Move code to Internal module and add tests

src/HCat.hs

@@ -9,27 +9,22 @@ import System.IO (hPutStrLn, stderr)
 import Control.Exception qualified as Exception
 import System.IO.Error qualified as IOError
 
-import Data.Text qualified as Text
 import Data.Text.IO qualified as TextIO
 
-parseArgs :: [String] -> Either String FilePath
-parseArgs args = case args of
-  [] -> Left "No filename given!"
-  [arg] -> Right arg
-  _ -> Left "Only a single file is supported"
+import HCat.Internal (parseArgs)
 
 runHCat :: IO ()
 runHCat = do
-  fileNameOrError <-  parseArgs <$> Env.getArgs
-  fileName <- eitherToError fileNameOrError
-  TextIO.readFile fileName >>= TextIO.putStr
+    fileNameOrError <- parseArgs <$> Env.getArgs
+    fileName <- eitherToError fileNameOrError
+    TextIO.readFile fileName >>= TextIO.putStr
 
 eitherToError :: Show a => Either a b -> IO b
 eitherToError = either (Exception.throwIO . IOError.userError . show) return
 
 handleError :: IOError -> IO ()
 handleError e = do
-  hPutStrLn stderr $ "I ran into an issue: " <> show e
-      
+    hPutStrLn stderr $ "I ran into an issue: " <> show e
+
 defaultMain :: IO ()
 defaultMain = Exception.catch runHCat handleError

src/HCat/Internal.hs

@@ -0,0 +1,8 @@
+-- | Internal module in order to facilitate testability.
+module HCat.Internal where
+
+parseArgs :: [String] -> Either String FilePath
+parseArgs args = case args of
+    [] -> Left "No filename given!"
+    [arg] -> Right arg
+    _ -> Left "Only a single file is supported"