import Data.Array

data Graph a w = Array a [(a,w)]

-- array (1,3) [(1,[(2,3),(3,2)])]

--Given a vertex list out adjacents

getAdjVertices :: (Ix a, Eq a) => a -> (Graph a w) -> [a]

getAdjVertices x arr | not (x `elem` (indices arr)) = error "No such Vertex"

                     | otherwise = extractV (arr ! 2)

                     where extractV [] = []

                           extractV (x:xs) = (fst x):extractV (xs)

[1 of 1] Compiling Main             ( programs\graph.hs, interpreted )

programs\graph.hs:16:47:

    Couldn't match expected type `Array a e0'

                with actual type `Graph a w'

    In the first argument of `indices', namely `arr'

    In the second argument of `elem', namely `(indices arr)'

    In the first argument of `not', namely `(x `elem` (indices arr))'

programs\graph.hs:17:46:

    Couldn't match expected type `Array Integer [(a, b0)]'

                with actual type `Graph a w'

    In the first argument of `(!)', namely `arr'

    In the first argument of `extractV', namely `(arr ! 2)'

    In the expression: extractV (arr ! 2)

Failed, modules loaded: none.

Prelude>