Introduce Run monad

A monad for nice, pure exceptions. I want to avoid working with IO
exceptions as much as possible.

src/Mima/IO.hs

@@ -1,17 +1,32 @@
 module Mima.IO
-  ( parseFile
+  ( Run
+  , doRun
+  , failWith
+  , parseFile
   ) where
 
+import           Control.Monad.Trans.Class
+import           Control.Monad.Trans.Except
 import qualified Data.Text.IO as T
 import           Text.Megaparsec
 
 import           Mima.Parser.Common
 
-parseFile :: Parser a -> FilePath -> IO (Maybe a)
+type Run a = ExceptT String IO a
+
+doRun :: Run () -> IO ()
+doRun r = do
+  result <- runExceptT r
+  case result of
+    Left errorMsg -> putStrLn errorMsg
+    Right ()      -> pure ()
+
+failWith :: String -> Run a
+failWith = except . Left
+
+parseFile :: Parser a -> FilePath -> Run a
 parseFile parser filepath = do
-  content <- T.readFile filepath
+  content <- lift $ T.readFile filepath
   case parse parser filepath content of
-    Right a           -> pure $ Just a
-    Left errorBundle  -> do
-      putStrLn $ errorBundlePretty errorBundle
-      pure Nothing
+    Right a           -> pure a
+    Left errorBundle  -> failWith $ errorBundlePretty errorBundle