module shallow

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

:: Expr :==
	([(String, Int)] -> Either String Int //Evaluator
	, String)                             //Printer

lit :: Int -> Expr
lit i = (const $ pure i, toString i)

var :: String -> Expr
var s =
	( maybe (Left $ "Undefined variable " +++ s) Right o lookup s
	, s
	)

(+.) infixl 6 :: Expr Expr -> Expr
(+.) (ea, pa) (eb, pb) =
	( \s->liftM2 (+) (ea s) (eb s)
	, pa +++ " + " +++ pb
	)

print :: Expr -> String
print (_, p) = p

eval :: Expr [(String, Int)]-> Either String Int
eval (e, _) s = e s

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