diff --git a/app/Main.hs b/app/Main.hs
index 769bbb1..e005532 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -151,4 +151,4 @@ run elms = do
 main :: IO ()
 main = do
   elms <- runInputT inputSettings $ run testElements
-  putStrLn $ show elms
+  putStrLn $ elementsToString elms
diff --git a/src/Cards.hs b/src/Cards.hs
index 6442aeb..534857d 100644
--- a/src/Cards.hs
+++ b/src/Cards.hs
@@ -5,6 +5,7 @@ module Cards
   , toDueCards
   , fromCards
   , elementsToString
+  , parseElements
   , Card -- Card stuff
   , sides
   , tier
@@ -24,6 +25,11 @@ import           Data.List
 import qualified Data.Map.Strict as Map
 import           Data.Time
 
+-- | Contains 'Card's and comments with a certain ordering.
+-- To update some 'Card's in an 'Elements', use 'toCards' or 'toDueCards' and
+-- 'fromCards'.
+-- 'Card's can be removed (not added), as long as the numbers
+-- associated to the remaining cards stay with their original card.
 data Elements = Elements (Map.Map Integer Element)
   deriving (Show)
 
@@ -31,19 +37,32 @@ data Element = ECard Card
              | EComment String
   deriving (Show)
 
+-- | A single index card with one or more sides.
 data Card = Card
   { sides :: [String]
+  -- ^ The sides of a 'Card'.
+  -- As opposed to real index cards, a 'Card' may have more or less than two
+  -- sides.
   , tier :: Tier
+  -- ^ The 'Tier' of a 'Card'.
   , lastChecked :: UTCTime
+  -- ^ The time a 'Card' was last looked at.
   , offset :: NominalDiffTime
+  -- ^ A random offset, used when determining whether a 'Card' needs to be
+  -- revised.
+  -- This is to "stretch out" cards revised in a short time frame.
   } deriving (Show)
 
+-- | The "level" of a 'Card' in a typical index card learning scheme.
+-- Represents the time which should elapse before the 'Card' is looked at again.
 data Tier = Unrevised
           | TenMin | TwentyMin | FortyMin
           | OneDay | TwoDays | FourDays | EightDays
           | SixteenDays | ThirtyTwoDays | SixtyFourDays
   deriving (Show, Eq, Ord, Enum, Bounded)
 
+-- | A 'Elements' containing some cards and some comments, for testing.
+-- Will be removed soon.
 testElements :: Elements
 testElements = Elements . Map.fromList. zip [1..] $
   [ card ["first card", "really"]
@@ -60,16 +79,26 @@ testElements = Elements . Map.fromList. zip [1..] $
  - Elements stuff
  -}
 
+-- | @'updateElements' a b@ adds the updated values from @b@ to @a@.
 updateElements :: Elements -> Elements -> Elements
 updateElements (Elements old) (Elements new) = Elements $ Map.union new old
 
+-- | Extract all 'Card's from an 'Elements'.
+-- Entries may be deleted or modified, as long as the numbers are not changed
+-- and stay associated to their original 'Card'.
 toCards :: Elements -> [(Integer, Card)]
 toCards (Elements elms) =
   [(key, card) | (key, Just card) <- mapSnd toCard $ Map.toList elms]
 
+-- | Extract all 'Card's which are due from an 'Elements'.
+-- Entries may be deleted or modified, as long as the numbers are not changed
+-- and stay associated to their original 'Card'.
 toDueCards :: UTCTime -> Elements -> [(Integer, Card)]
 toDueCards time = filter (isDue time . snd) . toCards
 
+-- | Convert a list of 'Card's back into an 'Elements'.
+-- As long as the same numbers are assosiated to the same cards as they were
+-- originally, this can safely be used to update the original 'Elements'.
 fromCards :: [(Integer, Card)] -> Elements
 fromCards = Elements . Map.fromList . mapSnd fromCard
 
@@ -91,6 +120,7 @@ fromCard = ECard
  - Card stuff
  -}
 
+-- | @'isDue' time card@ returns whether @card@ needs to be revised at @time@.
 isDue :: UTCTime -> Card -> Bool
 isDue time Card{tier=t, lastChecked=lc, offset=o} =
   diffUTCTime time lc >= o + tierDiff t
@@ -101,20 +131,25 @@ updateOffset :: Card -> IO Card
 updateOffset Card{sides=s, tier=t, lastChecked=lc} = do
   return Card{sides=s, tier=t, lastChecked=lc, offset=0}
 
+-- | Reset a card's 'Tier'.
+-- This also resets the 'lastChecked' and 'offset' times.
 reset :: UTCTime -> Card -> Card
 reset time Card{sides=s} =
   Card{sides=s, tier=minBound, lastChecked=time, offset=0}
 
+-- | Push a 'Card' to the next highest 'Tier'.
+-- Set the 'lastChecked' and 'offset' time accordingly.
+-- Uses the IO monad to access the global random number generator.
 update :: UTCTime -> Card -> IO Card
 update time Card{sides=s, tier=t} =
   updateOffset $ Card {sides=s, tier=boundedSucc t, lastChecked=time, offset=0}
 
--- helper function
 boundedSucc :: (Eq a, Bounded a, Enum a) => a -> a
 boundedSucc a
   | a == maxBound = a
   | otherwise     = succ a
 
+-- | Create a new card.
 createCard :: UTCTime -> [String] -> Card
 createCard time s =
   Card{sides=s, tier=minBound, lastChecked=time, offset=0}
@@ -123,6 +158,7 @@ createCard time s =
  - Tier stuff
  -}
 
+-- | The 'NominalDiffTime' associated with each 'Tier'.
 tierDiff :: Tier -> NominalDiffTime
 tierDiff Unrevised     = 0
 tierDiff TenMin        = 60 * 10
@@ -136,6 +172,7 @@ tierDiff SixteenDays   = 3600 * (16 * 24 - 8)
 tierDiff ThirtyTwoDays = 3600 * (32 * 24 - 8)
 tierDiff SixtyFourDays = 3600 * (64 * 24 - 8)
 
+-- | The name associated with each 'Tier'.
 tierName :: Tier -> String
 tierName Unrevised     = "unrevised"
 tierName TenMin        = "10min"
@@ -153,6 +190,8 @@ tierName SixtyFourDays = "64d"
  - Converting to String
  -}
 
+-- | Convert an 'Elements' to a string which can be parsed by 'parseElements'.
+-- This string can then be written to a text file for storage.
 elementsToString :: Elements -> String
 elementsToString (Elements e) =
   let elms = map snd $ Map.toList e
@@ -168,10 +207,12 @@ cardToString Card{sides=s, tier=t, lastChecked=lc, offset=o} =
              ", \"last_checked\": " ++ (show $ formatTime defaultTimeLocale "%s" lc) ++
              ", \"delay\": " ++ (show $ fromEnum o) ++
              "}"
-  in unlines $ info : intersperse "::" s
+  in unlines $ info : intersperse "::" s ++ [""] -- newline at the end
 
 {-
  - Parsing
  -}
 
--- TODO
+-- | Not yet implemented.
+parseElements :: String -> Maybe Elements
+parseElements = undefined