implementation module class

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

//Easy to add functionality
//Easy to add backends
//Type safety as in GADTs

class lit m :: Int -> m
class var m :: String -> m
class (+.) infixl 6 m :: m m -> m

:: Eval :== ([(String, Int)] -> Either String Int)
instance lit Eval where lit i = pure (pure i)
instance var Eval where var s = maybe (Left $ "Undefined variable " +++ s) Right o lookup s
instance +. Eval where (+.) a b = \s->liftM2 (+) (a s) (b s)

:: Print :== String
instance lit Print where lit i = toString i
instance var Print where var s = s
instance +. Print where (+.) a b = a +++ " + " +++ b

Start :: Eval
Start = (lit 5 +. lit 37)