diff --git a/rlp.cabal b/rlp.cabal
index 7b4a2c5..c5c699d 100644
--- a/rlp.cabal
+++ b/rlp.cabal
@@ -35,6 +35,7 @@ library
                     , containers
                     , microlens
                     , microlens-th
+                    , mtl
                     , template-haskell
                     -- required for happy
                     , array
diff --git a/src/Compiler/RLPC.hs b/src/Compiler/RLPC.hs
index 08d991e..2896f5a 100644
--- a/src/Compiler/RLPC.hs
+++ b/src/Compiler/RLPC.hs
@@ -1,10 +1,71 @@
 {-# LANGUAGE GeneralisedNewtypeDeriving #-}
+{-# LANGUAGE TemplateHaskell #-}
 module Compiler.RLPC
     ( RLPC(..)
+    , RLPCOptions(RLPCOptions)
+    , addFatal
+    , addWound
+    , Severity(..)
+    , SrcError(..)
+    , evalRLPCT
+    , evalRLPC
     )
     where
+----------------------------------------------------------------------------------
+import Control.Arrow            ((>>>))
+import Control.Monad.Reader
+import Control.Monad.Errorful
+import Data.Functor.Identity
+import Data.Coerce
+import Lens.Micro
+import Lens.Micro.TH
+----------------------------------------------------------------------------------
 
 -- TODO: fancy errors
-newtype RLPC a = RLPC { runRLPC :: Either String a }
-    deriving (Functor, Applicative, Monad)
+newtype RLPCT e m a = RLPC {
+        runRLPCT :: ReaderT RLPCOptions (ErrorfulT (SrcError e) m) a
+    }
+    deriving (Functor, Applicative, Monad, MonadReader RLPCOptions)
+
+type RLPC e = RLPCT e Identity
+
+evalRLPCT :: RLPCOptions
+          -> RLPCT e m a
+          -> m (Either (SrcError e) (a, [SrcError e]))
+evalRLPCT o = runRLPCT >>> flip runReaderT o >>> runErrorfulT
+
+evalRLPC :: RLPCOptions
+         -> RLPC e a
+         -> Either (SrcError e) (a, [SrcError e])
+evalRLPC o m = coerce $ evalRLPCT o m
+
+data RLPCOptions = RLPCOptions
+
+data SrcError e = SrcError
+    { _errLocation   :: (Int, Int)
+    , _errSeverity   :: Severity
+    , _errDiagnostic :: e
+    }
+
+deriving instance (Show e) => Show (SrcError e)
+
+data Severity = Error
+              | Warning
+              | Debug
+              deriving Show
+
+-- temporary until we have a new doc building system
+type ErrorDoc = String
+
+class Diagnostic e where
+    errorDoc :: e -> ErrorDoc
+
+-- makeLenses ''RLPCOptions
+makeLenses ''SrcError
+
+pure []
+
+instance MonadErrorful (SrcError e) (RLPC e) where
+    addWound = RLPC . lift . addWound
+    addFatal = RLPC . lift . addFatal
 
diff --git a/src/Control/Monad/Errorful.hs b/src/Control/Monad/Errorful.hs
new file mode 100644
index 0000000..8e309b6
--- /dev/null
+++ b/src/Control/Monad/Errorful.hs
@@ -0,0 +1,62 @@
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE TupleSections, PatternSynonyms #-}
+module Control.Monad.Errorful
+    ( ErrorfulT
+    , runErrorfulT
+    , Errorful
+    , runErrorful
+    , MonadErrorful(..)
+    )
+    where
+----------------------------------------------------------------------------------
+import Control.Monad.Trans
+import Control.Monad.Except
+import Data.Functor.Identity
+import Data.Coerce
+import Lens.Micro
+----------------------------------------------------------------------------------
+
+newtype ErrorfulT e m a = ErrorfulT { runErrorfulT :: m (Either e (a, [e])) }
+
+type Errorful e = ErrorfulT e Identity
+
+pattern Errorful :: (Either e (a, [e])) -> Errorful e a
+pattern Errorful a = ErrorfulT (Identity a)
+
+runErrorful :: Errorful e a -> Either e (a, [e])
+runErrorful m = coerce (runErrorfulT m)
+
+class (Applicative m) => MonadErrorful e m | m -> e where
+    addWound   :: e -> m ()
+    addFatal   :: e -> m a
+
+    -- not sure if i want to add this yet...
+    -- catchWound :: m a -> (e -> m a) -> m a
+
+instance (Applicative m) => MonadErrorful e (ErrorfulT e m) where
+    addWound e = ErrorfulT $ pure . Right $ ((), [e])
+    addFatal e = ErrorfulT $ pure . Left  $ e
+
+instance MonadTrans (ErrorfulT e) where
+    lift m = ErrorfulT (Right . (,[]) <$> m)
+
+instance (Functor m) => Functor (ErrorfulT e m) where
+    fmap f (ErrorfulT m) = ErrorfulT $ fmap (_1 %~ f) <$> m
+
+instance (Applicative m) => Applicative (ErrorfulT e m) where
+    pure a = ErrorfulT (pure . Right $ (a, []))
+
+    m <*> a = ErrorfulT (m' `apply` a')
+        where
+            m' = runErrorfulT m
+            a' = runErrorfulT a
+            -- TODO: strict concatenation
+            apply = liftA2 $ liftA2 (\ (f,e1) (x,e2) -> (f x, e1 ++ e2)) 
+
+instance (Monad m) => Monad (ErrorfulT e m) where
+    ErrorfulT m >>= k = ErrorfulT $ do
+        m' <- m
+        case m' of
+            Right (a,es) -> runErrorfulT (k a)
+            Left e       -> pure (Left e)
+
diff --git a/src/Core/Lex.x b/src/Core/Lex.x
index 042e73f..0b4f683 100644
--- a/src/Core/Lex.x
+++ b/src/Core/Lex.x
@@ -1,16 +1,19 @@
 {
+-- TODO: layout semicolons are not inserted at EOf.
 {-# LANGUAGE TemplateHaskell #-}
 module Core.Lex
     ( lexCore
     , lexCore'
     , CoreToken(..)
-    , lexTmp
-    , ParserError
+    , ParseError(..)
+    , Located(..)
+    , AlexPosn(..)
     )
     where
 import Data.Char (chr)
 import Debug.Trace
 import Core.Syntax
+import Compiler.RLPC
 import Lens.Micro
 import Lens.Micro.TH
 }
@@ -184,9 +187,6 @@ getContext = do
 
 type Lexer = AlexInput -> Int -> Alex (Located CoreToken)
 
-alexEOF :: Alex (Located CoreToken)
-alexEOF = Alex $ \ st@(AlexState { alex_pos = p }) -> Right (st, Located p TokenEOF)
-
 alexInitUserState :: AlexUserState
 alexInitUserState = AlexUserState [Layout 1]
 
@@ -200,13 +200,29 @@ lexStream = do
         Located _ TokenEOF  -> pure [l]
         _                   -> (l:) <$> lexStream
 
-lexCore :: String -> Either String [Located CoreToken]
-lexCore s = runAlex s (alexSetStartCode 0 *> lexStream)
+-- | The main lexer driver.
+lexCore :: String -> RLPC ParseError [Located CoreToken]
+lexCore s = case m of
+    Left e   -> addFatal err
+        where err = SrcError
+                { _errLocation = undefined -- TODO: location
+                , _errSeverity = Error
+                , _errDiagnostic = ParErrLexical e
+                }
+    Right ts -> pure ts
+    where
+        m = runAlex s (alexSetStartCode 0 *> lexStream)
 
-lexCore' :: String -> Either String [CoreToken]
+-- | @lexCore@, but the tokens are stripped of location info. Useful for
+-- debugging
+lexCore' :: String -> RLPC ParseError [CoreToken]
 lexCore' s = fmap f <$> lexCore s
     where f (Located _ t) = t
 
+data ParseError = ParErrLexical String
+                | ParErrParse
+                deriving Show
+
 lexWith :: (String -> CoreToken) -> Lexer
 lexWith f (p,_,_,s) l = pure $ Located p (f $ take l s)
 
@@ -266,7 +282,6 @@ getOffside = do
 doBol :: Lexer
 doBol (p,c,_,s) l = do
     off <- getOffside
-    col <- getSrcCol
     case off of
         LT      -> insRBraceV p
         EQ      -> insSemi p
@@ -277,13 +292,7 @@ letin (p,_,_,_) l = do
     popContext
     pure $ Located p TokenIn
 
-lexTmp :: IO [CoreToken]
-lexTmp = do
-    s <- readFile "/tmp/t.hs"
-    case lexCore' s of
-        Left e -> error e
-        Right a -> pure a
-
-data ParserError
+alexEOF :: Alex (Located CoreToken)
+alexEOF = Alex $ \ st@(AlexState { alex_pos = p }) -> Right (st, Located p TokenEOF)
 
 }
diff --git a/src/Core/Parse.y b/src/Core/Parse.y
index 342acf4..05e8d7e 100644
--- a/src/Core/Parse.y
+++ b/src/Core/Parse.y
@@ -1,4 +1,3 @@
--- TODO: resolve shift/reduce conflicts
 {
 module Core.Parse
     ( parseCore
@@ -8,6 +7,7 @@ module Core.Parse
     )
     where
 
+import Control.Monad        ((>=>))
 import Data.Foldable        (foldl')
 import Core.Syntax
 import Core.Lex
@@ -16,31 +16,33 @@ import Compiler.RLPC
 
 %name parseCore Module
 %name parseCoreExpr Expr
-%tokentype { CoreToken }
+%tokentype { Located CoreToken }
 %error { parseError }
-%monad { RLPC }
+%monad { RLPC ParseError }
 
 %token
-      let             { TokenLet }
-      letrec          { TokenLetrec }
-      module          { TokenModule }
-      where           { TokenWhere }
-      ','             { TokenComma }
-      in              { TokenIn }
-      litint          { TokenLitInt $$ }
-      varname         { TokenVarName $$ }
-      varsym          { TokenVarSym $$ }
-      conname         { TokenConName $$ }
-      consym          { TokenConSym $$ }
-      'λ'             { TokenLambda }
-      '->'            { TokenArrow }
-      '='             { TokenEquals }
-      '('             { TokenLParen }
-      ')'             { TokenRParen }
-      '{'             { TokenLBrace }
-      '}'             { TokenRBrace }
-      ';'             { TokenSemicolon }
-      eof             { TokenEOF }
+      let             { Located _ TokenLet }
+      letrec          { Located _ TokenLetrec }
+      module          { Located _ TokenModule }
+      where           { Located _ TokenWhere }
+      ','             { Located _ TokenComma }
+      in              { Located _ TokenIn }
+      litint          { Located _ (TokenLitInt $$) }
+      varname         { Located _ (TokenVarName $$) }
+      varsym          { Located _ (TokenVarSym $$) }
+      conname         { Located _ (TokenConName $$) }
+      consym          { Located _ (TokenConSym $$) }
+      'λ'             { Located _ TokenLambda }
+      '->'            { Located _ TokenArrow }
+      '='             { Located _ TokenEquals }
+      '('             { Located _ TokenLParen }
+      ')'             { Located _ TokenRParen }
+      '{'             { Located _ TokenLBrace }
+      '}'             { Located _ TokenRBrace }
+      vl              { Located _ TokenLBraceV }
+      vr              { Located _ TokenRBraceV }
+      ';'             { Located _ TokenSemicolon }
+      eof             { Located _ TokenEOF }
 
 %%
 
@@ -53,7 +55,14 @@ Eof             : eof           { () }
                 | error         { () }
 
 Program         :: { Program }
-Program         : '{' ScDefs Close              { Program $2 }
+Program         : VOpen ScDefs VClose              { Program $2 }
+
+VOpen           :: { () }
+VOpen           : vl                            { () }
+
+VClose          :: { () }
+VClose          : vr                            { () }
+                | error                         { () }
 
 ScDefs          :: { [ScDef] }
 ScDefs          : ScDef ';' ScDefs              { $1 : $3 }
@@ -67,15 +76,16 @@ ParList         : Var ParList                   { $1 : $2 }
                 | {- epsilon -}                 { [] }
 
 Expr            :: { Expr }
-Expr            : let '{' Bindings Close in Expr  { Let NonRec $3 $6 }
-                | letrec '{' Bindings Close in Expr { Let Rec $3 $6 }
+Expr            : LetExpr                       { $1 }
                 | 'λ' Binders '->' Expr         { Lam $2 $4 }
                 | Application                   { $1 }
                 | Expr1                         { $1 }
 
-Close           :: { () }
-Close           : '}'                           { () }
-                | error                         { () }
+LetExpr         :: { Expr }
+LetExpr         : let    VOpen Bindings VClose in Expr { Let NonRec $3 $6 }
+                | letrec VOpen Bindings VClose in Expr { Let Rec $3 $6 }
+                | let    '{'   Bindings '}'    in Expr { Let NonRec $3 $6 }
+                | letrec '{'   Bindings '}'    in Expr { Let Rec $3 $6 }
 
 Binders         :: { [Name] }
 Binders         : Var Binders                  { $1 : $2 }
@@ -115,14 +125,21 @@ Con             : '(' consym ')'                    { $2 }
                 | conname                           { $1 }
 
 {
-parseError :: [CoreToken] -> a
-parseError ts = error $ "parse error at token: " <> show (head ts)
+parseError :: [Located CoreToken] -> RLPC ParseError a
+parseError (Located (AlexPn _ x y) _ : _) = addFatal err
+    where err = SrcError
+            { _errLocation = (x, y)
+            , _errSeverity = Error
+            , _errDiagnostic = ParErrParse
+            }
 
-parseTmp :: IO (Module)
+parseTmp :: IO Module
 parseTmp = do
     s <- readFile "/tmp/t.hs"
-    case lexCore' s >>= runRLPC . parseCore of
-        Left e -> error e
-        Right a -> pure a
+    case parse s of
+        Left e -> error (show e)
+        Right (ts,_) -> pure ts
+    where
+        parse = evalRLPC RLPCOptions . (lexCore >=> parseCore)
 }