module test

import StdEnv
import Data.Maybe
import Data.Functor
import Control.Monad
import Control.Applicative

class expr v where
	lit :: i -> v i | toString i
	(+.) infixl 6 :: (v i) (v i) -> v i | + i

instance + (v a) | expr v & + a where
	+ l r = l +. r

eval :: (Maybe a) -> Maybe a
eval x = x
instance expr Maybe where
	lit i = Just i
	+. x y = (+) <$> x <*> y

:: Print a =: Print String
print :: (Print a) -> String
print (Print a) = a
instance expr Print where
	lit i = Print (toString i)
	+. (Print l) (Print r) = Print (l +++ "+" +++ r)

printEval :: (A.v: v a | expr v) -> (Maybe a, String)
//printEval f = (f, let (Print p) = f in p)
printEval f = (eval f, print f)

//Mag niet
//Start :: (Maybe Int, String)
//Start = printEval (lit 4 + lit 38)

//Mag wel
Start = let (Print f) = lit 4 + lit 38 in f