import System.IO
import GHC.Read
import Text.ParserCombinators.ReadPrec (lift,(<++))
import Text.ParserCombinators.ReadP (skipSpaces,skipMany,get)
 
mnk :: [Double] -> [Double] -> (Double,Double,Double)
mnk x y = (a,b,sigm)
          where n    = length x
                sx   = sum x
                sy   = sum y
                sx2  = sum $ map (^(2 :: Int)) x
                sxy  = sum $ zipWith (*) x y
                det  = sx2*(fromIntegral n)-sx*sx
                deta = (fromIntegral n)*sxy-sx*sy
                detb = sx2*sy-sx*sxy
                a    = deta/det
                b    = detb/det
                sigm = sum $ zipWith (\xi yi -> (yi - (a*xi+b))^(2 :: Int)) x y
 
 
data InputData = XY Double Double
               | Complete
 
instance GHC.Read.Read InputData where
 
        readPrec = (do
                        x <- readPrec
                        lift skipSpaces
                        y <- readPrec
                        return (XY x y))
 
                     <++ (do
                        lift $ skipMany get
                        return Complete)
 
 
main :: IO ()
main = do
    hSetBuffering stdout NoBuffering
    let dataInput i xs ys = do
            let strI = show i
            putStr $ "Enter x" ++ strI ++ " y" ++ strI ++ 
                        " or not a number to complete : " 
            inp <- readLn
            case inp of
                (XY x y) -> dataInput (i+1) (xs++[x]) (ys++[y])
                Complete -> return (xs,ys)
    (xs,ys) <- dataInput (0 :: Int) [] []
    if length xs < 2 then putStrLn "no data"
    else do
        let (a,b,sigm) = mnk xs ys
        putStrLn $ "\na=" ++ show a
        putStrLn $ "b=" ++ show b
        putStrLn $ "sigm=" ++ show sigm
 

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