[Haskell-beginners] Error while using indices and elem

21 Apr 2014

      Hi,

Do I need to define indices and elem for Graph a w here explicitly ?

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)

GHCI gives :

Prelude> :l programs\graph.hs
[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>

Regards.
Nishant