Improve readability

src/Operations/Import/Firefox.hs

@@ -12,7 +12,9 @@ Portability : POSIX
 
 Imports from firefox. Firefox needs to be closed when doing the import
 -}
-module Operations.Import.Firefox where
+module Operations.Import.Firefox
+  ( importFirefox )
+  where
 
 import Data.Monoid
        (Endo(..))
@@ -45,7 +47,19 @@ import System.Environment
 import GHC.Stack
 
 import Control.Lens
-       (foldMapOf, folded, has, ix, makeLenses, to, (%~), (&), (<>~), (^.))
+       ( Lens'
+       , foldMapOf
+       , folded
+       , has
+       , ix
+       , lens
+       , makeLenses
+       , to
+       , (%~)
+       , (&)
+       , (<>~)
+       , (^.)
+       )
 
 import qualified Database.SQLite.Simple as SQL
 
@@ -70,7 +84,8 @@ data Firefox
             }
   deriving stock (Show, Eq)
 
-makeLenses ''Firefox
+url :: Lens' Firefox Text
+url = lens _url (\f u -> f{_url = u})
 
 stores
   :: MonadResource m
@@ -101,11 +116,17 @@ makeLenses ''Update
 
 importFirefox :: BuukaM ()
 importFirefox = do
+  -- Collect all the imported bookmarks
   fs <- (`appEndo` []) <$> runResourceT (runConduit $ stores .| C.mapM bookmarks .| C.foldMap (\f -> Endo (++ f)))
+  -- Insert to the buuka store iff, the urls don't already exist in the store
+  -- The fold keeps track of a set of already seen entries. Every iteration
+  -- adds the current url to the known set of urls. Only if the url doesn't
+  -- exist in the set, will it be inserted to the store
   buukaU $ modify $ \oldState -> F.foldl' update (initialState oldState) fs ^. buuka
   where
     toEntry Firefox{..} = B.BuukaEntry{ B._url = B.URL _url, B._title = Just _title }
     update acc f = acc
       & seen <>~ (f ^. url . to S.singleton)
       & if has (seen . ix (f ^. url)) acc then id else buuka %~ (B.insert (toEntry f))
-    initialState oldState = Update oldState (foldMapOf (B._Buuka . folded . B.url . B._URL) S.singleton oldState)
+    initialState oldState = Update oldState (initialUrls oldState)
+    initialUrls = foldMapOf (B._Buuka . folded . B.url . B._URL) S.singleton