Re: [Haskell-cafe] Area from [(x,y)] using foldl

How about these type signatures. import Data.List poly1 = [(0,1),(5,0),(3,4)]::[(Double,Double)] areaPoly :: [(Double,Double)] -> Double areaPolyCalc :: (Double,(Double,Double)) -> (Double,Double) -> (Double,(Double,Double)) areaPoly (pt:pts) = 0.5 * (fst (foldl' areaPolyCalc (0,pt) pts)) areaPolyCalc (sum,(x,y)) (xNext,yNext) = (sum + (x * yNext - xNext * y),(xNext,yNext)) -- Regards, Casey