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|
module Environment ( Env
, setVar
, getVar
, defineVar
, manyBindings
, newEnv
, liftLispResult
, IOResult
) where
import Base (Env (..), Expr (..))
import Control.Applicative ((<$>))
import Control.Monad (mapM)
import Control.Monad.Except
import Data.IORef
import Data.Maybe (isJust)
import Error.Base (LispError (..), LispResult (..), unwrap)
newEnv :: IO Env
newEnv = newIORef []
type IOResult = ExceptT LispError IO
liftLispResult :: LispResult a -> IOResult a
liftLispResult (Left err) = throwError err
liftLispResult (Right val) = return val
isBound :: Env -> String -> IO Bool
isBound env var = do
ptr <- readIORef env
return $ isJust $ lookup var ptr
-- env modifiers
getVar :: Env -> String -> IOResult Expr
getVar env var = do
ptr <- liftIO $ readIORef env
maybe (throwError $ UnboundVariable var)
(liftIO . readIORef)
$ lookup var ptr
setVar :: Env -> String -> Expr -> IOResult ()
setVar env var val = do
ptr <- liftIO $ readIORef env
maybe (throwError $ UnboundVariable var)
(liftIO . flip writeIORef val)
$ lookup var ptr
defineVar :: Env -> String -> Expr -> IOResult ()
defineVar env var val = do
alreadyBound <- liftIO $ isBound env var
if alreadyBound
then setVar env var val
else liftIO $ do
newRef <- newIORef val
ptr <- readIORef env
writeIORef env $ (var, newRef):ptr
makeBind :: (String, Expr) -> IO (String, IORef Expr)
makeBind (var, val) = do
n <- newIORef val
return (var, n)
manyBindings :: Env -> [(String, Expr)] -> IO Env
manyBindings env binds = do
ptr <- readIORef env
newIORef =<< (++ ptr) <$> mapM makeBind binds
|
