Warning: this code is in progress and only works for triangles

polygon-area.hs

import System.IO

-- lst = processInput ["1043 770","551 990","681 463"]
lst = processInput ["458 695","621 483","877 469","1035 636","1061 825","875 1023","645 1033","
485 853"]

-- area :: [Double] -> Double
area d@(a:b:c:[]) = let s = sum d / 2 in round $ sqrt $ (s * (s-a) * (s-b) * (s-c) )

processInput = map (tuplintfy . words)

strToDouble = read :: String -> Double

dist (x1, y1) (x2, y2) = sqrt ( (x1 - x2)^2 + (y1 - y2)^2 )

distances coords@(p:_) = fn coords
    where fn (p1:p2:[]) = dist p1 p2 : dist p2 p : []
          fn (p1:p2:ps) = dist p1 p2 : fn (p2:ps)

tuplintfy :: [String] -> (Double, Double)
tuplintfy [x,y] = (strToDouble x, strToDouble y)

main :: IO ()
main = do
    n_temp <- getLine
    let n = read n_temp :: Int
    l <- getMultipleLines n
    putStrLn $ (show . area .distances . processInput) l

getMultipleLines :: Int -> IO [String]
getMultipleLines n
    | n <= 0 = return []
    | otherwise = do
        x <- getLine
        xs <- getMultipleLines (n-1)
        let ret = (x:xs)
        return ret