diff --git a/src/Language/SystemVerilog/Parser.hs b/src/Language/SystemVerilog/Parser.hs
index 6f0e49e..6c0d188 100644
--- a/src/Language/SystemVerilog/Parser.hs
+++ b/src/Language/SystemVerilog/Parser.hs
@@ -5,29 +5,32 @@ module Language.SystemVerilog.Parser
     ( parseFiles
     ) where
 
+import Control.Monad.Except
 import qualified Data.Map.Strict as Map
 import Language.SystemVerilog.AST (AST)
 import Language.SystemVerilog.Parser.Lex (lexFile, Env)
 import Language.SystemVerilog.Parser.Parse (parse)
 
 -- parses a compilation unit given include search paths and predefined macros
-parseFiles :: [FilePath] -> [(String, String)] -> [FilePath] -> IO [AST]
-parseFiles includePaths defines paths = do
+parseFiles :: [FilePath] -> [(String, String)] -> Bool -> [FilePath] -> IO (Either String [AST])
+parseFiles includePaths defines siloed paths = do
     let env = Map.map (\a -> (a, [])) $ Map.fromList defines
-    parseFiles' includePaths env paths
+    runExceptT (parseFiles' includePaths env siloed paths)
 
 -- parses a compilation unit given include search paths and predefined macros
-parseFiles' :: [FilePath] -> Env -> [FilePath] -> IO [AST]
-parseFiles' _ _ [] = return []
-parseFiles' includePaths env (path : paths) = do
-    (ast, env') <- parseFile' includePaths env path
-    asts <- parseFiles' includePaths env' paths
+parseFiles' :: [FilePath] -> Env -> Bool -> [FilePath] -> ExceptT String IO [AST]
+parseFiles' _ _ _ [] = return []
+parseFiles' includePaths env siloed (path : paths) = do
+    (ast, envEnd) <- parseFile' includePaths env path
+    let envNext = if siloed then env else envEnd
+    asts <- parseFiles' includePaths envNext siloed paths
     return $ ast : asts
 
 -- parses a file given include search paths, a table of predefined macros, and
 -- the file path
-parseFile' :: [String] -> Env -> FilePath -> IO (AST, Env)
+parseFile' :: [String] -> Env -> FilePath -> ExceptT String IO (AST, Env)
 parseFile' includePaths env path = do
-    (tokens, env') <- lexFile includePaths env path
-    let ast = parse tokens
+    result <- liftIO $ lexFile includePaths env path
+    (tokens, env') <- liftEither result
+    ast <- parse tokens
     return (ast, env')
diff --git a/src/Language/SystemVerilog/Parser/Lex.x b/src/Language/SystemVerilog/Parser/Lex.x
index a781b0b..4d72be9 100644
--- a/src/Language/SystemVerilog/Parser/Lex.x
+++ b/src/Language/SystemVerilog/Parser/Lex.x
@@ -511,16 +511,16 @@ alexInitUserState :: AlexUserState
 alexInitUserState = LS [] "" Map.empty [] []
 
 -- public-facing lexer entrypoint
-lexFile :: [String] -> Env -> FilePath -> IO ([Token], Env)
+lexFile :: [String] -> Env -> FilePath -> IO (Either String ([Token], Env))
 lexFile includePaths env path = do
     str <- readFile path
     let result = runAlex str $ setEnv >> alexMonadScan >> get
     return $ case result of
-        Left msg -> error $ "Lexical Error: " ++ msg
+        Left msg -> Left msg
         Right finalState ->
             if null $ lsCondStack finalState
-                then (reverse $ lsToks finalState, lsEnv finalState)
-                else error $ "unfinished conditional directives: " ++
+                then Right (reverse $ lsToks finalState, lsEnv finalState)
+                else Left $ path ++ ": unfinished conditional directives: " ++
                         (show $ length $ lsCondStack finalState)
     where
         setEnv = do
@@ -541,7 +541,7 @@ lexicalError :: String -> Alex a
 lexicalError msg = do
     (pn, _, _, _) <- alexGetInput
     pos <- toTokPos pn
-    alexError $ msg ++ ", at " ++ show pos
+    alexError $ "Lexical error: " ++ msg ++ ", at " ++ show pos
 
 -- get the current user state
 get :: Alex AlexUserState
diff --git a/src/Language/SystemVerilog/Parser/Parse.y b/src/Language/SystemVerilog/Parser/Parse.y
index f20aba5..b1d5c85 100644
--- a/src/Language/SystemVerilog/Parser/Parse.y
+++ b/src/Language/SystemVerilog/Parser/Parse.y
@@ -13,11 +13,13 @@
 {
 module Language.SystemVerilog.Parser.Parse (parse) where
 
+import Control.Monad.Except
 import Language.SystemVerilog.AST
 import Language.SystemVerilog.Parser.ParseDecl
 import Language.SystemVerilog.Parser.Tokens
 }
 
+%monad { ExceptT String IO }
 %name parse
 %tokentype { Token }
 %error { parseError }
@@ -1194,10 +1196,10 @@ DimFn :: { DimFn }
 
 {
 
-parseError :: [Token] -> a
+parseError :: [Token] -> ExceptT String IO a
 parseError a = case a of
-  []              -> error "Parse error: no tokens left to parse."
-  Token t s p : _ -> error $ "Parse error: unexpected token '" ++ s ++ "' (" ++ show t ++ ") at " ++ show p ++ "."
+  []              -> throwError $ "Parse error: no tokens left to parse."
+  Token t s p : _ -> throwError $ "Parse error: unexpected token '" ++ s ++ "' (" ++ show t ++ ") at " ++ show p ++ "."
 
 genItemsToGenItem :: [GenItem] -> GenItem
 genItemsToGenItem [x] = x
diff --git a/src/sv2v.hs b/src/sv2v.hs
index 891b049..ec73afc 100644
--- a/src/sv2v.hs
+++ b/src/sv2v.hs
@@ -22,16 +22,15 @@ main :: IO ()
 main = do
     job <- readJob
     -- parse the input files
-    let includePaths = incdir job
     let defines = map splitDefine $ define job
-    let singleton = \x -> [x]
-    let toFileLists = if siloed job then map singleton else singleton
-    astLists <- mapM
-        (parseFiles includePaths defines)
-        (toFileLists $ files job)
-    let asts = concat astLists
-    -- convert the files
-    let asts' = convert (exclude job) asts
-    -- print the converted files out
-    hPrint stdout $ concat asts'
-    exitSuccess
+    result <- parseFiles (incdir job) defines (siloed job) (files job)
+    case result of
+        Left msg -> do
+            hPutStr stderr $ msg ++ "\n"
+            exitFailure
+        Right asts -> do
+            -- convert the files
+            let asts' = convert (exclude job) asts
+            -- print the converted files out
+            hPrint stdout $ concat asts'
+            exitSuccess