diff --git a/src/Mima/Parse/Common.hs b/src/Mima/Parse/Common.hs
index a782bff..f6b0aa1 100644
--- a/src/Mima/Parse/Common.hs
+++ b/src/Mima/Parse/Common.hs
@@ -14,6 +14,14 @@ module Mima.Parse.Common
   , asLargeValue
   , asSmallValue
   , fixedWidthHexAddress
+  -- * Nice error messages
+  , defaultPosState
+  , WeedError
+  , WeedErrorBundle
+  -- ** Remembering an element's offset
+  , WithOffset
+  , errorAt
+  , errorAt'
   ) where
 
 import           Data.Char
@@ -119,3 +127,26 @@ fixedWidthHexAddress :: Parser MimaAddress
 fixedWidthHexAddress = label "fixed-width hexadecimal address"
                      $ asLargeValue
                      $ fixedWidthHex 5
+
+{- Nice error messages -}
+
+defaultPosState :: FilePath -> T.Text -> PosState T.Text
+defaultPosState filename input = PosState
+  { pstateInput      = input
+  , pstateOffset     = 0
+  , pstateSourcePos  = initialPos filename
+  , pstateTabWidth   = defaultTabWidth
+  , pstateLinePrefix = ""
+  }
+
+type WeedError = ParseError T.Text Void
+type WeedErrorBundle = ParseErrorBundle T.Text Void
+
+data WithOffset a = WithOffset Int a
+  deriving (Show)
+
+errorAt :: WithOffset a -> String -> WeedError
+errorAt wo errorMsg = errorAt' wo [errorMsg]
+
+errorAt' :: WithOffset a -> [String] -> WeedError
+errorAt' (WithOffset o _) = FancyError o . Set.fromList . map ErrorFail
diff --git a/src/Mima/Parse/Weed.hs b/src/Mima/Parse/Weed.hs
new file mode 100644
index 0000000..8e8996f
--- /dev/null
+++ b/src/Mima/Parse/Weed.hs
@@ -0,0 +1,47 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module Mima.Parse.Weed
+  ( Weed
+  , runWeed
+  , critical
+  , harmless
+  ) where
+
+import qualified Data.List.NonEmpty as NE
+import           Data.Monoid
+
+data Weed e a = Weed (Endo [e]) (Either e a)
+
+instance Functor (Weed e) where
+  fmap f (Weed e a) = Weed e $ fmap f a
+
+instance Applicative (Weed e) where
+  pure = Weed mempty . pure
+  (Weed es1 (Left e1)) <*> (Weed es2 (Left e2)) = Weed (es1 <> Endo (e1:) <> es2) (Left e2)
+  (Weed es1 f)         <*> (Weed es2 a)         = Weed (es1 <> es2) (f <*> a)
+
+instance Monad (Weed e) where
+  (Weed es1 v) >>= f =
+    case f <$> v of
+      Left e -> Weed es1 (Left e)
+      Right (Weed es2 a) -> Weed (es1 <> es2) a
+
+runWeed :: Weed e a -> Either (NE.NonEmpty e) a
+-- Since the Endos never remove an element and we add an extra
+-- element, this list is never empty.
+--
+-- I've tried to figure out nicer types for this, but if I want to
+-- keep the Endo trick, the tradeoff isn't worth it. The problem here
+-- is that I can't easily check if 'es' is 'mempty' with these
+-- endofunctors.
+runWeed (Weed es (Left e))  = Left $ NE.fromList $ appEndo es [e]
+runWeed (Weed es (Right a)) =
+  case appEndo es [] of
+    (x:xs) -> Left $ x NE.:| xs
+    []     -> Right a
+
+critical :: e -> Weed e a
+critical e = Weed mempty (Left e)
+
+harmless :: e -> Weed e ()
+harmless e = Weed (Endo (e:)) (Right ())