transition to linked lexical environments
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a6cdb3b073
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49
interp.hs
49
interp.hs
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@ -25,24 +25,32 @@ data Value = IntV Integer
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| TupleV [Value]
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| ListV [Value]
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| Builtin BIF
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| FnV [(Pattern, AST)] -- pattern->body bindings
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| FnV Env [(Pattern, AST)] -- closure pattern->body bindings
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deriving (Eq)
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type Env = M.Map String Value -- an environment
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type Env = [M.Map String Value] -- lexical environment (linked list)
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type InterpState = StateT ([Handle], Env) IO -- interpreter state (open handles, global env)
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lookup :: Env -> String -> Maybe Value
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lookup env name = M.lookup name env
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emptyEnv = [M.empty]
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-- look up a binding from the bottom up
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lookup :: Env -> String -> Maybe Value
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lookup [] _ = Nothing
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lookup (env:xs) name =
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case M.lookup name env of
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Nothing -> lookup xs name
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Just x -> Just x
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-- bind in the local environment
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bind :: Env -> String -> Value -> Env
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bind env name value = M.insert name value env
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bind (env:xs) name value = (M.insert name value env):xs
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instance Show Value where
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show (IntV i) = show i
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show (StrV s) = show s
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show (TupleV v) = "(" ++ intercalate "," (map show v) ++ ")"
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show (ListV v) = show v
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show (FnV _) = "<fn>"
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show (FnV _ _) = "<fn>"
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show (StreamV _) = "<stream>"
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show (Builtin _) = "<built-in>"
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show UnitV = "()"
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@ -79,13 +87,13 @@ _itos (IntV i) = return $ StrV $ show i
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_itos v = error $ "itos: not an int: " ++ show v
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initialState = ([stdout, stdin],
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M.fromList [("id", FnV [(VarP "x", Var "x")]),
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[M.fromList [("id", FnV emptyEnv [(VarP "x", Var "x")]),
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("stdout", StreamV 0),
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("putstr", Builtin $ BIF _putstr),
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("putstrln", Builtin $ BIF (\x -> _putstr $ x +$ StrV "\n")),
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("print", Builtin $ BIF _print),
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("itos", Builtin $ BIF _itos),
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("getline", Builtin $ BIF _getline)])
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("getline", Builtin $ BIF _getline)]])
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eval :: AST -> InterpState Value
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@ -121,9 +129,9 @@ eval (Defun name fn) = do
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eval fn >>= \fn' ->
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put (s, bind env name fn') >> return fn'
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Just oldfn -> -- add pattern to old fn
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let FnV oldpats = oldfn
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let FnV cls oldpats = oldfn
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Lambda [(pat, body)] = fn
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newfn = FnV (oldpats ++ [(pat, body)]) in
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newfn = FnV cls (oldpats ++ [(pat, body)]) in
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put (s, bind env name newfn) >> return newfn
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eval (Def name v') = do
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@ -133,22 +141,28 @@ eval (Def name v') = do
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return v
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eval (Lambda pats) =
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return $ FnV pats
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get >>= \(_,env) ->
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return $ FnV env pats
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eval (Add l r) = do { l <- eval l; r <- eval r; return $ l +$ r }
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eval (Sub l r) = do { l <- eval l; r <- eval r; return $ l -$ r }
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eval (Mul l r) = do { l <- eval l; r <- eval r; return $ l *$ r }
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eval (Div l r) = do { l <- eval l; r <- eval r; return $ l /$ r }
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eval (Call name arg) = get >>= \(_,env) ->
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eval (Call name arg) = get >>= \(h,env) ->
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case lookup env name of
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Just fn@(FnV _) -> eval arg >>= apply fn
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Just fn@(FnV cls _) -> do
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arg' <- eval arg
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put (h,cls) -- enter closure env
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let v = apply fn arg'
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put (h,env) -- restore env
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v
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Just fn@(Builtin _) -> eval arg >>= apply fn
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Nothing -> error $ "call: name " ++ name ++ " doesn't exist or is not a function"
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eval x = error $ "eval: unhandled: " ++ show x
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patternBindings :: Pattern -> Value -> Maybe Env
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patternBindings :: Pattern -> Value -> Maybe (M.Map String Value)
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patternBindings (VarP n) v = Just $ M.fromList [(n, v)]
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patternBindings (IntP n) (IntV v)
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@ -191,16 +205,17 @@ patternBindings (TupleP _) _ = Nothing -- not a tuple
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-- applies a function
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apply :: Value -> Value -> InterpState Value
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apply (FnV pats) arg =
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apply (FnV _ pats) arg =
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apply' pats
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where
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apply' [] = error $ "argument " ++ show arg ++ " doesn't satisfy any patterns"
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apply' ((pat, body):xs) =
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case patternBindings pat arg of
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Just env' -> -- satisfies
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Just bindings -> -- satisfies
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do
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(s,env) <- get
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put (s, M.union env' env)
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let newenv = bindings:env
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put (s, newenv)
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eval body
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Nothing -> -- doesn't satisfy this pattern
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apply' xs
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