Use internal CacheFileName newtype for referencing where the cache lives

src/Unused/Cache.hs

@@ -10,23 +10,26 @@ import Data.Vector (toList)
 import qualified Data.ByteString.Lazy as BS
 import Unused.Cache.DirectoryFingerprint (sha)
 
+newtype CacheFileName = CacheFileName String
+type Cache = ReaderT CacheFileName IO
+
 cached :: (FromRecord a, ToRecord a) => String -> IO [a] -> IO [a]
 cached context f =
     runReaderT fromCache =<< cacheFileName context
   where
     fromCache = maybe (writeCache =<< liftIO f) return =<< readCache
 
-writeCache :: ToRecord a => [a] -> ReaderT String IO [a]
+writeCache :: ToRecord a => [a] -> Cache [a]
 writeCache [] = return []
 writeCache contents = do
     liftIO $ createDirectoryIfMissing True cacheDirectory
-    fileName <- ask
+    (CacheFileName fileName) <- ask
     liftIO $ BS.writeFile fileName $ encode contents
     return contents
 
-readCache :: FromRecord a => ReaderT String IO (Maybe [a])
+readCache :: FromRecord a => Cache (Maybe [a])
 readCache = do
-    fileName <- ask
+    (CacheFileName fileName) <- ask
     exists <- liftIO $ doesFileExist fileName
 
     if exists
@@ -35,11 +38,11 @@ readCache = do
   where
     processCsv = either (const Nothing) (Just . toList)
 
-cacheFileName :: String -> IO String
+cacheFileName :: String -> IO CacheFileName
 cacheFileName context = do
     putStrLn "\n\nCalculating cache fingerprint... "
     currentSha <- sha
-    return $ cacheDirectory ++ "/" ++ context ++ "-" ++ currentSha ++ ".csv"
+    return $ CacheFileName $ cacheDirectory ++ "/" ++ context ++ "-" ++ currentSha ++ ".csv"
 
 cacheDirectory :: String
 cacheDirectory = "tmp/unused"