diff --git a/configifier.cabal b/configifier.cabal
--- a/configifier.cabal
+++ b/configifier.cabal
@@ -1,5 +1,5 @@
 name:                configifier
-version:             0.0.8
+version:             0.1.0
 synopsis:            parser for config files, shell variables, command line args.
 description:         See <https://github.com/zerobuzz/configifier/blob/master/README.md README>
 license:             AGPL-3
diff --git a/src/Data/Configifier.hs b/src/Data/Configifier.hs
--- a/src/Data/Configifier.hs
+++ b/src/Data/Configifier.hs
@@ -117,7 +117,7 @@
 
 data Source =
       YamlString SBS
-    | YamlFile FilePath  -- FIXME: { yamlFilePath :: FilePath, yamlFileOptional :: Bool }
+    | YamlFile { yamlFileOptional :: Bool, yamlFilePath :: FilePath }
     | ShellEnv [(String, String)]
     | CommandLine [String]
   deriving (Eq, Ord, Show, Typeable)
@@ -208,7 +208,7 @@
 
     get :: Source -> IO tm
     get (YamlString sbs)     = run $ parseConfigFile sbs
-    get (YamlFile fpath)     = parseConfigFileWithIncludes fpath >>= run
+    get (YamlFile opt fpath) = parseConfigFileWithIncludes opt fpath >>= run
     get (ShellEnv env)       = run $ parseShellEnv env
     get (CommandLine args)   = run $ parseCommandLine args
 
@@ -223,18 +223,15 @@
 --
 -- File path arguments are optiona; config paths to non-existent
 -- files are silently dropped.
-defaultSources :: [FilePath] -> IO [Source]
-defaultSources filePaths = do
-    files <- YamlFile   <$$> filterM doesFileExist filePaths
-    env   <- ShellEnv    <$> (getEnvironment >>= withShellEnvPrefix)
+defaultSources' :: String -> [FilePath] -> IO [Source]
+defaultSources' progName filePaths = do
+    files <- YamlFile False <$$> filterM doesFileExist filePaths
+    env   <- ShellEnv . withShellEnvPrefix progName <$> getEnvironment
     args  <- CommandLine <$> getArgs
     return $ files ++ [env] ++ readUserConfigFiles [args]
 
--- | Require that all shell env variables start with executable name.
--- (This is just a call to 'requireShellEnvPrefix'' with the result of
--- 'progName'.)
-withShellEnvPrefix :: Env -> IO Env
-withShellEnvPrefix env = (`withShellEnvPrefix'` env) <$> getProgName
+defaultSources :: [FilePath] -> IO [Source]
+defaultSources filePaths = getProgName >>= (`defaultSources'` filePaths) . (<> "_")
 
 
 -- * corner cases
@@ -254,24 +251,24 @@
   where
     f :: Source -> [Source]
     f s@(YamlString _)   = [s]
-    f s@(YamlFile _)     = [s]
+    f s@(YamlFile _ _)   = [s]
     f s@(ShellEnv _)     = [s]
     f (CommandLine args) = filter (not . (== CommandLine [])) $ g [] args
 
     g :: [String] -> [String] -> [Source]
     g acc [] = [CommandLine (reverse acc)]
     g acc fresh@(freshHead:freshTail) = case popArg fresh of
-        Right (("config", v), fresh') -> CommandLine (reverse acc) : YamlFile v : g [] fresh'
+        Right (("config", v), fresh') -> CommandLine (reverse acc) : YamlFile True v : g [] fresh'
         _ -> g (freshHead : acc) freshTail
 
 -- | Require prefix for shell env variables.  This function will chop
 -- off the given prefix of all env entries, and filter all entries
 -- that do not have this prefix.
-withShellEnvPrefix' :: String -> Env -> Env
-withShellEnvPrefix' prefix = catMaybes . map f
+withShellEnvPrefix :: String -> Env -> Env
+withShellEnvPrefix prefix = catMaybes . map f
   where
     f (k, v) = if prefix `isPrefixOf` k
-        then Just (take (length prefix) k, v)
+        then Just (drop (length prefix) k, v)
         else Nothing
 
 
@@ -284,8 +281,13 @@
     trunc s = if SBS.length s > l then SBS.take l s <> "..." else s
 
 -- | See "Data.Yaml.Include".
-parseConfigFileWithIncludes :: (FromJSON (TaggedM cfg)) => FilePath -> IO (Either Error (TaggedM cfg))
-parseConfigFileWithIncludes fpath = mapLeft (InvalidYamlFile fpath) <$> decodeFileEither fpath
+parseConfigFileWithIncludes :: (Monoid (TaggedM cfg), FromJSON (TaggedM cfg)) =>
+      Bool -> FilePath -> IO (Either Error (TaggedM cfg))
+parseConfigFileWithIncludes optional fpath = f optional <$> decodeFileEither fpath
+  where
+    f _     (Right v) = Right v
+    f True  (Left _)  = Right mempty
+    f False (Left e)  = Left $ InvalidYamlFile fpath e
 
 renderConfigFile :: (Freeze cfg, t ~ Tagged cfg, ToJSON (TaggedM cfg)) => t -> SBS
 renderConfigFile = Yaml.encode . thaw
diff --git a/tests/Data/ConfigifierSpec.hs b/tests/Data/ConfigifierSpec.hs
--- a/tests/Data/ConfigifierSpec.hs
+++ b/tests/Data/ConfigifierSpec.hs
@@ -209,7 +209,7 @@
 
     it "(\"l\" :> Int :*> \"l'\" :> Int)" $
         let t :: forall c . ( c ~ ToConfigCode ("l" :> Int :*> "l'" :> Int)
-                            , ToVal c '["l"] ~ Just Int  -- (redundant)
+                            , ToVal c '["l"] ~ 'Just Int  -- (redundant)
                             , ToConfig c Id ~ (Id Int :*> Id Int)  -- (redundant)
                             ) => IO ()
             t = do
@@ -223,7 +223,7 @@
 
     it "(\"l\" :> Int :*> Maybe (\"l'\" :> Int))" $
         let t :: forall c . ( c ~ ToConfigCode ("l" :> Int :*> Maybe ("l'" :> Int))
-                            , ToVal c '["l"] ~ Just Int  -- (redundant)
+                            , ToVal c '["l"] ~ 'Just Int  -- (redundant)
                             , ToConfig c Id ~ (Id Int :*> MaybeO (Id Int))  -- (redundant)
                             ) => IO ()
             t = do
@@ -357,15 +357,15 @@
 readUserConfigFilesSpec :: Spec
 readUserConfigFilesSpec = describe "readUserConfigFiles" $ do
     it "finds --config args" $
-        readUserConfigFiles [CommandLine ["--config=FILE"]] `shouldBe` [YamlFile "FILE"]
+        readUserConfigFiles [CommandLine ["--config=FILE"]] `shouldBe` [YamlFile True "FILE"]
 
     it "keeps surrounding args intact" $ do
         readUserConfigFiles [CommandLine ["1", "2", "--config=FILE", "3", "4"]] `shouldBe`
-            [CommandLine ["1", "2"], YamlFile "FILE", CommandLine ["3", "4"]]
+            [CommandLine ["1", "2"], YamlFile True "FILE", CommandLine ["3", "4"]]
         readUserConfigFiles [CommandLine ["--config=FILE", "2"]] `shouldBe`
-            [YamlFile "FILE", CommandLine ["2"]]
+            [YamlFile True "FILE", CommandLine ["2"]]
         readUserConfigFiles [CommandLine ["1", "--config=FILE"]] `shouldBe`
-            [CommandLine ["1"], YamlFile "FILE"]
+            [CommandLine ["1"], YamlFile True "FILE"]
 
 
 stringAsCharList :: Spec