The awesome code of https://gist.github.com/4638535 implemented in Haskell, but sadly without GUI.

Shootyear.hs

module Main where
import Data.Maybe

isShootYear yr = [mod yr 400 == 0, mod yr 4 == 0, mod yr 100 /= 0] == [True, True, False]

isShootYearMsg True  = "är ett skottår"
isShootYearMsg False = "är inte ett skottår"

maybeRead :: Read a => String -> Maybe a
maybeRead s = case reads s of
    [(x, "")] -> Just x
    _         -> Nothing

main = do
    putStrLn "Ett år tak!"
    yrstr <- getLine
    let yr = maybeRead yrstr
    if isJust yr
        then sequence_ [putStrLn $ (isShootYearMsg $ isShootYear $ fromJust yr), putStrLn "", main]
        else putStrLn "det där var fan inte ett år\n" >> main