More comments in DB.hs

Some clean up to replace use of `either` when `first` would do.

level07/src/FirstApp/DB.hs

@@ -89,6 +89,11 @@ runDb
   -> AppM a
 runDb a = do
   db <- asks ( dbConn . envDb )
+  -- We use the liftDb function to take the IO (Either SQLiteResponse a) and
+  -- convert it into an `m (Either Error a)` so that it matches the requirements
+  -- to be in our AppM, we then lean on the ExceptT functionality and use our
+  -- helper to either `throwError` with any DB errors that have made it this far
+  -- or simply return the desired value.
   liftDb db >>= throwL
   -- Or alternatively, if you hate variables...
   -- asks (dbConn.envDb) >>= ( liftDb >=> throwL )

level07/src/FirstApp/Main.hs

@@ -6,12 +6,13 @@ module FirstApp.Main
   ) where
 
 import           Control.Monad.Except               (ExceptT (ExceptT),
-                                                     runExceptT, withExceptT)
+                                                     runExceptT)
 import           Control.Monad.IO.Class             (liftIO)
 
 import           Network.Wai
 import           Network.Wai.Handler.Warp           (run)
 
+import           Data.Bifunctor                     (first)
 import           Data.Either                        (Either (..), either)
 
 import           Data.Text                          (Text)
@@ -68,8 +69,7 @@ prepareAppReqs = runExceptT $ do
   pure $ Env cfg db
   where
     toStartUpErr e =
-      withExceptT e   -- apply our error constructor to unify the error types to StartUpError
+      ExceptT . fmap (first e)
-      . ExceptT       -- convert our function to an ExceptT with the constructor
 
     -- Take our possibly failing configuration/db functions with their unique
     -- error types and turn them into a consistently typed ExceptT. We can then
@@ -134,6 +134,7 @@ mkAddRequest
   -> Either Error RqType
 mkAddRequest ti c = AddRq
   <$> mkTopic ti
+  -- Got string types...
   <*> (mkCommentText . decodeUtf8 $ LBS.toStrict c)
 
 mkViewRequest