grap.hs

import qualified Data.Map as M

data Vertex = Vertex String deriving Eq

instance Show Vertex where
  show (Vertex s) = "Vertex " ++ s

type Graph = [(Vertex, [Vertex])]

graph :: Graph
graph =
  [ (v1, [v2, v3])
  , (v2, [v1])
  , (v3, [v1, v4])
  , (v4, [v3])
  ] where
  v1 = Vertex "v1"
  v2 = Vertex "v2"
  v3 = Vertex "v3"
  v4 = Vertex "v4"

isAdjacent :: Vertex -> Vertex -> Either String Bool
isAdjacent v w =
  case lookup v graph of
    Nothing  -> Left $ "Vertex not found in graph: " ++ show v
    Just adj -> Right $ w `elem` adj