import Data.List

import Data.Array

import Control.Monad

type Node = Int

type Graph = Array Int [ Node ]

dfs' :: Graph -> Node -> [ Node ] -> [ Node ]

dfs' g v vis = dfsfun lst where

lst = g ! v

dfsfun [] = vis

dfsfun ( x : xs )

| elem x vis = dfsfun xs

| otherwise = dfs' g y ( v : vis )

--set the flag true if the graph is direct otherwise false

buildGraph :: ( Int , Int ) -> [ ( Node , Node ) ] -> Bool -> Graph

buildGraph bnds xs f

| f = accumArray ( flip (:) ) [] bnds xs --direct graph a->b

| otherwise = accumArray ( flip (:) ) [] bnds xss where --indirect a -> b and b -> a

xss = foldr ( \ ( x , y ) b -> ( y , x ) : b ) xs xs

dfsSearch :: Graph -> Int -> [ Node ]

dfsSearch g v = dfs' g v []

Lets create a indirect graph

ghci>let g = buildGraph ( 0 , 5 ) [ ( 0 , 1 ) , ( 0 , 2 ) , ( 0 , 3 ) , ( 0 , 4 ) ] False

ghci>g

array (0,5) [(0,[4,3,2,1]),(1,[0]),(2,[0]),(3,[0]),(4,[0]),(5,[])]

Here I am getting only 0 but it should return [ 0 , 1 , 2 , 3 , 4 ] . What is happening here when i am passing 0 as root node to function , at first level i get

lst = [ 4 , 3, 2, 1 ] . First element of this list is 4 so 0 is added to vis list and now 4 is passed to dfs' as parent. When it goes down , we get lst = [0] and since 0 is member of vis list so it returns the vis as result . When we write dfs in c/c++ then it returns to calling function and again loops through remaining element which i am not able visualize in Haskell. Could some one please help me how to solve this issue .