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-- In a form-encoded string, the same key may appear several times, with or
-- without values, e.g. key&key=1&key=2. What type might you use to represent
-- the values associated with a key in this sort of string? Write a parser that
-- correctly captures all of the information.
-- I will use Map String [String] where the values are lists of values for a
-- urlencoded-key. Empty list indicates no values for the key.
{-- From examples/examples/ch10/Parse.hs and modified:
-- removed unused code,
-- made Parse an instance of Applicative,
-- used the do notation as this chapter is "Programming with monads".
--}
import Data.Map
data ParseState = ParseState {
string :: String
} deriving (Show)
newtype Parse a = Parse {
runParse :: ParseState -> (a, ParseState)
}
(==>) :: Parse a -> (a -> Parse b) -> Parse b
firstParser ==> makeSecondParser = Parse chainedParser
where chainedParser initState = runParse (makeSecondParser firstResult) newState
where
(firstResult, newState) = runParse firstParser initState
identity :: a -> Parse a
identity a = Parse (\s -> (a, s))
instance Functor Parse where
fmap f parser = parser ==> \result ->
identity (f result)
instance Applicative Parse where
pure = identity
parFunc <*> parVal =
parVal ==> \v ->
parFunc ==> \f ->
pure (f v)
instance Monad Parse where
return = pure
(>>=) = (==>)
parse :: Parse a -> String -> a
parse parser initState = fst (runParse parser (ParseState initState))
getState :: Parse ParseState
getState = Parse (\s -> (s, s))
putState :: ParseState -> Parse ()
putState s = Parse (\_ -> ((), s))
parseChar :: Parse (Maybe Char)
parseChar = do
initState <- getState
case string initState of
[] ->
return Nothing
(char:remainder) -> do
putState newState
return (Just char)
where newState = initState { string = remainder }
peekChar :: Parse (Maybe Char)
peekChar = (safeHead . string) <$> getState
where safeHead [] = Nothing
safeHead (x:xs) = Just x
parseWhile :: (Char -> Bool) -> Parse String
parseWhile p = do
mp <- (fmap p <$> peekChar)
if mp == Just True
then do
mc <- parseChar
case mc of
Nothing -> return []
Just c -> (c:) <$> parseWhile p
else return []
{-- End of code from examples --}
parseKey :: Parse String
parseKey = parseWhile isKeyEnd
where isKeyEnd c = (c /= '&' && c /= '=')
parseValue :: Parse String
parseValue = do
c <- peekChar
case c of
Nothing -> return ""
Just '&' -> do
-- No value.
-- Consume the end character so that the next parseKey starts from the
-- first character of the next key.
parseChar
return ""
Just '=' -> do
-- Consume the '='
parseChar
v <- parseWhile isValueEnd
-- Consume the end character
parseChar
return v
where isValueEnd c = (c /= '&')
parseKeyValuePairs :: Parse (Map String [String])
parseKeyValuePairs = do
k <- parseKey
case k of
"" -> return empty
_ -> do
v <- parseValue
case v of
"" -> (insertWith (++) k []) <$> parseKeyValuePairs
_ -> (insertWith (++) k [v]) <$> parseKeyValuePairs
-- ghci> :l 15_a_3.hs
-- [1 of 2] Compiling Main ( 15_a_3.hs, interpreted )
-- Ok, one module loaded.
-- ghci> parse parseKeyValuePairs ""
-- fromList []
-- ghci> parse parseKeyValuePairs "key"
-- fromList [("key",[])]
-- ghci> parse parseKeyValuePairs "key&"
-- fromList [("key",[])]
-- ghci> parse parseKeyValuePairs "key="
-- fromList [("key",[])]
-- ghci> parse parseKeyValuePairs "key=123"
-- fromList [("key",["123"])]
-- ghci> parse parseKeyValuePairs "key=123&"
-- fromList [("key",["123"])]
-- ghci> parse parseKeyValuePairs "key&key=1&key=2"
-- fromList [("key",["1","2"])]
-- ghci> parse parseKeyValuePairs "key&foo=4&key=2&foo=&bar&key=&foo=3&key=1&foo&key=0"
-- fromList [("bar",[]),("foo",["4","3"]),("key",["2","1","0"])]
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