I am having hard time understanding the following code.  The code is from Applicative Parser library:  http://hackage.haskell.org/packages/archive/uu-parsinglib/2.5.5.2/doc/html/src/Text-ParserCombinators-UU-BasicInstances.html

 instance (Show a,  loc `IsLocationUpdatedBy` a) => Provides  (Str  a loc)  (a -> Bool, String, a)  a where

    splitState (p, msg, a) k (Str  tts   msgs pos  del_ok) 

          = let ins exp =       (5, k a (Str tts (msgs ++ [Inserted (show a)  pos  exp]) pos  False))

                del exp =       (5, splitState (p,msg, a) 

                                    k
                                    (Str (tail tts) 

                                         (msgs ++ [Deleted  (show(head tts))  pos  exp]) 

                                         (advance pos (head tts))

                                         True ))
            in case tts of

               (t:ts)  ->  if p t 

                           then  show_symbol ("Accepting symbol: " ++ show t ++ " at position: " ++ show pos ++"\n") 

                                 (Step 1 (k t (Str ts msgs (advance pos t) True)))

                           else  Fail [msg] (ins: if del_ok then [del] else [])

               []      ->  Fail [msg] [ins]



Specifically I am having hard time understanding how the "ins exp" and "del exp" in the "let" is related to "ins:"  "del_ok", [del], [ins].
I don't understand how given the "let" expression the following expressions are expanded

 Fail [msg] (ins: if del_ok then [del] else [])

and

Fail [msg] [ins]

Appreciate any help.


Thanks

Daryoush