module deep

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

:: Expr
	= Lit Int
	| Var String
	| (+.) infixl 6 Expr Expr

eval :: Expr -> ([(String, Int)] -> Either String Int)
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)

print :: Expr -> String
print (Lit i) = toString i
print (Var s) = s
print (a +. b) = print a +++ " + " +++ print b

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