Add a function to return the list of edges in a graph

CHANGELOG.md

@@ -7,6 +7,8 @@ Notable changes to this project are documented in this file. The format is based
 Breaking changes:
 
 New features:
+- Added an `edges` function that returns a list of all edges in the graph (#17 by @MaybeJustJames)
+- Added `toMap` to unwrap `Graph` (#18)
 
 Bugfixes:
 
@@ -19,7 +21,6 @@ Breaking changes:
 
 New features:
 - Added `Foldable` and `Traversable` instances for `Graph` (#16 by @MaybeJustJames)
-- Added `toMap` to unwrap `Graph` (#18)
 
 Bugfixes:
 

src/Data/Graph.purs

@@ -2,21 +2,25 @@
 
 module Data.Graph
   ( Graph
+  , Edge
   , unfoldGraph
   , fromMap
   , toMap
   , vertices
+  , edges
   , lookup
   , outEdges
   , topologicalSort
   ) where
 
 import Prelude
+
 import Data.Bifunctor (lmap)
 import Data.CatList (CatList)
 import Data.CatList as CL
 import Data.Foldable (class Foldable, foldl, foldr, foldMap)
-import Data.List (List(..))
+import Data.FoldableWithIndex (foldlWithIndex)
+import Data.List (List(..), (:))
 import Data.List as L
 import Data.Map (Map)
 import Data.Map as M
@@ -42,6 +46,9 @@ instance traversableGraph :: Traversable (Graph k) where
   traverse f (Graph m) = Graph <$> (traverse (\(v /\ ks) -> (_ /\ ks) <$> (f v)) m)
   sequence = traverse identity
 
+-- | An Edge between 2 nodes in a Graph
+type Edge k = { start :: k, end :: k }
+
 -- | Unfold a `Graph` from a collection of keys and functions which label keys
 -- | and specify out-edges.
 unfoldGraph
@@ -54,9 +61,9 @@ unfoldGraph
   -> (k -> v)
   -> (k -> out k)
   -> Graph k v
-unfoldGraph ks label edges =
+unfoldGraph ks label theEdges =
   Graph (M.fromFoldable (map (\k ->
-            Tuple k (Tuple (label k) (L.fromFoldable (edges k)))) ks))
+            Tuple k (Tuple (label k) (L.fromFoldable (theEdges k)))) ks))
 
 -- | Create a `Graph` from a `Map` which maps vertices to their labels and
 -- | outgoing edges.
@@ -72,6 +79,17 @@ toMap (Graph g) = g
 vertices :: forall k v. Graph k v -> List v
 vertices (Graph g) = map fst (M.values g)
 
+-- | List all edges in a graph
+edges :: forall k v. Graph k v -> List (Edge k)
+edges (Graph g) = foldlWithIndex edges' Nil g
+  where
+    edges' :: k -> List (Edge k) -> Tuple v (List k) -> List (Edge k)
+    edges' src acc (_ /\ dests) =
+      foldl (mkEdge src) acc dests
+
+    mkEdge :: k -> List (Edge k) -> k -> List (Edge k)
+    mkEdge src acc dest = { start: src, end: dest } : acc
+
 -- | Lookup a vertex by its key.
 lookup :: forall k v. Ord k => k -> Graph k v -> Maybe v
 lookup k (Graph g) = map fst (M.lookup k g)

test/Main.purs

@@ -2,22 +2,54 @@ module Test.Main where
 
 import Prelude
 
-import Effect (Effect, foreachE)
-import Effect.Console (logShow)
 import Data.Foldable (foldr)
+import Data.Graph (Graph, Edge, edges, fromMap, unfoldGraph, topologicalSort)
+import Data.List (filter, length, toUnfoldable, range, (:), List(Nil))
+import Data.Map (Map)
+import Data.Map as M
 import Data.Maybe (Maybe(Just))
 import Data.Traversable (traverse)
-import Data.Graph (Graph, unfoldGraph, topologicalSort)
-import Data.List (filter, toUnfoldable, range, (:), List(Nil))
+import Data.Tuple (Tuple)
+import Data.Tuple.Nested ((/\))
+import Effect (Effect, foreachE)
+import Effect.Console (logShow)
+
+-- An example graph:
+--       0
+--      / \
+--     1  2
+--      \/
+--      3
+
+example1 :: Graph Int Int
+example1 =
+  fromMap example1'
+  where
+  example1' :: Map Int (Tuple Int (List Int))
+  example1' =
+    M.fromFoldable
+      [ (0 /\ (0 /\ (1 : 2 : Nil)))
+      , (1 /\ (1 /\ (3 : Nil)))
+      , (2 /\ (2 /\ (3 : Nil)))
+      , (3 /\ (3 /\ Nil))
+      ]
+
+showEdge :: forall k. Show k => Edge k -> String
+showEdge { start, end } =
+  "(" <> show start <> " --> " <> show end <> ")"
 
 main :: Effect Unit
 main = do
-  let double x | x * 2 < 100000 = [x * 2]
-               | otherwise      = []
-      graph :: Graph Int Int
-      graph = unfoldGraph (range 1 100000) identity double
+  let
+    double x
+      | x * 2 < 100000 = [ x * 2 ]
+      | otherwise = []
+
+    graph :: Graph Int Int
+    graph = unfoldGraph (range 1 100000) identity double
   foreachE (toUnfoldable (topologicalSort graph)) logShow
   logShow
     $ filter (_ /= 0) <<< foldr (:) Nil
-    <$> traverse (\n -> if n `mod` 2 == 0 then Just 0 else Just n) graph
-
+        <$> traverse (\n -> if n `mod` 2 == 0 then Just 0 else Just n) graph
+  logShow (length (edges graph))
+  logShow $ map showEdge $ edges example1