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

[Casey Hawthorne]

Sorry, I forgot to add that if the polygon is very far from the
origin, you may have overflow or increased round off error; it is
better to translate the polygon back to the origin, before doing the
area calculation.


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