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

Casey Hawthorne caseyh at istar.ca
Sun Nov 8 17:44:29 EST 2009

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))

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