diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog for harg
 
+## 0.2.0.0 [2019.09.06]
+
+- Trigger a parser failure when any option in the sources fails to parse
+  NOTE: this introduces a breaking change, in that some parsers that failed silently
+        and selected the default (if applicable) will now fail.
+
 ## 0.1.3.0 [2019.08.28]
 
 - Add `manyParser` to parse list of options separated by delimiter
diff --git a/README.lhs b/README.lhs
--- a/README.lhs
+++ b/README.lhs
@@ -756,6 +756,7 @@
 
 - Better errors using `optparse-applicative`'s internals
 - Allow user to pass `optparse-applicative` preferences
+- Write tests
 - ~~Be able to provide and get back the same type for multiple subcommands~~
 - ~~Integrate config files (e.g. JSON using aeson)~~
 
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -756,6 +756,7 @@
 
 - Better errors using `optparse-applicative`'s internals
 - Allow user to pass `optparse-applicative` preferences
+- Write tests
 - ~~Be able to provide and get back the same type for multiple subcommands~~
 - ~~Integrate config files (e.g. JSON using aeson)~~
 
diff --git a/harg.cabal b/harg.cabal
--- a/harg.cabal
+++ b/harg.cabal
@@ -1,7 +1,7 @@
 cabal-version: 2.2
 
 name:           harg
-version:        0.1.3.0
+version:        0.2.0.0
 synopsis:       Haskell program configuration using higher kinded data
 description:    Please see the README on GitHub at <https://github.com/alexpeits/harg#readme>
 homepage:       https://github.com/alexpeits/harg
diff --git a/src/Options/Harg/Cmdline.hs b/src/Options/Harg/Cmdline.hs
--- a/src/Options/Harg/Cmdline.hs
+++ b/src/Options/Harg/Cmdline.hs
@@ -25,13 +25,13 @@
   -> a (Compose Opt f)      -- ^ Target configuration options
   -> Optparse.Parser (a f)
 mkOptparseParser sources opts
-  = let
-      srcOpts
-        = foldl'
-            (B.bzipWith (<|>))
-            (B.bmap (const (Compose Nothing)) opts)
-            sources
-    in B.bsequence $ B.bzipWith mkParser srcOpts opts
+  = B.bsequence $ B.bzipWith mkParser srcOpts opts
+  where
+    srcOpts
+      = foldl'
+          (B.bzipWith (<|>))
+          (B.bmap (const (Compose Nothing)) opts)
+          sources
 
 -- | Create a 'Optparse.Parser' for a single option, using the accumulated
 -- source results.
@@ -70,23 +70,23 @@
   -> f a
   -> Compose Optparse.Parser f a
 toFlagParser sources (Compose opt@Opt{..}) active
-  =
-    let
-      mDef
-        = case getCompose sources of
-            Nothing -> _optDefault
-            Just x  -> Just x
-      modifiers
-        = foldMap (fromMaybe mempty)
-            [ Optparse.long <$> _optLong
-            , Optparse.short <$> _optShort
-            , Optparse.help <$> ppHelp opt
-            ]
-      in Compose $ case mDef of
-           Nothing ->
-             Optparse.flag' active modifiers
-           Just def ->
-             Optparse.flag def active modifiers
+  = Compose
+    $ case mDef of
+        Nothing ->
+          Optparse.flag' active modifiers
+        Just def ->
+          Optparse.flag def active modifiers
+  where
+    mDef
+      = case getCompose sources of
+          Nothing -> _optDefault
+          Just x  -> Just x
+    modifiers
+      = foldMap (fromMaybe mempty)
+          [ Optparse.long <$> _optLong
+          , Optparse.short <$> _optShort
+          , Optparse.help <$> ppHelp opt
+          ]
 
 -- | Create a 'Optparse.Parser' for a 'ArgumentOpt', which results in an
 -- @optparse-applicative@ 'Optparse.argument'.
diff --git a/src/Options/Harg/Config.hs b/src/Options/Harg/Config.hs
--- a/src/Options/Harg/Config.hs
+++ b/src/Options/Harg/Config.hs
@@ -25,11 +25,11 @@
   -> c (Compose Opt f)
   -> Optparse.Parser (c f)
 mkConfigParser HargCtx{..} conf
-  = let
-      (_, envC)
-        = accumSourceResults
+  = mkOptparseParser envC conf
+  where
+    (_, envC)
+      = accumSourceResults
         $ runSource (EnvSourceVal _hcEnv) conf
-    in mkOptparseParser envC conf
 
 -- | Run two option parsers in parallel and return the result of the
 -- first one. This is used with the configuration parser being the first
@@ -41,13 +41,12 @@
   -> Optparse.Parser (c (f :: Type -> Type))
   -> Optparse.Parser (a (g :: Type -> Type))
   -> IO (c f)
-getConfig HargCtx{..} confParser optParser
-  = do
-      let
-        parser
-          = (,) <$> confParser <*> optParser
-        parserInfo
-          = Optparse.info (Optparse.helper <*> parser) mempty
-        res
-          = Optparse.execParserPure Optparse.defaultPrefs parserInfo _hcArgs
-      fst <$> Optparse.handleParseResult res
+getConfig HargCtx{..} confParser optParser = do
+  let
+    parser
+      = (,) <$> confParser <*> optParser
+    parserInfo
+      = Optparse.info (Optparse.helper <*> parser) mempty
+    res
+      = Optparse.execParserPure Optparse.defaultPrefs parserInfo _hcArgs
+  fst <$> Optparse.handleParseResult res
diff --git a/src/Options/Harg/Operations.hs b/src/Options/Harg/Operations.hs
--- a/src/Options/Harg/Operations.hs
+++ b/src/Options/Harg/Operations.hs
@@ -17,7 +17,7 @@
 import           Options.Harg.Het.HList          (AssocListF, MapAssocList(..))
 import           Options.Harg.Het.Prod           ((:*)(..))
 import           Options.Harg.Het.Variant        (VariantF)
-import           Options.Harg.Pretty             (ppWarning, ppError)
+import           Options.Harg.Pretty             (ppOptErrors)
 import           Options.Harg.Sources            ( accumSourceResults
                                                  , DefaultSources, defaultSources
                                                  , HiddenSources, hiddenSources
@@ -27,7 +27,6 @@
 import           Options.Harg.Types              (HargCtx(..), getCtx, Opt, OptError)
 import           Options.Harg.Util               (toDummyOpts, allToDummyOpts, compose)
 
-
 -- | Run the option parser and combine with values from the specified sources,
 -- passing the context explicitly.
 execOptWithCtx
@@ -43,21 +42,27 @@
   -> c Opt    -- ^ Source options
   -> a Opt    -- ^ Target configuration options
   -> IO (a Identity)
-execOptWithCtx ctx conf opts
-  = do
-      let
-        configParser = mkConfigParser ctx $ compose Identity (conf :* hiddenSources)
-        dummyParser = mkOptparseParser [] (toDummyOpts @String opts)
-      config <- getConfig ctx configParser dummyParser
-      sourceVals <- getSource ctx config
-      let
-        (errs, sources)
-          = accumSourceResults
-          $ runSource sourceVals (compose Identity opts)
-        parser
-          = mkOptparseParser sources (compose Identity opts)
-      (res, _) <- execParser ctx ((,) <$> parser <*> configParser) errs
-      pure res
+execOptWithCtx ctx conf opts = do
+  let
+    configParser
+      = mkConfigParser ctx $ compose Identity (conf :* hiddenSources)
+    dummyParser
+      = mkOptparseParser [] (toDummyOpts @String opts)
+  config <- getConfig ctx configParser dummyParser
+  sourceVals <- getSource ctx config
+  let
+    (errs, sources)
+      = accumSourceResults
+        $ runSource sourceVals (compose Identity opts)
+    optParser
+      = mkOptparseParser sources (compose Identity opts)
+    -- parser that includes the configuration options, otherwise parsing
+    -- will find more options and fail
+    allParser
+      = (,) <$> optParser <*> configParser
+  fst <$> if null errs
+          then execParser ctx allParser
+          else failParser allParser errs
 
 -- | Run the option parser and combine with values from the specified sources
 execOpt
@@ -72,10 +77,9 @@
   => c Opt  -- ^ Source options
   -> a Opt  -- ^ Target configuration options
   -> IO (a Identity)
-execOpt conf opts
-  = do
-      ctx <- getCtx
-      execOptWithCtx ctx conf opts
+execOpt conf opts = do
+  ctx <- getCtx
+  execOptWithCtx ctx conf opts
 
 -- | Run the option parser only with default sources (environment variables),
 -- passing the context explicitly.
@@ -118,25 +122,30 @@
   -> c Opt    -- ^ Source options
   -> AssocListF ts xs Opt  -- ^ Target options associated with subcommands
   -> IO (VariantF xs Identity)
-execCommandsWithCtx ctx conf opts
-  = do
-      let
-        configParser = mkConfigParser ctx $ compose Identity (conf :* hiddenSources)
-        (_, dummyCommands)
-          = mapSubcommand () (allToDummyOpts @String opts)
-        dummyParser
-          = Optparse.subparser (mconcat dummyCommands)
+execCommandsWithCtx ctx conf opts = do
+  let
+    configParser
+      = mkConfigParser ctx $ compose Identity (conf :* hiddenSources)
+    (_, dummyCommands)
+      = mapSubcommand () (allToDummyOpts @String opts)
+    dummyParser
+      = Optparse.subparser (mconcat dummyCommands)
 
-      config <- getConfig ctx configParser dummyParser
-      sourceVals <- getSource ctx config
+  config <- getConfig ctx configParser dummyParser
+  sourceVals <- getSource ctx config
 
-      let
-        (errs, commands)
-          = mapSubcommand sourceVals (mapAssocList (compose Identity) opts)
-        parser
-          = Optparse.subparser (mconcat commands)
-      (res, _) <- execParser ctx ((,) <$> parser <*> configParser) errs
-      pure res
+  let
+    (errs, commands)
+      = mapSubcommand sourceVals (mapAssocList (compose Identity) opts)
+    optParser
+      = Optparse.subparser (mconcat commands)
+    -- parser that includes the configuration options, otherwise parsing
+    -- will find more options and fail
+    allParser
+      = (,) <$> optParser <*> configParser
+  fst <$> if null errs
+          then execParser ctx allParser
+          else failParser allParser errs
 
 -- | Run the subcommand parser and combine with values from the specified
 -- sources
@@ -154,10 +163,9 @@
   => c Opt  -- ^ Source options
   -> AssocListF ts xs Opt  -- ^ Target options associated with subcommands
   -> IO (VariantF xs Identity)
-execCommands conf opts
-  = do
-      ctx <- getCtx
-      execCommandsWithCtx ctx conf opts
+execCommands conf opts = do
+  ctx <- getCtx
+  execCommandsWithCtx ctx conf opts
 
 -- | Run the subcommand parser only with default sources (environment
 -- variables), passing the context explicitly.
@@ -195,17 +203,27 @@
 execParser
   :: HargCtx
   -> Optparse.Parser a
+  -> IO a
+execParser HargCtx{..} parser
+  = Optparse.handleParseResult (execParserPure _hcArgs parser)
+
+failParser
+  :: Optparse.Parser a
   -> [OptError]
   -> IO a
-execParser HargCtx{..} parser errs
-  = do
-      let
-        res = execParserPure _hcArgs parser
-      case res of
-        Optparse.Success a
-          -> ppWarning errs >> pure a
-        _
-          -> ppError errs >> Optparse.handleParseResult res
+failParser parser errs
+  = Optparse.handleParseResult (Optparse.Failure failure)
+  where
+    failure
+      = Optparse.parserFailure
+          Optparse.defaultPrefs
+          parserInfo
+          (Optparse.ErrorMsg errStr)
+          []
+    parserInfo
+      = Optparse.info (Optparse.helper <*> parser) Optparse.forwardOptions
+    errStr
+      = ppOptErrors errs
 
 -- | Run the optparse-applicative parser and return the
 -- 'Optparse.ParserResult'
diff --git a/src/Options/Harg/Pretty.hs b/src/Options/Harg/Pretty.hs
--- a/src/Options/Harg/Pretty.hs
+++ b/src/Options/Harg/Pretty.hs
@@ -12,32 +12,11 @@
 ppHelp Opt{..}
   = (<> ppEnvVar _optEnvVar) <$> _optHelp
 
-ppWarning
-  :: [OptError]
-  -> IO ()
-ppWarning []
-  = pure ()
-ppWarning err
-  =  putStrLn "Parser succeeded with warnings:"
-  >> ppOptErrors err
-  >> putStrLn ""
-
-ppError
-  :: [OptError]
-  -> IO ()
-ppError []
-  = pure ()
-ppError err
-  =  putStrLn "Parser errors:"
-  >> ppOptErrors err
-  >> putStrLn ""
-
 ppOptErrors
   :: [OptError]
-  -> IO ()
+  -> String
 ppOptErrors
-  = putStrLn
-  . intercalate "\n"
+  = intercalate "\n\n"
   . map ppOptError
   . nubBy cmpOptErr
   where
@@ -45,10 +24,11 @@
       =  _optLong l == _optLong r && sl == sr && dl == dr
     ppOptError :: OptError -> String
     ppOptError (OptError (SomeOpt opt) src desc)
-      =  "\t"
+      =  "option "
       <> fromMaybe "<no opt name>" (_optLong opt)
-      <> "\t\t"
+      <> ": "
       <> desc
+      <> "\n\t"
       <> ppSource src
       <> ppEnvVar (_optEnvVar opt)
 
diff --git a/src/Options/Harg/Sources/JSON.hs b/src/Options/Harg/Sources/JSON.hs
--- a/src/Options/Harg/Sources/JSON.hs
+++ b/src/Options/Harg/Sources/JSON.hs
@@ -28,15 +28,14 @@
 
 instance GetSource JSONSource Identity where
   type SourceVal JSONSource = JSONSourceVal
-  getSource _ctx (JSONSource (Identity (ConfigFile path)))
-    = do
-        contents <- readFileLBS path
-        case JSON.eitherDecode contents of
-          Right json
-            -> pure $ JSONSourceVal json
-          Left err
-            -> printErrAndExit
-               $ "Error decoding " <> path <> " to JSON: " <> err
+  getSource _ctx (JSONSource (Identity (ConfigFile path))) = do
+    contents <- readFileLBS path
+    case JSON.eitherDecode contents of
+      Right json
+        -> pure $ JSONSourceVal json
+      Left err
+        -> printErrAndExit
+            $ "Error decoding " <> path <> " to JSON: " <> err
   getSource _ctx (JSONSource (Identity NoConfigFile))
     = pure JSONSourceNotRequired
 
@@ -59,16 +58,16 @@
   -> a (Compose Opt f)
   -> a (Compose SourceRunResult f)
 runJSONSource json opt
-  = let
-      res :: JSON.Result (a Maybe)
-      res
-        = JSON.fromJSON json
-      toSuccess :: Maybe x -> Compose SourceRunResult f x
-      toSuccess mx
-        = Compose $ pure <$> maybe OptNotFound OptParsed mx
-      toFailure :: Compose Opt f x -> Compose SourceRunResult f x
-      toFailure _
-        = Compose $ pure <$> OptNotFound
-    in case res of
-         JSON.Success v -> B.bmap toSuccess v
-         JSON.Error _e  -> B.bmap toFailure opt
+  = case res of
+      JSON.Success v -> B.bmap toSuccess v
+      JSON.Error _e  -> B.bmap toFailure opt
+  where
+    res :: JSON.Result (a Maybe)
+    res
+      = JSON.fromJSON json
+    toSuccess :: Maybe x -> Compose SourceRunResult f x
+    toSuccess mx
+      = Compose $ pure <$> maybe OptNotFound OptParsed mx
+    toFailure :: Compose Opt f x -> Compose SourceRunResult f x
+    toFailure _
+      = Compose $ pure <$> OptNotFound
diff --git a/src/Options/Harg/Sources/YAML.hs b/src/Options/Harg/Sources/YAML.hs
--- a/src/Options/Harg/Sources/YAML.hs
+++ b/src/Options/Harg/Sources/YAML.hs
@@ -55,20 +55,20 @@
   -> a (Compose Opt f)
   -> a (Compose SourceRunResult f)
 runYAMLSource yaml opt
-  = let
-      res :: Either YAML.ParseException (a Maybe)
-      res
-        = YAML.decodeEither' yaml
-      toSuccess :: Maybe x -> Compose SourceRunResult f x
-      toSuccess mx
-        = Compose $ pure <$> maybe OptNotFound OptParsed mx
-      toFailure
-        :: YAML.ParseException
-        -> Compose Opt f x
-        -> Compose SourceRunResult f x
-      toFailure exc (Compose o)
-        = Compose
+  = case res of
+      Right v  -> B.bmap toSuccess v
+      Left exc -> B.bmap (toFailure exc) opt
+  where
+    res :: Either YAML.ParseException (a Maybe)
+    res
+      = YAML.decodeEither' yaml
+    toSuccess :: Maybe x -> Compose SourceRunResult f x
+    toSuccess mx
+      = Compose $ pure <$> maybe OptNotFound OptParsed mx
+    toFailure
+      :: YAML.ParseException
+      -> Compose Opt f x
+      -> Compose SourceRunResult f x
+    toFailure exc (Compose o)
+      = Compose
         $ OptFoundNoParse (toOptError o (Just "YAMLSource") (displayException exc))
-    in case res of
-         Right v  -> B.bmap toSuccess v
-         Left exc -> B.bmap (toFailure exc) opt
diff --git a/src/Options/Harg/Subcommands.hs b/src/Options/Harg/Subcommands.hs
--- a/src/Options/Harg/Subcommands.hs
+++ b/src/Options/Harg/Subcommands.hs
@@ -98,21 +98,17 @@
     ) => ExplSubcommands n (t ': ts) (x ': xs) as where
 
   explMapSubcommand n srcs (ACons opt opts)
-    = let
-        (errs, sc)
-          = subcommand
-        (errs', rest)
-          = hgcastWith (proof @as @x @xs)
+    = (thisErr ++ restErr, sc : rest)
+    where
+      (thisErr, sc)
+        = subcommand
+      (restErr, rest)
+        = hgcastWith (proof @as @x @xs)
           $ explMapSubcommand
               @(S n) @ts @xs @(as ++ '[x])
               (SS n) srcs opts
-
-      in (errs ++ errs', sc : rest)
-
-    where
       subcommand
         = let
-            -- TODO: accumulate errors
             (errs, src)
               = accumSourceResults $ runSource srcs opt
             parser
@@ -121,6 +117,6 @@
               = symbolVal (Proxy :: Proxy t)
             cmd
               = Optparse.command tag
-              $ injectPosF n
-              <$> Optparse.info (Optparse.helper <*> parser) mempty
+                $ injectPosF n
+                  <$> Optparse.info (Optparse.helper <*> parser) mempty
           in (errs, cmd)
diff --git a/src/Options/Harg/Util.hs b/src/Options/Harg/Util.hs
--- a/src/Options/Harg/Util.hs
+++ b/src/Options/Harg/Util.hs
@@ -76,12 +76,11 @@
   :: (FilePath -> IO a)
   -> FilePath
   -> IO a
-readFileWith f path
-  = do
-      exists <- doesFileExist path
-      if exists
-        then readFile_
-        else printErrAndExit ("File not found: " <> path)
+readFileWith f path = do
+  exists <- doesFileExist path
+  if exists
+    then readFile_
+    else printErrAndExit ("File not found: " <> path)
   where
     readFile_
       = f path