module Main where

import Control.Monad

primPlusInt :: (Monad m) => Int -> Int -> m Int

primPlusInt a b = return (a + b)

primMinusInt :: (Monad m) => Int -> Int -> m Int

primMinusInt a b = return (a - b)

class Eval a b | a -> b where
 eval :: (Monad m) => a -> m b

instance Eval Int Int where
  eval = return

instance Eval Bool Bool where
  eval = return

instance Eval Char Char where
  eval = return

instance (Eval a b) => Eval [a] [b] where
  eval [] = return []
  eval (x:xs) = do
    x' <- eval x
    xs' <- eval xs
    return (x':xs')

data (Eval a Int, Eval b Int) => FADD a b = FADD a b

instance (Eval a Int, Eval b Int) => Eval (FADD a b) Int where
 eval (FADD a b) = do
   a' <- eval a
   b' <- eval b
   primPlusInt a' b'

data (Eval a Int, Eval b Int) => FSUB a b = FSUB a b

instance (Eval a Int, Eval b Int) => Eval (FSUB a b) Int where
 eval (FSUB a b) = do
   a' <- eval a
   b' <- eval b
   primMinusInt a' b'


data Ffun a b c = Ffun a b c

instance (Eval a Int, Eval b x, Eval c x) => Eval (Ffun a b c) x where
  eval (Ffun a b c) = fun a b c


fun a b c = do
 p <- eval a
 if p < (0::Int) then eval b else eval c

main = do
  putStrLn "bla"
  fun (FADD (2::Int) (3::Int)) (FADD (6::Int) (8::Int)) (FSUB (3::Int) (9::Int))
    >>= putStrLn . show
  fun (FADD (2::Int) (3::Int)) True False
    >>= putStrLn . show
  fun (FADD (2::Int) (3::Int)) "Neg" "Pos"
    >>= putStrLn . show

bla
-6
False
"Pos"