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

[Casey Hawthorne]

How about these BETTER type signatures.

-- Area of a Polygon
    
import Data.List

type X = Double
type Y = Double
type Area = Double

    
poly1 = [(0,1),(5,0),(3,4)]::[(X,Y)]


areaPoly :: [(X,Y)] -> Area


areaPolyCalc :: (Area,(X,Y)) -> (X,Y) -> (Area,(X,Y))




<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