Implement a tiny cat version

We only support a single file which gets written to stdout. Non-existing
files or other errors are printed to stderr and the program terminates.

app/hcat.hs

@@ -3,4 +3,4 @@ module Main (main) where
 import HCat qualified
 
 main :: IO ()
-main = HCat.runHCat
+main = HCat.defaultMain

src/HCat.hs

@@ -1,6 +1,32 @@
 module HCat (
     runHCat,
+    defaultMain,
 ) where
 
+import System.Environment qualified as Env
+import System.IO (hPutStrLn, stderr)
+
+import Control.Exception qualified as Exception
+import System.IO.Error qualified as IOError
+
+parseArgs :: [String] -> Either String FilePath
+parseArgs args = case args of
+  [] -> Left "No filename given!"
+  [arg] -> Right arg
+  _ -> Left "Only a single file is supported"
+
 runHCat :: IO ()
-runHCat = pure ()
+runHCat = do
+  fileNameOrError <-  parseArgs <$> Env.getArgs
+  fileName <- eitherToError fileNameOrError
+  readFile fileName >>= 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
+      
+defaultMain :: IO ()
+defaultMain = Exception.catch runHCat handleError