diff --git a/LICENSE b/LICENSE
--- a/LICENSE
+++ b/LICENSE
@@ -1,4 +1,4 @@
-Copyright (c) 2010-2016 Brett Lajzer
+Copyright (c) 2010-2018 Brett Lajzer
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/Main.hs b/Main.hs
--- a/Main.hs
+++ b/Main.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | This is the command-line executable "dib". Since Dib proper is a
@@ -167,16 +167,16 @@
       exitCode <- system buildString
       D.setCurrentDirectory ".."
       processExitCode exitCode
-      
+
       calTimeStr <- getDibCalendarTimeStr
       tsFile <- openFile timestampFile WriteMode
       hPutStr tsFile calTimeStr
       hClose tsFile
-      
+
       let versionStr = showVersion version
       verFile <- openFile versionFile WriteMode
       hPutStr verFile versionStr
-      hClose verFile 
+      hClose verFile
 
 requoteArg :: String -> String
 requoteArg arg = requoteArgInternal arg False
@@ -191,7 +191,7 @@
   D.createDirectoryIfMissing False ".dib"
   needToRebuild <- checkDibTimestamps
   rebuild needToRebuild
-  let quotes = if os =="mingw32" then "\"\"" else "" 
+  let quotes = if os =="mingw32" then "\"\"" else ""
   system $ quotes ++ correctExe ++ quotes ++ " +RTS -N -RTS " ++ args
 
 shouldHandleInit :: [String] -> Bool
diff --git a/dib.cabal b/dib.cabal
--- a/dib.cabal
+++ b/dib.cabal
@@ -1,5 +1,5 @@
 name:           dib
-version:        0.6.1
+version:        0.7.0
 cabal-version:  >= 1.6
 category:		Development
 build-type:     Simple
@@ -17,7 +17,7 @@
 source-repository this
   type: git
   location: https://github.com/blajzer/dib.git
-  tag: 0.6.1
+  tag: 0.7.0
 
 library
   build-depends:   base >= 4.4 && < 4.10, text, containers, mtl, directory, time, process, filepath, cereal, bytestring, digest
@@ -31,4 +31,3 @@
   ghc-options:     -Wall
   hs-source-dirs:  .
   main-is:         Main.hs
-
diff --git a/src/Dib.hs b/src/Dib.hs
--- a/src/Dib.hs
+++ b/src/Dib.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | = Introduction
@@ -43,14 +43,14 @@
 --
 -- An example of using the C Builder to build an executable called "myProject" with
 -- its source code in the "src/" directory is as follows:
--- 
+--
 -- @
 -- module Main where
--- 
+--
 -- import Dib
 -- import Dib.Builders.C
 -- import qualified Data.Text as T
--- 
+--
 -- projectInfo = defaultGCCConfig {
 --   outputName = "myProject",
 --   targetName = "myProject",
@@ -60,17 +60,17 @@
 --   outputLocation = ObjAndBinDirs "obj" ".",
 --   includeDirs = ["src"]
 -- }
--- 
+--
 -- project = makeCTarget projectInfo
 -- clean = makeCleanTarget projectInfo
--- 
+--
 -- targets = [project, clean]
--- 
+--
 -- main = dib targets
 -- @
--- 
--- This was generated with @dib --init c myProject gcc src@. 
 --
+-- This was generated with @dib --init c myProject gcc src@.
+--
 -- A build script is expected to declare the available 'Target's and then pass them
 -- to the 'dib' function. Only the top-level 'Target's need to be passed to 'dib';
 -- it will scrape out the dependencies from there. The first 'Target' in the list
@@ -92,7 +92,8 @@
   dib,
   getArgDict,
   addEnvToDict,
-  makeArgDictLookupFunc
+  makeArgDictLookupFunc,
+  makeArgDictLookupFuncChecked
   ) where
 
 import Dib.Gatherers
@@ -132,29 +133,41 @@
   hSetBuffering stderr LineBuffering
   args <- Env.getArgs
   numProcs <- GHC.getNumProcessors
+
+  -- Validate that we have at least one target
+  if targets == [] then putStrLn $ "ERROR: Invalid configuration, no targets defined." else do
+
   let allTargets = gatherAllTargets targets
   let buildArgs = parseArgs args allTargets numProcs
   let selectedTarget = buildTarget buildArgs
   let theTarget = L.find (\(Target name _ _ _ _) -> name == selectedTarget) allTargets
-  if isNothing theTarget
-    then putStrLn $ "ERROR: Invalid target specified: \"" ++ T.unpack selectedTarget ++ "\"" else
-    do
-      dbLoadStart <- getCurrentTime
-      (tdb, cdb, tcdb) <- loadDatabase
-      dbLoadEnd <- getCurrentTime
 
-      startTime <- getCurrentTime
-      (_, s) <- runBuild (runTarget (fromJust theTarget)) (BuildState buildArgs selectedTarget tdb cdb tcdb Set.empty Map.empty)
-      endTime <- getCurrentTime
+  -- Validate targets
+  let targetErrors = validateTargets allTargets
+  if isJust targetErrors then putStrLn $ "ERROR: Invalid targets:\n" ++ fromJust targetErrors else do
 
-      dbSaveStart <- getCurrentTime
-      saveDatabase (getTargetTimestampDB s) (getChecksumDB s) (getTargetChecksumDB s)
-      dbSaveEnd <- getCurrentTime
+  -- Validate that we're trying to build something that exists
+  if isNothing theTarget then putStrLn $ "ERROR: Invalid target specified: \"" ++ T.unpack selectedTarget ++ "\"" else do
 
-      putStrLn $ "DB load/save took " ++ show (diffUTCTime dbLoadEnd dbLoadStart) ++ "/" ++ show (diffUTCTime dbSaveEnd dbSaveStart) ++ " seconds."
-      putStrLn $ "Build took " ++ show (diffUTCTime endTime startTime) ++ " seconds."
-      return ()
+  -- load the database
+  dbLoadStart <- getCurrentTime
+  (tdb, cdb, tcdb) <- loadDatabase
+  dbLoadEnd <- getCurrentTime
 
+  -- run the build
+  startTime <- getCurrentTime
+  (_, s) <- runBuild (runTarget (fromJust theTarget)) (BuildState buildArgs selectedTarget tdb cdb tcdb Set.empty Map.empty)
+  endTime <- getCurrentTime
+
+  -- save the database
+  dbSaveStart <- getCurrentTime
+  saveDatabase (getTargetTimestampDB s) (getChecksumDB s) (getTargetChecksumDB s)
+  dbSaveEnd <- getCurrentTime
+
+  -- output build stats
+  putStrLn $ "DB load/save took " ++ show (diffUTCTime dbLoadEnd dbLoadStart) ++ "/" ++ show (diffUTCTime dbSaveEnd dbSaveStart) ++ " seconds."
+  putStrLn $ "Build took " ++ show (diffUTCTime endTime startTime) ++ " seconds."
+
 gatherAllTargetsInternal :: [Target] -> Set.Set Target -> Set.Set Target
 gatherAllTargetsInternal (t:ts) s =
   let (recurse, newSet) = if Set.notMember t s then (True, Set.insert t s) else (False, s)
@@ -167,6 +180,12 @@
       targetsMinusInitial = L.filter (\x -> x /= head t) allTargets
   in head t : targetsMinusInitial
 
+validateTargets :: [Target] -> Maybe String
+validateTargets ts =
+  let targetErrors = L.foldl' (\acc t -> acc ++ validate t) "" ts
+      validate (Target name _ _ stages gatherers) = if length stages > 0 && length gatherers == 0 then (T.unpack name) ++ ": target requires at least one gatherer since it specifies at least one stage.\n" else ""
+  in if targetErrors == "" then Nothing else Just targetErrors
+
 extractVarsFromArgs :: [String] -> ArgDict
 extractVarsFromArgs args = L.foldl' extractVarsFromArgsInternal Map.empty $ map (L.break (== '=')) args
   where
@@ -205,6 +224,19 @@
 makeArgDictLookupFunc :: String -> String -> ArgDict -> String
 makeArgDictLookupFunc arg defVal dict = fromMaybe defVal $ Map.lookup arg dict
 
+-- | Makes a function that can be used to look up a value in the argument
+-- dictionary, returning a default value if the argument does not exist, and
+-- checking success against a list of valid values.
+-- Returns an error string on Left, and success string on Right.
+makeArgDictLookupFuncChecked :: String -> String -> [String] -> ArgDict -> Either String String
+makeArgDictLookupFuncChecked arg defVal validValues dict =
+    let partialResult = makeArgDictLookupFunc arg defVal dict
+        result = L.find (== partialResult) validValues
+    in if isJust result then
+        Right $ fromJust result
+      else
+        Left $ "ERROR: invalid value \"" ++ partialResult ++ "\" for argument \"" ++ arg ++ "\". Expected one of: [" ++ L.intercalate  ", " validValues ++ "]"
+
 printSeparator :: IO ()
 printSeparator = putStrLn "============================================================"
 
@@ -319,8 +351,9 @@
       joinedDest = T.concat $ L.intersperse ":" d
   in (joinedDest, Hash.crc32 (TE.encodeUtf8 joinedSrc))
 
-buildFoldFunc :: Either [SrcTransform] T.Text -> Target -> BuildM (Either [SrcTransform] T.Text)
-buildFoldFunc (Left _) t@(Target name _ _ _ _) = do
+buildFoldFunc :: StageResults -> Target -> BuildM StageResults
+buildFoldFunc l@(Left _) _ = return l
+buildFoldFunc (Right _) t@(Target name _ _ _ _) = do
   buildState <- get
   let oldTargetName = getCurrentTargetName buildState
   put $ putCurrentTargetName buildState name
@@ -329,33 +362,31 @@
   put $ putCurrentTargetName newBuildState oldTargetName
   return result
 
-buildFoldFunc r@(Right _) _ = return r
-
-isLeft :: Either a b -> Bool
-isLeft (Left _) = True
-isLeft _ = False
+isRight :: Either a b -> Bool
+isRight (Right _) = True
+isRight _ = False
 
-runTarget :: Target -> BuildM (Either [SrcTransform] T.Text)
+runTarget :: Target -> BuildM StageResults
 runTarget t@(Target name _ deps _ _) = do
   buildState <- get
   let outdatedTargets = filter (not.targetIsUpToDate buildState) deps
-  depStatus <- foldM buildFoldFunc (Left []) outdatedTargets
-  if isLeft depStatus then do
+  depStatus <- foldM buildFoldFunc (Right []) outdatedTargets
+  if isRight depStatus then do
       result <- runTargetInternal t
       writePendingDBUpdates
       return result
     else
       buildFailFunc depStatus name
 
-buildFailFunc :: Either [SrcTransform] T.Text -> T.Text -> BuildM (Either [SrcTransform] T.Text)
-buildFailFunc (Right err) name = do
+buildFailFunc :: StageResults -> T.Text -> BuildM StageResults
+buildFailFunc (Left err) name = do
   liftIO printSeparator
   liftIO $ putStr $ "ERROR: Error building target \"" ++ T.unpack name ++ "\": "
   liftIO $ putStrLn $ T.unpack err
-  return $ Right ""
-buildFailFunc (Left _) _ = return $ Right ""
+  return $ Left ""
+buildFailFunc (Right _) _ = return $ Left ""
 
-runTargetInternal :: Target -> BuildM (Either [SrcTransform] T.Text)
+runTargetInternal :: Target -> BuildM StageResults
 runTargetInternal t@(Target name hashFunc _ stages gatherers) = do
   buildState <- get
   let tcdb = getTargetChecksumDB buildState
@@ -364,10 +395,10 @@
   gatheredFiles <- liftIO $ runGatherers gatherers
   let srcTransforms = map (flip OneToOne "") gatheredFiles
   liftIO $ putStrLn $ "==== Target: \"" ++ T.unpack name ++ "\""
-  stageResult <- foldM stageFoldFunc (Left srcTransforms) $ zip stages $ repeat forceRebuild
-  if isLeft stageResult then targetSuccessFunc t else buildFailFunc stageResult name
+  stageResult <- foldM stageFoldFunc (Right srcTransforms) $ zip stages $ repeat forceRebuild
+  if isRight stageResult then targetSuccessFunc t else buildFailFunc stageResult name
 
-targetSuccessFunc :: Target -> BuildM (Either [SrcTransform] T.Text)
+targetSuccessFunc :: Target -> BuildM StageResults
 targetSuccessFunc t@(Target name hashFunc _ _ _) = do
   buildState <- get
   let updatedTargets = Set.insert t $ getUpToDateTargets buildState
@@ -375,13 +406,13 @@
   put $ putTargetChecksumDB (putUpToDateTargets buildState updatedTargets) updatedChecksums
   liftIO $ putStrLn $ "Successfully built target \"" ++ T.unpack name ++ "\""
   liftIO $ putStrLn ""
-  return $ Left []
+  return $ Right []
 
-stageFoldFunc :: Either [SrcTransform] T.Text -> (Stage, Bool) -> BuildM (Either [SrcTransform] T.Text)
-stageFoldFunc (Left t) (s, force) = runStage s force t
-stageFoldFunc r@(Right _) _ = return r
+stageFoldFunc :: StageResults -> (Stage, Bool) -> BuildM StageResults
+stageFoldFunc (Right t) (s, force) = runStage s force t
+stageFoldFunc l@(Left _) _ = return l
 
-workerThreadFunc :: (SrcTransform -> IO (Either SrcTransform T.Text)) -> MVar [SrcTransform] -> MVar (Either [SrcTransform] T.Text, [BuildM ()]) -> MVar (Either [SrcTransform] T.Text, [BuildM ()]) -> MVar Int -> IO ()
+workerThreadFunc :: (SrcTransform -> IO StageResult) -> MVar [SrcTransform] -> MVar (StageResults, [BuildM ()]) -> MVar (StageResults, [BuildM ()]) -> MVar Int -> IO ()
 workerThreadFunc sf q r f c = do
   queue <- takeMVar q
   if null queue then do
@@ -402,14 +433,14 @@
       taskResult <- sf workItem
       let dbThunk = updateDatabase taskResult workItem
       resultAcc <- takeMVar r
-      let combine right@(Right _) _ = right
-          combine (Left ml) (Left v) = Left (v : ml)
-          combine (Left _) (Right v) = Right v
+      let combine left@(Left _) _ = left
+          combine (Right ml) (Right v) = Right (v : ml)
+          combine (Right _) (Left v) = Left v
       let newResultAcc = (\(res, thunks) -> (combine res taskResult, dbThunk : thunks)) resultAcc
       putMVar r newResultAcc
       workerThreadFunc sf q r f c
 
-stageHelper :: (SrcTransform -> IO (Either SrcTransform T.Text)) -> Int -> [SrcTransform] -> Either [SrcTransform] T.Text -> BuildM (Either [SrcTransform] T.Text)
+stageHelper :: (SrcTransform -> IO StageResult) -> Int -> [SrcTransform] -> StageResults -> BuildM StageResults
 stageHelper f m i r = do
   finalResultMVar <- liftIO newEmptyMVar
   resultMVar <- liftIO $ newMVar (r, []) -- (overall result, database thunks)
@@ -423,13 +454,13 @@
       sequence_ $ snd result
       return $ fst result
 
-runStage :: Stage -> Bool -> [SrcTransform] -> BuildM (Either [SrcTransform] T.Text)
+runStage :: Stage -> Bool -> [SrcTransform] -> BuildM StageResults
 runStage s@(Stage name _ _ extraDeps f) force m = do
   liftIO $ putStrLn $ "-- Stage: \"" ++ T.unpack name ++ "\""
   depScannedFiles <- liftIO $ processMappings s m
   (targetsToBuild, upToDateTargets) <- partitionMappings depScannedFiles extraDeps force
   bs <- get
-  result <- stageHelper f (getMaxBuildJobs bs) targetsToBuild (Left $ map transferUpToDateTarget upToDateTargets)
+  result <- stageHelper f (getMaxBuildJobs bs) targetsToBuild (Right $ map transferUpToDateTarget upToDateTargets)
   updateDatabaseExtraDeps result extraDeps
 
 -- These might not be quite correct. I guessed at what made sense.
@@ -445,11 +476,11 @@
   mapM d transMap
 
 updateDatabase :: Either l r -> SrcTransform -> BuildM ()
-updateDatabase (Right _) _ = return ()
-updateDatabase (Left _) (OneToOne s d) = updateDatabaseHelper [s] [d]
-updateDatabase (Left _) (OneToMany s ds) = updateDatabaseHelper [s] ds
-updateDatabase (Left _) (ManyToOne ss d) = updateDatabaseHelper ss [d]
-updateDatabase (Left _) (ManyToMany ss ds) = updateDatabaseHelper ss ds
+updateDatabase (Left _) _ = return ()
+updateDatabase (Right _) (OneToOne s d) = updateDatabaseHelper [s] [d]
+updateDatabase (Right _) (OneToMany s ds) = updateDatabaseHelper [s] ds
+updateDatabase (Right _) (ManyToOne ss d) = updateDatabaseHelper ss [d]
+updateDatabase (Right _) (ManyToMany ss ds) = updateDatabaseHelper ss ds
 
 updateDatabaseHelper :: [T.Text] -> [T.Text] -> BuildM ()
 updateDatabaseHelper srcFiles destFiles = do
@@ -464,9 +495,9 @@
   put $ putChecksumDB (putPendingDBUpdates buildstate updatedPDBU) updatedCDB
   return ()
 
-updateDatabaseExtraDeps :: Either [SrcTransform] T.Text -> [T.Text] -> BuildM (Either [SrcTransform] T.Text)
-updateDatabaseExtraDeps result@(Right _) _ = return result
-updateDatabaseExtraDeps result@(Left _) deps = do
+updateDatabaseExtraDeps :: StageResults -> [T.Text] -> BuildM StageResults
+updateDatabaseExtraDeps result@(Left _) _ = return result
+updateDatabaseExtraDeps result@(Right _) deps = do
   buildstate <- get
   let pdbu = getPendingDBUpdates buildstate
   timestamps <- liftIO $ mapM getTimestamp deps
diff --git a/src/Dib/Builders/C.hs b/src/Dib/Builders/C.hs
--- a/src/Dib/Builders/C.hs
+++ b/src/Dib/Builders/C.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | A builder for C/C++ code.
@@ -17,9 +17,11 @@
 import Dib.Gatherers
 import Dib.Target
 import Dib.Types
+import Dib.Util
 import Dib.Scanners.CDepScanner
 
 import Data.List as L
+import Data.Monoid
 import Data.Word
 import System.Process (system)
 import System.Directory as D
@@ -184,13 +186,13 @@
 
 remapObjFile :: BuildLocation -> T.Text -> T.Text
 remapObjFile InPlace f = f
-remapObjFile (BuildDir d) f = d `T.snoc` F.pathSeparator `T.append` massageFilePath f
-remapObjFile (ObjAndBinDirs d _) f = d `T.snoc` F.pathSeparator `T.append` massageFilePath f
+remapObjFile (BuildDir d) f = d `T.snoc` F.pathSeparator <> massageFilePath f
+remapObjFile (ObjAndBinDirs d _) f = d `T.snoc` F.pathSeparator <> massageFilePath f
 
 remapBinFile :: BuildLocation -> T.Text -> T.Text
 remapBinFile InPlace f = f
-remapBinFile (BuildDir d) f = d `T.snoc` F.pathSeparator `T.append` f
-remapBinFile (ObjAndBinDirs _ d) f = d `T.snoc` F.pathSeparator `T.append` f
+remapBinFile (BuildDir d) f = d `T.snoc` F.pathSeparator <> f
+remapBinFile (ObjAndBinDirs _ d) f = d `T.snoc` F.pathSeparator <> f
 
 -- | Given a 'CTargetInfo', will make the directories required to build the project.
 makeBuildDirs :: CTargetInfo -> IO ()
@@ -210,7 +212,7 @@
 -- | Given a 'CTargetInfo', produces a 'Target'
 makeCTarget :: CTargetInfo -> Target
 makeCTarget info =
-  let includeDirString = includeOption info `T.append` T.intercalate (" " `T.append` includeOption info) (includeDirs info)
+  let includeDirString = includeOption info <> T.intercalate (" " <> includeOption info) (includeDirs info)
       makeBuildString s t = T.unpack $ T.concat [compiler info, " ", inFileOption info, " ", s, " ", outFileOption info, " ", t, " ", includeDirString, " ", commonCompileFlags info, " ", getCorrectCompileFlags info s]
       makeLinkString ss t = T.unpack $ T.concat [linker info, " ", T.unwords ss, " ", outFileOption info, " ", t, " ", linkFlags info]
       makeArchiveString ss t = T.unpack $ T.concat [archiver info, " ", archiverFlags info, " ", t, " ", T.unwords ss]
@@ -221,21 +223,21 @@
         putStrLn $ "Building: " ++ T.unpack sourceFile
         exitCode <- system buildString
         handleExitCode exitCode t buildString
-      buildCmd _ = return $ Right "Unhandled SrcTransform."
+      buildCmd _ = return $ Left "Unhandled SrcTransform."
 
       linkCmd (ManyToOne ss t) = do
         let linkString = makeLinkString ss t
         putStrLn $ "Linking: " ++ T.unpack t
         exitCode <- system linkString
         handleExitCode exitCode t linkString
-      linkCmd _ = return $ Right "Unhandled SrcTransform."
+      linkCmd _ = return $ Left "Unhandled SrcTransform."
 
       archiveCmd (ManyToOne ss t) = do
         let archiveString = makeArchiveString ss t
         putStrLn $ "Archiving: " ++ T.unpack t
         exitCode <- system archiveString
         handleExitCode exitCode t archiveString
-      archiveCmd _ = return $ Right "Unhandled SrcTransform."
+      archiveCmd _ = return $ Left "Unhandled SrcTransform."
 
       buildDirGatherer = makeCommandGatherer $ makeBuildDirs info
       cppStage = Stage "compile" (map (changeExt "o" (outputLocation info))) (cDepScanner (map T.unpack $ includeDirs info)) (extraCompileDeps info) buildCmd
@@ -244,13 +246,9 @@
   in Target (targetName info) (cTargetHash info) [] [cppStage, if staticLibrary info then archiveStage else linkStage] [buildDirGatherer, makeFileTreeGatherer (srcDir info) (matchExtensionsExcluded [".cpp", ".c"] [excludeFiles $ exclusions info])]
 
 changeExt :: T.Text -> BuildLocation -> SrcTransform -> SrcTransform
-changeExt newExt b (OneToOne l _) = OneToOne l $ remapObjFile b $ T.append (T.dropWhileEnd (/='.') l) newExt
+changeExt newExt b (OneToOne l _) = OneToOne l $ remapObjFile b $ (T.dropWhileEnd (/='.') l)  <> newExt
 changeExt _ _ _ = undefined
 
-handleExitCode :: ExitCode -> T.Text -> String -> IO (Either SrcTransform T.Text)
-handleExitCode ExitSuccess t _ = return $ Left $ OneToOne t ""
-handleExitCode (ExitFailure _) _ e = return $ Right $ T.pack (show e)
-
 combineTransforms :: T.Text -> [SrcTransform] -> [SrcTransform]
 combineTransforms t st = [ManyToOne sources t]
   where sources = foldl' (\l (OneToOne s _) -> l ++ [s]) [] st
@@ -261,7 +259,7 @@
   let cleanCmd (OneToOne s _) = do
         putStrLn $ "removing: " ++ T.unpack s
         D.removeFile (T.unpack s)
-        return $ Left $ OneToOne "" ""
+        return $ Right $ OneToOne "" ""
       cleanCmd _ = error "Should never hit this."
 
       objDir InPlace = srcDir info
@@ -269,10 +267,10 @@
       objDir (ObjAndBinDirs d _) = d
 
       programFile InPlace = outputName info
-      programFile (BuildDir d) = d `T.snoc` F.pathSeparator `T.append` outputName info
-      programFile (ObjAndBinDirs _ d) = d `T.snoc` F.pathSeparator `T.append` outputName info
+      programFile (BuildDir d) = d `T.snoc` F.pathSeparator <> outputName info
+      programFile (ObjAndBinDirs _ d) = d `T.snoc` F.pathSeparator <> outputName info
 
       cleanStage = Stage "clean" id return [] cleanCmd
       objectGatherer = makeFileTreeGatherer (objDir $ outputLocation info) (matchExtension ".o")
       programGatherer = makeSingleFileGatherer (programFile $ outputLocation info)
-  in Target ("clean-" `T.append` targetName info) (const 0) [] [cleanStage] [objectGatherer, programGatherer]
+  in Target ("clean-" <> targetName info) (const 0) [] [cleanStage] [objectGatherer, programGatherer]
diff --git a/src/Dib/Builders/Copy.hs b/src/Dib/Builders/Copy.hs
--- a/src/Dib/Builders/Copy.hs
+++ b/src/Dib/Builders/Copy.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | A trivial builder that copies a directory tree from one location to another.
@@ -13,15 +13,15 @@
 import qualified System.Directory as D
 import System.FilePath as P
 
-copyFunc :: SrcTransform -> IO (Either SrcTransform T.Text)
+copyFunc :: SrcTransform -> IO StageResult
 copyFunc (OneToOne s t) = do
   let unpackedTarget = T.unpack t
   let unpackedSource = T.unpack s
   D.createDirectoryIfMissing True $ takeDirectory unpackedTarget
   putStrLn $ "Copying: " ++ unpackedSource ++ " -> " ++ unpackedTarget
   D.copyFile unpackedSource unpackedTarget
-  return $ Left (OneToOne t "")
-copyFunc _ = return $ Right "Unexpected SrcTransform"
+  return $ Right (OneToOne t "")
+copyFunc _ = return $ Left "Unexpected SrcTransform"
 
 remapFile :: String -> String -> SrcTransform -> SrcTransform
 remapFile src dest (OneToOne s _) = OneToOne s $ T.pack $ dest </> makeRelative src (T.unpack s)
diff --git a/src/Dib/Builders/Simple.hs b/src/Dib/Builders/Simple.hs
--- a/src/Dib/Builders/Simple.hs
+++ b/src/Dib/Builders/Simple.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | The Simple builder allows you to execute a 'OneToOne' trasformation
@@ -10,6 +10,7 @@
 
 import Dib.Gatherers
 import Dib.Types
+import Dib.Util
 
 import qualified Data.Text as T
 import qualified System.Directory as D
@@ -17,7 +18,7 @@
 import System.Exit
 import System.FilePath as P
 
-buildFunc :: (String -> String -> String) -> SrcTransform -> IO (Either SrcTransform T.Text)
+buildFunc :: (String -> String -> String) -> SrcTransform -> IO StageResult
 buildFunc func (OneToOne s t) = do
   let unpackedTarget = T.unpack t
   let unpackedSource = T.unpack s
@@ -26,7 +27,7 @@
   let buildCmd = func unpackedSource unpackedTarget
   exitCode <- system buildCmd
   handleExitCode exitCode t buildCmd
-buildFunc _ _ = return $ Right "Unexpected SrcTransform"
+buildFunc _ _ = return $ Left "Unexpected SrcTransform"
 
 remapFile :: String -> String -> T.Text -> SrcTransform -> SrcTransform
 remapFile src dest ext (OneToOne s _) = OneToOne s $ T.pack $ dest </> makeRelative src (T.unpack (changeExt s ext))
@@ -34,11 +35,6 @@
 
 changeExt :: T.Text -> T.Text -> T.Text
 changeExt path = T.append (T.dropWhileEnd (/='.') path)
-
---TODO: move to a utility module and factor out of C builder
-handleExitCode :: ExitCode -> T.Text -> String -> IO (Either SrcTransform T.Text)
-handleExitCode ExitSuccess t _ = return $ Left $ OneToOne t ""
-handleExitCode (ExitFailure _) _ e = return $ Right $ T.pack (show e)
 
 -- | The 'makeSimpleTarget' function generates a target.
 -- It takes a name, source directory, destination directory, destination extension,
diff --git a/src/Dib/Gatherers.hs b/src/Dib/Gatherers.hs
--- a/src/Dib/Gatherers.hs
+++ b/src/Dib/Gatherers.hs
@@ -1,5 +1,5 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | Module that exposes all of the various 'GatherStrategy' types and functions
diff --git a/src/Dib/Scanners/CDepScanner.hs b/src/Dib/Scanners/CDepScanner.hs
--- a/src/Dib/Scanners/CDepScanner.hs
+++ b/src/Dib/Scanners/CDepScanner.hs
@@ -1,5 +1,5 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving, ExistentialQuantification, KindSignatures, FlexibleContexts #-}
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | C dependency scanner. Runs a stripped-down pre-processor to scan for
@@ -13,7 +13,7 @@
 import qualified Data.Text as T
 import qualified System.Directory as Dir
 import qualified System.FilePath as F
-import Control.Applicative
+import Control.Applicative()
 import Control.Monad.State.Lazy
 
 data Dependency = Dependency String String
@@ -150,4 +150,3 @@
 cDepScanner includeDirs (OneToOne i o) = getDepsForFile includeDirs (T.unpack i) >>= \d -> return $ ManyToOne (i:d) o
 cDepScanner includeDirs (OneToMany i o) = getDepsForFile includeDirs (T.unpack i) >>= \d -> return $ ManyToMany (i:d) o
 cDepScanner _ _ = error "Unimplemented. Implement this if it is a valid relationship."
-
diff --git a/src/Dib/Stage.hs b/src/Dib/Stage.hs
--- a/src/Dib/Stage.hs
+++ b/src/Dib/Stage.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | Module exposing the 'Stage' type and related type wrappers, along with a
@@ -15,4 +15,4 @@
 
 -- | A stage that does nothing and just passes the 'SrcTransform's through.
 emptyStage :: Stage
-emptyStage = Stage "empty" id return [] (return.Left)
+emptyStage = Stage "empty" id return [] (return.Right)
diff --git a/src/Dib/Target.hs b/src/Dib/Target.hs
--- a/src/Dib/Target.hs
+++ b/src/Dib/Target.hs
@@ -1,4 +1,4 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | Module that exposes the 'Target' data type and a handful of convenience functions
diff --git a/src/Dib/Types.hs b/src/Dib/Types.hs
--- a/src/Dib/Types.hs
+++ b/src/Dib/Types.hs
@@ -2,14 +2,14 @@
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 {-# OPTIONS_HADDOCK hide, prune #-}
 
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | Various types used in dib. Due to certain circumstances, this module does
 -- not export any of these types directly; other modules do.
 module Dib.Types where
 
-import Control.Applicative
+import Control.Applicative()
 import Control.Monad.State as S
 import qualified Data.Map as Map
 import qualified Data.Serialize as Serialize
@@ -70,9 +70,14 @@
 -- a 'SrcTransform' with dependencies included in the input.
 type DepScanner = (SrcTransform -> IO SrcTransform)
 
+-- | Type wrapper for the result of a 'StageFunc'.
+-- Left indicates an error while Right indicates success.
+type StageResult = Either T.Text SrcTransform
+type StageResults = Either T.Text [SrcTransform]
+
 -- | Type wrapper for a function that given a 'SrcTransform', produces either
 -- a 'SrcTransform' containing the output files in the input, or an error.
-type StageFunc = (SrcTransform -> IO (Either SrcTransform T.Text))
+type StageFunc = (SrcTransform -> IO StageResult)
 
 -- | Type describing a build stage.
 -- Takes a name, an 'InputTransformer', 'DepScanner', additional dependencies, and the builder function.
diff --git a/src/Dib/Util.hs b/src/Dib/Util.hs
--- a/src/Dib/Util.hs
+++ b/src/Dib/Util.hs
@@ -1,12 +1,17 @@
--- Copyright (c) 2010-2016 Brett Lajzer
+-- Copyright (c) 2010-2018 Brett Lajzer
 -- See LICENSE for license information.
 
 -- | Module containing utility functions and common functionality.
 module Dib.Util (
-  getSubDirectories
+  getSubDirectories,
+  handleExitCode
   ) where
 
+import Dib.Types
+
+import System.Exit (ExitCode(ExitSuccess, ExitFailure))
 import Control.Monad
+import qualified Data.Text as T
 import qualified System.Directory as D
 
 -- | Given a directory, returns the list of subdirectories.
@@ -16,3 +21,8 @@
   let filteredDirs = filter (\x -> x /= "." && x /= "..") contents
   dirs <- filterM (\(x, _) -> D.doesDirectoryExist x) $ zip (map (root ++) filteredDirs) filteredDirs
   return $ map snd dirs
+
+-- | A utility function for handling 'ExitCode's in 'StageFunction's.
+handleExitCode :: ExitCode -> T.Text -> String -> IO StageResult
+handleExitCode ExitSuccess t _ = return $ Right $ OneToOne t ""
+handleExitCode (ExitFailure _) _ e = return $ Left $ T.pack (show e)