import Control.Applicative
import Data.Numbers.Primes -- $ cabal install primes

data Delta a = Delta [String] a [String] a deriving (Show)

value :: (Delta a) -> a
value (Delta _ x _ _) = x

similar :: (Delta a) -> a
similar (Delta _ _ _ y) = y

instance (Eq a) => Eq (Delta a) where
    s == ss = (value s) == (value ss)

instance Functor Delta where
    fmap f (Delta xs x ys y) = Delta xs (f x) ys (f y)

instance Applicative Delta where
    pure f                                      = Delta [] f [] f
    (Delta lf f lg g) <*> (Delta lx x ly y) = Delta (lf ++ lx) (f x) (lg ++ ly) (g y)

mu :: Delta (Delta a) -> Delta a
mu (Delta lx (Delta llx x _ _) ly (Delta _ _ lly y)) = Delta (lx ++ llx) x (ly ++ lly) y

instance Monad Delta where
    return x = Delta [] x [] x
    s >>= f  = mu $ fmap f s


returnS :: (Show s) => s -> Delta s
returnS x = Delta [(show x)] x [(show x)] x

returnSS :: (Show s) => s -> s -> Delta s
returnSS x y = Delta [(show x)] x [(show y)] y

-- samples

generator :: Int -> Delta [Int]
generator x = let intList = [1..x] in
                  returnS intList

primeFilter :: [Int] -> Delta [Int]
primeFilter xs = let primeList    = filter isPrime xs
                     refactorList = filter even xs    in
                 returnSS primeList refactorList

count :: [Int] -> Delta Int
count xs = let primeCount = length xs in
           returnS primeCount

primeCount :: Int -> Delta Int
primeCount x = generator x >>= primeFilter >>= count