An example of type family

Graph.hs

{-# LANGUAGE TypeFamilies #-}

class Graph g where
  type Vertex g
  data Edge g
  src, tgt :: Edge g -> Vertex g
  outEdges :: g -> Vertex g -> [Edge g]

newtype G1 = G1 [Edge G1]
instance Graph G1 where
  type Vertex G1 = Int
  data Edge G1 = MkEdge1 (Vertex G1) (Vertex G1)
  src = undefined
  tgt = undefined
  outEdges = undefined

newtype G2 = G2 [Edge G2]
instance Graph G2 where
  type Vertex G2 = String
  data Edge G2 = MkEdge2 Int (Vertex G2) (Vertex G2)
  src = undefined
  tgt = undefined
  outEdges = undefined

neighbours g v = map tgt (outEdges g v)