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)])]
You probably meant to make "Graph a w" a type synonym but you used "data" thus it is a type constructor with one data constructor named Array which takes two parameters : an "a" and a list of pairs of "a" and "w".
You wanted to write :
> type Graph a w = Array a [(a,w)]
which should work better (indices will work on it like on an Array from Data.Array).