(***) Generate K-regular simple graphs with N nodes
In a K-regular graph all nodes have a degree of K; i.e. the number of edges incident in each node is K.
This solution generates all possible graphs with n nodes and n * k / 2 edges, filters the k regular graphs, then collects all non-isomorphic graphs using graph canonization.
data Graph a = Graph [a] [(a, a)] deriving (Show, Eq) data Adjacency a = Adj [(a, [a])] deriving (Show, Eq) regular :: Int -> Int -> [Graph Int] regular n k | r == 1 || n <= k || n < 0 || k < 0 =  | otherwise = map (adjToGraph . fst) $ foldr (\x xs -> if any ((==) (snd x) . snd) xs then xs else x : xs)  $ zip a $ map canon a where a = filter (\(Adj a) -> all ((==) k . length . snd) a) $ map (graphToAdj . Graph [1..n]) $ perm e q e = map (\xs -> (head xs, last xs)) $ perm [1..n] 2 (q, r) = (n * k) `quotRem` 2 perm n k = foldr (\x xs -> [i : s | i <- n, s <- xs, i `notElem` s, asc (i : s)]) [] [1..k] asc xs = all (uncurry (<)) $ zip xs $ tail xs graphToAdj :: (Eq a) => Graph a -> Adjacency a graphToAdj (Graph  _) = Adj  graphToAdj (Graph (x:xs) ys) = Adj ((x, ys >>= f) : zs) where f (a, b) | a == x = [b] | b == x = [a] | otherwise =  Adj zs = graphToAdj (Graph xs ys) adjToGraph :: (Eq a) => Adjacency a -> Graph a adjToGraph (Adj ) = Graph   adjToGraph (Adj ((v, a):vs)) = Graph (v : xs) ((a >>= f) ++ ys) where f x = if (v, x) `elem` ys || (x, v) `elem` ys then  else [(v, x)] Graph xs ys = adjToGraph (Adj vs) canon :: (Eq a, Ord a) => Adjacency a -> String canon (Adj a) = minimum $ map f $ perm n where n = length a v = map fst a perm n = foldr (\x xs -> [i : s | i <- [1..n], s <- xs, i `notElem` s]) [] [1..n] f p = let n = zip v p in show [(snd x, sort id $ map (\x -> snd $ head $ snd $ break ((==) x . fst) n) $ snd $ find a x) | x <- sort snd n] sort f n = foldr (\x xs -> let (lt, gt) = break ((<) (f x) . f) xs in lt ++ [x] ++ gt)  n find a x = let (xs, ys) = break ((==) (fst x) . fst) a in head ys