basics of new environment set up

lambda.cabal

@@ -28,6 +28,7 @@ executable lambda
                      , parsec
                      , mtl
                      , haskeline
+                     , containers
   hs-source-dirs:      src
   default-language:    Haskell2010
 

src/Lambda/Environment.hs

@@ -1,8 +1,49 @@
+{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, FlexibleInstances #-}
 module Lambda.Environment where
 
-import           Lambda.Types
-
 import qualified Data.Map.Strict as Map
+import           Data.IORef
+import           Control.Monad.Except
+import           System.IO
+import           Lambda.Types hiding (Env, isBound, getVar, setVar, defineVar, bindVars)
+
+type Environment = IORef (Map.Map String (IORef LispVal))
+
+nullEnv :: IO Environment
+nullEnv = newIORef Map.empty
+
+isBound :: Environment -> String -> IO Bool
+isBound e s = readIORef e >>= return . Map.member s
+
+getVar :: Environment -> String -> IOThrowsError LispVal
+getVar e s = do
+  env <- liftIO $ readIORef e
+  case Map.lookup s env of
+    Just v  -> liftIO $ readIORef v
+    Nothing -> throwError $ UnboundVar "Trying to get an unbound variable..." s
+
+updateVar :: Environment -> String -> LispVal -> IOThrowsError LispVal
+updateVar e s l = do
+  env <- liftIO $ readIORef e
+  case Map.lookup s env of
+    Just v  -> liftIO $ writeIORef v l
+    Nothing -> throwError $ UnboundVar "Trying to update an unbound variable.." s
+  return l
+
+defineVar :: Environment -> String -> LispVal -> IOThrowsError LispVal
+defineVar e s l = do
+  defined <- liftIO $ isBound e s
+  if defined then updateVar e s l
+  else liftIO $ do
+    newVar <- newIORef l
+    modifyIORef' e $ Map.insert s newVar
+    return l
 
-foo :: Int
-foo = 1
+bindVars :: Environment -> [(String, LispVal)] -> IO Environment
+bindVars e ls = do
+  updates <- mapM mkRef ls >>= return . Map.fromList
+  modifyIORef' e (Map.union updates)
+  return e
+    where
+      mkRef :: (String, LispVal) -> IO (String, IORef LispVal)
+      mkRef (s, l) = newIORef l >>= return . (,) s

src/Lambda/Types.hs

@@ -45,9 +45,9 @@ showVal (Vector contents)      = "(" ++ unwordsList contents ++ ")"
 showVal (IOFunc _)             = "<IO primitive>"
 showVal (Port _)               = "<IO port>"
 showVal (PrimitiveFunc _)      = "<primitive>"
-showVal (Func {params = args,
-               vararg = varargs,
-               body = body,
+showVal (Func {params = args ,
+vararg = varargs             ,
+body = body                  ,
                closure = env}) = "(lambda (" ++ unwords (map show args) ++
                  (case varargs of
                     Nothing -> ""
@@ -138,5 +138,3 @@ bindVars envRef bindings = readIORef envRef >>= extendEnv bindings >>= newIORef
   where extendEnv bindings env = liftM (++ env) (mapM addBinding bindings)
         addBinding (var, value) = do ref <- newIORef value
                                      return (var, ref)
-
-