leap.hs

module LeapYear where


data Check a = Check (a -> Bool)

isLeap (Check check) n = check n

instance Monad (Check) where
    return a = Check (\_ -> True)
    (Check check) >>= cont = Check (\n -> isLeap (cont n) n ) -- check n && cont n
    
divby b = Check( \ n -> mod n b == 0)
except (Check check) = Check( \ n ->not $ check n )
unless (Check check) = Check( \ n ->check n )



isLeapYear n  =
    isLeap 
    $ do
        divby 4
        except $ divby 100
        unless $ divby 400