Add Edge instance for List (#3)

* For `Cons a tail` this adds a node `a` with an edge to `tail` 
* For `Nil` it adds a node `Nil` to the graph

Author: swapnilraj

src/Data/GenericGraph.purs

@@ -1,6 +1,7 @@
 module Data.GenericGraph where
 
 import Control.Semigroupoid ((>>>))
+import Data.List(List(..))
 import Data.Array (concat, foldr, (!!), (:))
 import Data.DotLang (Edge(..), EdgeType(..), Graph, Node(..), graphFromElements, changeNodeId, nodeId)
 import Data.DotLang.Attr (FillStyle(..))
@@ -71,6 +72,11 @@ instance arrayEdges :: Edges a => Edges (Array a) where
   edges [] = Root (Just $ Node "[]" []) []
   edges a = Root (Just $ Node ("array") []) (edges <$> a)
 
+instance listEdges :: Edges a => Edges (List a) where
+  edges Nil = Root (Just $ Node ("Nil") []) []
+  edges (Cons a as) = case edges a of
+                            (Root n _) -> Root n [edges as]
+
 instance genericReprArgument :: Edges a => GenericEdges (Argument a) where
   genericEdges' (Argument a) = edges a
 

test/Main.purs

@@ -8,6 +8,7 @@ import Data.Foldable (foldr)
 import Data.Generic.Rep (class Generic)
 import Data.Generic.Rep.Show (genericShow)
 import Data.GenericGraph (class Edges, genericEdges, genericToGraph)
+import Data.List(List(..)) as L
 import Effect (Effect)
 import Effect.Aff (Aff)
 import Effect.Aff.Class (liftAff)
@@ -91,6 +92,14 @@ main = runTest do
       equal
         "digraph {root [style=invis]; 0 [label=\"Cons'\"]; 4 [label=\"1\"]; 1 [label=\"Cons'\"]; 3 [label=\"2\"]; 2 [label=\"Nil\"]; root -> 0 []; 0 -> 4 []; 0 -> 1 []; 1 -> 3 []; 1 -> 2 []; }"
         (toText $ toGraph $ Cons' 1 (Cons' 2 Nil))
+    test "prelude.list Cons" do
+      equal
+        "digraph {root [style=invis]; 0 [label=\"1\"]; 1 [label=\"2\"]; 2 [label=\"6\"]; 3 [label=\"Nil\"]; root -> 0 []; 0 -> 1 []; 1 -> 2 []; 2 -> 3 []; }"
+        (toText $ genericToGraph $ L.Cons 1 (L.Cons 2 (L.Cons 6 L.Nil)))
+    test "prelude.list Nil" do
+          equal
+            "digraph {root [style=invis]; 0 [label=\"Nil\"]; root -> 0 []; }"
+            (toText $ genericToGraph $ (L.Nil :: L.List Int))
     test "tree" do
       equal
         "digraph {root [style=invis]; 0 [label=\"Node'\"]; 9 [label=\"Leaf'\"]; 8 [label=\"3\"]; 1 [label=\"Node'\"]; 4 [label=\"Node'\"]; 7 [label=\"Leaf'\"]; 6 [label=\"5\"]; 5 [label=\"Leaf'\"]; 3 [label=\"4\"]; 2 [label=\"Leaf'\"]; root -> 0 []; 0 -> 9 []; 0 -> 8 []; 0 -> 1 []; 1 -> 4 []; 4 -> 7 []; 4 -> 6 []; 4 -> 5 []; 1 -> 3 []; 1 -> 2 []; }"