When developing my QuickCheck-2 test-suite for graphviz, I wrote the following Arbitrary instance for FGL graphs (which needs FlexibleInstances): ,---- | instance (Graph g, Arbitrary n, Arbitrary e, Show n, Show e) => Arbitrary (g n e) where | arbitrary = do ns <- liftM nub arbitrary | let nGen = elements ns | lns <- mapM makeLNode ns | trace ("Nodes: " ++ show lns) (return ()) | les <- listOf $ makeLEdge nGen | trace ("Edges: " ++ show les) (return ()) | return $ mkGraph lns les | where | makeLNode n = liftM ((,) n) arbitrary | makeLEdge nGen = liftM3 (,,) nGen nGen arbitrary | | shrink gr = map (flip delNode gr) (nodes gr) `---- However, when I try to run this, I occasionally get irrefutable pattern match failures as follows: ,---- | *Data.GraphViz.Testing.Instances.FGL Data.Graph.Inductive.Tree> sample (arbitrary :: Gen (Gr Int Char)) | | | 0:0->[] | | 0:-2->[] | 1:0->[('\a',0)] | 2:0->[] | | -4:-3->[('U',-3),('#',1)] | -3:3->[] | 1:-1->[('}',-3)] | | -8:8->[] | -3:2->[] | -1:-5->[('\US',-3),('&',0)] | 0:5->[('F',-1),('p',4)] | 4:-1->[] | | -2:8->[('\177',-2),('(',-2),('d',-2),('4',-2),('D',-2),('\US',-2),('d',-2),('u',-2)] | | -16:11->[] | -2:-2->[] | 0:11->[('@',1)] | 1:13->[('u',11)] | 9:-11->[('\231',11)] | 11:12->[('\226',1)] | 16:15->[] | | -10:2->[] | -4:8->[] | 1:30->[] | 26:26->[('<',1),('K',-4)] | 31:-21->[] | | -35:51->[('@',-29)] | -29:21->[('\132',-11)] | -11:-31->[('j',61)] | -4:40->[('a',-29)] | 0:6->[('z',-35),('9',28),('\170',-11),('\SUB',28)] | 23:8->[('P',-29),('(',61),('\\',28)] | 28:60->[] | 61:44->[('q',61)] | *** Exception: Data/Graph/Inductive/Graph.hs:250:26-59: Irrefutable pattern failed for pattern (Data.Maybe.Just (pr, _, la, su), g') `---- The actual error comes from the definition of insEdge: ,---- | -- | Insert a 'LEdge' into the 'Graph'. | insEdge :: DynGraph gr => LEdge b -> gr a b -> gr a b | insEdge (v,w,l) g = (pr,v,la,(l,w):su) & g' | where (Just (pr,_,la,su),g') = match v g `---- with the Graph instance for Tree-based graphs using this for its mkGraph method: ,---- | mkGraph vs es = (insEdges' . insNodes vs) empty | where | insEdges' g = foldl' (flip insEdge) g es `---- So, is this really a bug in FGL, or am I using mkGraph wrong? On another note, why doesn't the PatriciaTree graph type have a Show instance? :( -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com