class Expr e where
    intVal  :: Int -> e Int
    boolVal :: Bool -> e Bool
    add     :: e Int -> e Int -> e Int
    isZero  :: e Int -> e Bool
    if'     :: e Bool -> e t -> e t -> e t

newtype Eval a = Eval {runEval :: a}

instance Expr Eval where
    intVal x  = Eval x
    boolVal x = Eval x
    add x y   = Eval $ runEval x + runEval y
    isZero x  = Eval $ runEval x == 0
    if' x y z = if (runEval x) then y else z 

newtype Print a = Print {printExpr :: String}

instance Expr Print where
    intVal x  = Print $ show x
    boolVal x = Print $ show x
    add x y   = Print $ printExpr x ++ "+" ++ printExpr y
    isZero x  = Print $ "isZero(" ++ printExpr x ++ ")"
    if' x y z = Print $ "if (" ++ printExpr x ++ ") then (" ++
        printExpr y ++ ") else (" ++ printExpr z ++ ")"