hw3: impl parser and interpreter

hw3/WhileInterp.hs

@@ -140,16 +140,38 @@ boolP = do
     "false" -> BoolVal False
     "skip" -> BoolVal False -- Treating the command 'skip' as a synonym for false, for ease of parsing
 
-numberP = error "TBD"
+numberP = do
+  n <- many1 digit
+  return $ IntVal (read n)
 
-varP = error "TBD"
+varP = do
+  x <- many1 letter
+  return $ Var x
 
-ifP = error "TBD"
+ifP = do
+  string "if"
+  e1 <- exprP
+  string "then"
+  e2 <- exprP
+  string "else"
+  e3 <- exprP
+  string "endif"
+  return $ If e1 e2 e3
 
-whileP = error "TBD"
+whileP = do
+  string "while"
+  e1 <- exprP
+  string "do"
+  e2 <- exprP
+  string "endwhile"
+  return $ While e1 e2
 
 -- An expression in parens, e.g. (9-5)*2
-parenP = error "TBD"
+parenP = do
+  char '('
+  e <- exprP
+  char ')'
+  return $ e
 
 
 -- This function will be useful for defining binary operations.
@@ -159,19 +181,52 @@ parenP = error "TBD"
 -- The first case is done for you.
 applyOp :: Binop -> Value -> Value -> Either ErrorMsg Value
 applyOp Plus (IntVal i) (IntVal j) = Right $ IntVal $ i + j
-applyOp _ _ _ = error "TBD"
+applyOp Minus (IntVal i) (IntVal j) = Right $ IntVal $ i - j
+applyOp Times (IntVal i) (IntVal j) = Right $ IntVal $ i * j
+applyOp Divide (IntVal i) (IntVal j) = Right $ IntVal $ i `div` j
+applyOp Gt (IntVal i) (IntVal j) = Right $ BoolVal $ i > j
+applyOp Ge (IntVal i) (IntVal j) = Right $ BoolVal $ i >= j
+applyOp Lt (IntVal i) (IntVal j) = Right $ BoolVal $ i < j
+applyOp Le (IntVal i) (IntVal j) = Right $ BoolVal $ i <= j
+applyOp _ v1 v2 = Left $ "Non-integer value pair: '" ++ show v1 ++ "', '" ++ show v2 ++ "' used in binary operation"
 
 
 -- As with the applyOp method, the semantics for this function
 -- should return Either values.  Left <error msg> indicates an error,
 -- whereas Right <something> indicates a successful execution.
 evaluate :: Expression -> Store -> Either ErrorMsg (Value, Store)
+evaluate (Val v) s = Right (v, s)
+evaluate (Var x) s = case Map.lookup x s of
+  Just v -> Right (v, s)
+  Nothing -> Left $ "Variable " ++ x ++ " not found"
+evaluate (Assign x e) s = do
+  (v,s') <- evaluate e s
+  return (v, Map.insert x v s')
+evaluate (Sequence e1 e2) s = do
+  (_,s1) <- evaluate e1 s
+  (v2,s') <- evaluate e2 s1
+  return (v2, s')
 evaluate (Op o e1 e2) s = do
   (v1,s1) <- evaluate e1 s
   (v2,s') <- evaluate e2 s1
   v <- applyOp o v1 v2
   return (v, s')
-evaluate _ _ = error "TBD"
+evaluate (If e1 e2 e3) s = do
+  (v1, s1) <- evaluate e1 s
+  (v', s') <- case v1 of
+    BoolVal True -> evaluate e2 s1
+    BoolVal False -> evaluate e3 s1
+    _ -> Left $ "Non-boolean value '" ++ show v1 ++ "' used as conditional"
+  return (v', s')
+evaluate (While e1 e2) s = do
+  (v1,s1) <- evaluate e1 s
+  case v1 of
+    BoolVal True -> do
+      (_,s2) <- evaluate e2 s1
+      (v', s') <- evaluate (While e1 e2) s2
+      return (v', s')
+    BoolVal False -> Right $ (BoolVal False, s)
+    _ -> Left $ "Non-boolean value '" ++ show v1 ++ "' used as while condition"
 
 
 -- Evaluates a program with an initially empty state