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

[Casey Hawthorne]

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))







<Spoiler Alert! -- Functions Below!>




















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