refactor for boolean blindness

src/Lib/Error.hs

@@ -55,3 +55,8 @@ handleS9ErrNuclear :: MonadIO m => S9ErrT m a -> m a
 handleS9ErrNuclear action = runExceptT action >>= \case
     Left e -> throwIO e
     Right a -> pure a
+
+errOnNothing :: MonadHandler m => Status -> Text -> Maybe a -> m a 
+errOnNothing status res entity = case entity of
+    Nothing -> sendResponseStatus status res
+    Just a -> pure a

src/Lib/Registry.hs

@@ -16,6 +16,7 @@ import           Yesod.Core
 
 import           Lib.Semver
 import           Lib.Types.Semver
+import           Data.Char
 
 type Registry = HashMap String (HashMap AppVersion FilePath)
 
@@ -50,6 +51,11 @@ type S9PK = Extension "s9pk"
 type SYS_EXTENSIONLESS = Extension ""
 type PNG = Extension "png"
 
+data Extensions = S9PK | PNG | EMPTY
+    deriving (Read)
+instance Show Extensions where
+    show a = fmap toLower $ show a
+
 instance IsString (Extension a) where
     fromString = Extension
 

src/Lib/Types/FileSystem.hs

@@ -0,0 +1,6 @@
+module Lib.Types.FileSystem where
+
+    import           Startlude
+
+    data FileExistence = Existent | NonExistent
+        deriving (Eq, Show)