From e4867d1f76874882921ec9501d20834a4d96eb5a Mon Sep 17 00:00:00 2001
From: Michael Andreen <harv@ruin.nu>
Date: Wed, 15 Mar 2006 10:18:39 +0000
Subject: [PATCH] ErrorT instead..

---
 Interpret.hs | 33 +++++++++++++++------------------
 examples/fac | 13 ++++++++-----
 examples/fib | 15 ++++-----------
 3 files changed, 27 insertions(+), 34 deletions(-)

diff --git a/Interpret.hs b/Interpret.hs
index 5afcefb..948903e 100644
--- a/Interpret.hs
+++ b/Interpret.hs
@@ -7,7 +7,7 @@ import Control.Concurrent.MVar
 import Data.Map as Map
 import Prelude hiding (lookup)
 
-emptyState = State{variables=[empty], functions=(empty)}
+emptyState = State{variables=[empty], functions=(empty), ret=(VInt 0)}
 
 data Value = VInt Integer | VBool Bool deriving Eq
 
@@ -19,16 +19,17 @@ instance Show Value where
 type Variables = [Map Ident Value]
 type Function = ([Decl],[Stm])
 
-data State = State {variables::Variables,functions::(Map Ident Function),ret::(MVar Value)}
+data State = State {variables::Variables,functions::(Map Ident Function),ret::Value}
+
+type EvalM = ErrorT String (StateT State IO)
 
 interpret :: [Func] -> [Stm] -> IO ()
 interpret fun st = do
-	mv <- newEmptyMVar
-	runStateT (do mapM Interpret.addFunction fun; mapM execute st) emptyState{ret=mv}
+	runStateT (runErrorT  (do mapM Interpret.addFunction fun; mapM_ execute st)) emptyState
 	return ()
 
 --eval :: (MonadState Variables m) => Exp -> m Value
-eval :: Exp -> StateT State IO Value
+eval :: Exp -> EvalM Value
 eval (EBool b) = return (VBool b)
 eval (EInt n) = return (VInt n)
 eval (EVar i) = getVariableValue i
@@ -49,10 +50,10 @@ eval (ENot e) = do
 	(VBool b) <- eval e
 	return $ VBool $ not b
 eval EReadI = do
-	s <- lift $ getWord
+	s <- liftIO $ getWord
 	return $ VInt $ read s
 eval EReadB = do
-	s <- lift $ getWord
+	s <- liftIO $ getWord
 	return $ VBool $ if (read s == 0) then False else True
 eval (EFunc i as) = do
 	vs <- mapM eval as
@@ -61,9 +62,9 @@ eval (EFunc i as) = do
 	let m = foldr (\((Decl t i),v) m -> insert i v m) empty $ zip ds vs
 		in modify (\s -> s{variables=[m]})
 	mapM_ execute ss `catchError` (\_ -> return ())
-	v <- lift $ takeMVar $ ret state
+	s <- get
 	put state
-	return v
+	return $ ret s
 
 getWord :: IO String
 getWord =  do
@@ -95,10 +96,7 @@ findVariable :: (MonadState State m) => Ident -> Variables -> m Value
 findVariable i [] = fail $ "Variable "++show i++" not found in any scope."
 findVariable i (m:ms) = if member i m then lookup i m else findVariable i ms
 
---setVariableValue :: (MonadState Variables m) => Ident -> Exp -> m Value 
---setVariableValue :: (MonadState Variables m) => Ident -> Exp -> m Value 
-
-setVariableValue :: Ident -> Exp -> StateT State IO Value
+setVariableValue :: Ident -> Exp -> EvalM Value
 setVariableValue i e = do
 	v <- eval e
 	state <- get
@@ -116,7 +114,7 @@ pushAndPop s = do
 	modify (\s -> s { variables = tail $ variables s})
 
 -- execute :: (MonadState Variables m) => Stm -> m ()
-execute :: Stm -> StateT State IO ()
+execute :: Stm -> EvalM ()
 execute (SNoop) = return ()
 execute (SExp e) = eval e >> return ()
 execute (SIf b s s') = do
@@ -124,7 +122,7 @@ execute (SIf b s s') = do
 	pushAndPop $ if b' then execute s else execute s'
 execute (SPrint e) = do
 	e' <- eval e
-	lift $ print e'
+	liftIO $ print e'
 execute (SBlock ss) = pushAndPop $ mapM execute ss
 execute (SWhile e s) = do
 	(VBool b) <- eval e
@@ -138,9 +136,8 @@ execute (SDecl t i e) =do
 	let (m:ms) = variables state in modify (\s -> s{variables=insert i v m:ms })
 execute (SReturn e) = do
 	v <- eval e
-	s <- get
-	lift $ putMVar (ret s) v
-	throwError $ userError "Returning.."
+	modify (\s -> s{ret=v})
+	throwError "Returning.."
 
 addFunction :: (MonadState State m) => Func -> m ()
 addFunction (Func _ i d ss) = modify (\s -> s{functions=insert i (d,ss) (functions s) })
diff --git a/examples/fac b/examples/fac
index 946e847..b29a3d1 100644
--- a/examples/fac
+++ b/examples/fac
@@ -1,5 +1,8 @@
-int fac = 1;
-int n = readInt;
-n++;
-while( n-- > 1) fac = fac*n;
-print fac;
+print fac(readInt);
+
+int fac(int n){
+	if (n <= 0) return 1;
+
+	return n*fac(n-1);
+}
+
diff --git a/examples/fib b/examples/fib
index 3bad4e9..12f5dc9 100644
--- a/examples/fib
+++ b/examples/fib
@@ -1,13 +1,6 @@
-/*
-tests while, decr and assignment.
-*/
-int n1 = 0;
-int n2 = 1;
-int n = readInt;
+print fib(readInt);
 
-while(n-- > 0){
-  int temp = n1+n2;
-  n1 = n2;
-  n2 = temp;
+int fib(int n){
+	if (n <= 1) return 1;
+	return fib(n-1)+fib(n-2);
 }
-print n2;
-- 
2.39.2