  print $ G.edges manualGraph2
  print ""
  print $ "Are graphs equal? " <> show (G.equal manualGraph2 manualGraph)
  print ""
  print "nodemap"
  print nodemap
  print ""
  print "labelledNodes"
  print labelledNodes