diff --git a/rewrite-inspector.cabal b/rewrite-inspector.cabal
--- a/rewrite-inspector.cabal
+++ b/rewrite-inspector.cabal
@@ -1,5 +1,5 @@
 name: rewrite-inspector
-version: 0.1.0.8
+version: 0.1.0.9
 cabal-version: >=1.10
 build-type: Simple
 license: BSD3
@@ -42,8 +42,8 @@
                       microlens-th,
                       data-default  >= 0.7.1.1
 
-
-    default-language: Haskell2010
+    ghc-options:        -Wall
+    default-language:   Haskell2010
     default-extensions: ScopedTypeVariables
                         ViewPatterns
                         LambdaCase
diff --git a/src/BrickUI.hs b/src/BrickUI.hs
--- a/src/BrickUI.hs
+++ b/src/BrickUI.hs
@@ -5,22 +5,30 @@
 
   Basic functionality for the terminal user-inteface (TUI).
 -}
-{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE OverloadedStrings, FlexibleInstances #-}
 
+{-# OPTIONS_GHC -fno-warn-orphans       #-}
+
 module BrickUI where
 
-import System.Environment  (getArgs)
-import Control.Applicative ((<|>))
-import Control.Monad       (void, (>>))
+import Prelude hiding (fail)
+
+import System.Environment     (getArgs)
+import Control.Applicative    ((<|>))
+import Control.Monad          (void, (>>))
+import Control.Monad.Fail     (MonadFail (..))
+import Control.Monad.IO.Class (liftIO)
+
+import Data.Either         (fromRight)
 import Data.List           (sortOn)
-import Data.Maybe          (isJust, listToMaybe, catMaybes)
+import Data.Maybe          (listToMaybe, catMaybes)
 import Lens.Micro
 
 import Brick
   ( App (..), BrickEvent (..), EventM, Next, Widget (..)
   , CursorLocation (..), cursorLocationName, cursorsL
   , VisibilityRequest (..), visibilityRequestsL
-  , hSize, vSize, imageL
+  , hSize, vSize
   , continue, halt
   , str, vBox, hBox
   )
@@ -38,19 +46,19 @@
 -- | Entry point for the TUI.
 runTerminal :: forall term. Diff term => FilePath -> IO ()
 runTerminal ftheme = do
-  res <- loadCustomizations ftheme (defaultTheme $ userStyles @term)
-  case res of
-    Left err ->
-      error $ "[theme.ini] Configuration file is malformed: " ++ err
-    Right theme -> do
-      [fname] <- getArgs
-      hist    <- readHistory @term fname
+  let dtheme = defaultTheme (userStyles @term)
+  theme <- fromRight dtheme <$> loadCustomizations ftheme dtheme
+  args <- getArgs
+  case args of
+    [fname] -> do
+      hist <- readHistory @term fname
       void $ B.customMain
         -- Vty configuration
         (V.mkVty $ V.defaultConfig { V.vtime = Just 100, V.vmin  = Just 1 })
         Nothing                             -- event channel
         (app @term (themeToAttrMap theme))  -- the Brick application
         (createVizStates @term hist)        -- initial state
+    _ -> error "Usage: clash-term <history_file>"
 
 -- | The 'Brick.App' configuration.
 app :: Diff term => B.AttrMap -> App (VizStates term) NoCustomEvent Name
@@ -111,11 +119,11 @@
     diff
       | v@(VizState (st:_) _ curE _ curO _ _) <- getCurrentState vs
       = let
-          showE vn = showCode (vs^.scroll)
-                              (min 80 $ getCodeWidth vs)
-                              (vs^.formData^.opts)
-                              (st^.ctx)
-                              (getSearchString vs)
+          showE = showCode (vs^.scroll)
+                           (min 80 $ getCodeWidth vs)
+                           (vs^.formData^.opts)
+                           (st^.ctx)
+                           (getSearchString vs)
           nextE = step v ^. curExpr
           (visL, visR) | v^.curOccur < v^.leftN
                        = (visibleCursors curO, invisibleCursors)
@@ -125,10 +133,10 @@
           hBoxSpaced 2
             [ B.viewport LeftViewport B.Both $
                 visL $
-                  withBorder "Before" $ showE LeftViewport curE
+                  withBorder "Before" $ showE curE
             , B.viewport RightViewport B.Both $
                 visR $
-                  withBorder "After" $ showE RightViewport nextE
+                  withBorder "After" $ showE nextE
             ]
 
       | otherwise
@@ -145,7 +153,7 @@
     searchMatches = C.vCenter $ str (n ++ " out of " ++ tot ++ " matches")
       where
         (n, tot)
-          | v@(VizState (_:_) _ curE _ _ _ _) <- getCurrentState vs
+          | v@(VizState (_:_) _ _ _ _ _ _) <- getCurrentState vs
           , let lr = v^.leftN + v^.rightN
           , lr > 0
           = (show (v^.curOccur + 1), show lr)
@@ -180,12 +188,16 @@
 
 -- * Event handling.
 
+-- | Allow pattern matches in EventM monadic do blocks.
+instance MonadFail (EventM Name) where
+  fail = liftIO . fail
+
 -- | Lookup terminal size and store in the current state.
 lookupSize :: EventM Name (VizStates term -> VizStates term)
 lookupSize = do
   out    <- V.outputIface <$> B.getVtyHandle
-  bounds <- V.displayBounds out
-  return $ (width .~ fst bounds) . (height .~ snd bounds)
+  (w, h) <- V.displayBounds out
+  return $ (width .~ w) . (height .~ h)
 
 -- | Update number of occurrences of searched string in both viewports.
 updateOcc :: Diff term => VizStates term -> VizStates term
@@ -346,7 +358,7 @@
 visibleCursors n p = Widget (hSize p) (vSize p) $ do
   res <- B.render p
   let crs  = map fst
-           $ sortOn ((\case (SearchResult i) -> i) . snd)
+           $ sortOn ((\case {SearchResult i -> i; _ -> 0}) . snd)
            $ catMaybes
            $ map (\c ->  case cursorLocationName c of
               Just s@(SearchResult _) -> Just (c, s)
diff --git a/src/Pretty.hs b/src/Pretty.hs
--- a/src/Pretty.hs
+++ b/src/Pretty.hs
@@ -12,8 +12,7 @@
 
 module Pretty where
 
-import Control.Arrow (first, second)
-
+import Control.Arrow             (first)
 import Data.List                 (nub, isPrefixOf, findIndices, sortOn)
 import Data.Text                 (unpack)
 import Data.Text.Prettyprint.Doc ( layoutPretty, layoutCompact
@@ -42,11 +41,11 @@
   -> String        -- ^ the string to search for
   -> term          -- ^ code to search in
   -> Int           -- ^ total number of occurrences
-countOcc opts searchString
+countOcc pOpts searchString
   = foldl (\cur d -> cur + countDoc d) 0
   . myForm @term []
   . layoutCompact
-  . ppr' @term opts
+  . ppr' @term pOpts
   where
     countDoc :: MyDoc -> Int
     countDoc = \case
@@ -56,7 +55,7 @@
 
 -- | Render the given expression as a 'Brick.Widget'.
 showCode
-  :: forall n term
+  :: forall term
    . Diff term
   => Bool          -- ^ whether to scroll to focused region
   -> Int           -- ^ maximum line width
@@ -65,12 +64,12 @@
   -> String        -- ^ the string to search for
   -> term          -- ^ code to display
   -> Widget Name   -- ^ the Brick widget to display
-showCode scroll w opts ctx0 searchString
+showCode toScroll w pOpts ctx0 searchString
   = vBox
-  . fmap (render scroll searchString 0) . split . (MLine 0 :)
+  . fmap (render toScroll searchString 0) . split . (MLine 0 :)
   . myForm @term ctx0
   . layoutPretty (LayoutOptions (AvailablePerLine w 0.8))
-  . ppr' @term opts
+  . ppr' @term pOpts
 
 -- | A simpler document type, specific to our use-case.
 data MyDoc = MString String
@@ -113,7 +112,7 @@
 
 -- | Render a single line in Brick (highlighting when marked).
 render :: Bool -> String -> Int -> (Int, [MyDoc]) -> Widget Name
-render scroll searchString n0 (i, xs) = B.padLeft (B.Pad i)
+render toScroll searchString n0 (i, xs) = B.padLeft (B.Pad i)
                                    $ hBox
                                    $ render1 n0 xs
   where
@@ -124,7 +123,7 @@
 
     f :: Int -> MyDoc -> (Widget Name, Int)
     f n = \case
-      MMod attrs w -> first (modify scroll (nub attrs)) (f n w)
+      MMod attrs w -> first (modify toScroll (nub attrs)) (f n w)
       MString s    -> highlightSearch n s searchString
       _            -> error "render1"
 
@@ -152,9 +151,9 @@
 
 -- | Add a list of stylistic modifications to a 'Widget'.
 modify :: Bool -> [String] -> Widget n -> Widget n
-modify scroll = foldr (.) id . fmap mod1 . sortOn (\case "Type" -> 1; _ -> 0)
+modify toScroll = foldr (.) id . fmap mod1 . sortOn (\case "Type" -> 1; _ -> 0)
   where
-    mod1 "focus" = (if scroll then B.visible else id) . B.withDefAttr "focus"
+    mod1 "focus" = (if toScroll then B.visible else id) . B.withDefAttr "focus"
     mod1 attr    = B.withDefAttr (B.attrName attr)
 
 -- | Highlight searched occurrences inside the given 'Widget'.