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
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