functor TOPSORT( structure G: GraphSig )=
struct
    type Graph = G.Graph
    type Vertex = G.Vertex

    exception Cycle

local
    fun member [] _ = false
      | member (h :: t) x = x = h orelse member t x
in
    fun topSort g =
        let fun sort [] path sorted = sorted
              | sort (x :: xs) path sorted =
                if member path x 
                     then raise Cycle
                else if member sorted x
                     then sort xs path sorted
                else let val afterx = sort 
                                        (G.adj g x) 
                                        (x :: path) 
                                         sorted
                     in
                         sort xs path (x :: afterx)
                     end
        in
            sort (G.vertices g) [] []
        end
end
end ;