implementation module exist

import StdEnv
import Data.Func
import Data.Functor
import Data.Either
import Data.List
import Data.Maybe
import Control.Applicative
import Control.Monad

class eval m  :: m -> ([(String, Int)] -> Either String Int)
class print m :: m -> String

instance eval Expr
where
	eval (Lit i) = const $ pure i
	eval (Var s) = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
	eval (a +. b) = \s->liftM2 (+) (eval a s) (eval b s)
	eval (Ext e) = eval e

instance print Expr
where
	print (Lit i) = toString i
	print (Var s) = s
	print (a +. b) = print a +++ " + " +++ print b
	print (Ext e) = print e

Start = eval (Lit 5 +. Lit 37) []